Human microbiocenose. Microflora of mouth cavity and its age-dependent changes.

Pathologic flora of the mouth cavity. Phusospirochetosis. Streptococcus mutans and its role in the caries aetiology Dysbacteriosis, causes of its appearance. Principles of microbiologic diagnostics.  Pathogenic flora of mouth cavity. Fuzospirochetosis. Streptococcus mutans, its role in development caries.

Basic principles of chemotherapy. Chemotherapeutical drugs. Methods of examonation of bacterial susceptibility to chemotherapeutical drugs. Microbial antagonism. Antibiotics. Methods of determination of bacterial susceptibility to antibiotics.

The doctrine about infection. Pathogenicity and virulence of bacteria and their role in stomatologic diseases development.

 

normal microflora of the human body and the methods of studying. dysbacteriosis, the causes of its beginning, prophylaxis, and treatment

 

Microflora of the body

Microbiocenosis is microbial community of different bacterial populations, which colonize certain biotope. 

Biotope is an area with relatively homogenous conditions where microbial population can survive.

Ecological niche is the  place  or status of microbes in their biotic environment.  

Constant (obligate, resident, indigenous, autochthonous) microflora is native, no imported  one of different biotopes.

Transient (temporary, facultative, allochthonous) microflora is not aboriginal, it is acquired one.

Before birth, the human body has no normal flora. During the birth process, the body comes in contact with microbes in the external environment. Later with the initial feedings and exposure to an expanding environment some microorganisms find their way to a permanent residence in many parts of the body

Most organisms in the external environment apparently do not find the body to be a favourable habitat. Charactenstic features of different body areas, such as temperature, oxygen availability, nutrient availability, natural inhibitors and pH influence the population that is able to survive and establish itself. Because these conditions vary from site to site in the body different sites acquire considerably different organisms as their normal flora. Once the normal flora is established it benefits the body by preventing the overgrowth of undesirable organisms. Destruction of the normal flora frequently disrupts the status quo resulting in the growth of harmful organisms. This can be seen after the prolonged administration of broad spectrum antibiotics. For example if the normal flora of the intestinal tract and vagina  are largely destroyed, the yeast Candida albicans which is unaffected by these antibacterial antibiotics can grow unchecked to become the major organism in these areas. It then cm infect the mucous membranes and the skin, causing a severe inflammation. Another complication of antibiotic therapy is a severe gastroenteritis known as pseudomembranous colitis. This syndrome has been associated with several antimicrobial agents but the antibiotics clindamycin and lincomycin have been incriminated most often. The mechanism of this diarrhoea was elucidated when it was observed that the use of these antibiotics resulted in an overgrowth in the intestine of an organism identified is Clostridium difficile. This organism produces in enterotoxin that causes the gastroenteritis but it can do so only when antibiotic therapy destroys much of the other normal intestinal flora permitting it to grow unchecked.

Our normal flora can be categorized is helpful (mutualutic symbionts) harmless (commensals) or potentially harmful (opportunists) However these groups are not mutually exclusive. Under certain circumstances even a mutualism cause harm and, thus, become a pathogen. Therefore these categories are of value only in describing the usual role of the organism in relation to its host.

In a mutualistic relationship the microbe and the host benefit one another. This type of relationship is common in the plant kingdom and is essential in ruminants such as cattle in which microbes are necessary for digestion of the cellulose in plant material. Few such relation ships exist in humans, however. Probably the only good example of mutualism in humans is found in the normal flora of the large intestine where enteric organisms synthesize vitamin K and the vitamins of the B complex,  enabling them to be absorbed through the intestinal wall and contribute to humautrition. However considering that our normal flora provides us with protection by interfering with the growth of potentially harmful organisms much of our normal flora could be considered mutualistic symbionts.

The microbe that lives on and benefits from its host without either benefiting or harming the host is called a commensal. Most of the organisms that make up the normal flora of a healthy individual could be categorized as commensals.

Opportunists (microbes that are potential pathogens) are of greatest interest to us. These organisms seem to lack the ability to invade and cause disease in healthy individuals but may be able to colonize as pathogens in ill or injured persons. Staphylococcus aureus is a good example of an opportunist. Many people (about 25 %) carry staphylococci in their nasopharynx without suffering any illness. However if these people acquire respiratory tract infections such as measles or influenza the staphylococci can invade the lung and cause severe pneumonia. Accidental contamination of the bladder with E coli or Enterococcus faecalis during a catheterization procedure also can lead to opportunistic infection. Both these organisms ire part of the normal flora of the large intestine and usually do not produce urinary tract infections. How ever if they gain access to the urethra or are transplanted mechanically to an environment in which they can grow they can cause disease. The most startling examples of opportunistic infections are associated with the viral infection known as acquired immunodeficiency syndrome (AIDS). This syndrome is caused by a virus that destroys certain subsets of T cells inhibiting the body’s ability to mount in immune response. As a result infection by the virus causing AIDS is characterized by severe and eventually fatal infections or malignancies that do not occur in individuals with functional immune systems.

Because much of our normal flora can cause disease under the proper conditions these organisms could be considered opportunists. This is particularly true in elderly and debilitated individuals and in patients receiving immunosuppressive drug therapy to prevent rejection of organ transplants. Opportunists are especially important as causes of nosocomial infections (ic those acquired during hospitalization).  Under appropriate circumstances most of the organisms that constitute our normal flora can cause disease.

Another group of bacteria that is not really part of our normal flora consists of pathogenic organisms that can exist in a large percentage of the population without causing disease. This group includes such organisms is Neisseria meningitidis (also called the meningococcus) the causative agent of epidemic meningitis. Many individuals carry this organism in their respiratory tracts without ever having meningitis, yet they can spread the bacterium to nonimmune individuals and cause disease. Streptococcus pneumoniae (the pneumococcus) the major cause of lobar pneumonia also is carried by 10 % to 20 % of normal healthy individuals. Persons who harbor bacteria such is these without ever exhibiting overt symptoms of disease are referred to as carriers.

Thus, although our normal flora can be beneficial by preventing the growth of potential pathogens, it also can be a reservoir from which endemic and epidemic diseases are spread.

A diverse microbial flora is associated with the skin and mucous membranes of every human being from shortly after birth until death. The human body, which contains about 10¹³ cells, routinely harbors about 10¹⁴ bacteria.This bacterial population constitutes the normal microbial flora. The normal microbial flora is relatively stable, with specific genera populating various body regions during particular periods in an individual’s life. Microorganisms of the normal flora may aid the host (by competing for microenvironments more effectively than such pathogens as Salmonella spp or by producing nutrients the host can use), may harm the host (by causing dental caries, abscesses, or other infectious diseases), or may exist as commensals (inhabiting the host for long periods without causing detectable harm or benefit). Even though most elements of the normal microbial flora inhabiting the human skin, nails, eyes, oropharynx, genitalia, and gastrointestinal tract are harmless in healthy individuals, these organisms frequently cause disease in compromised hosts. Viruses and parasites are not considered members of the normal microbial flora by most investigators because they are not commensals and do not aid the host.

 

 

Human microflora is the result of a mutual adaptation of micro- and macro-organisms in the process of evolution. Most bacteria of the normal and constant microflora of the human body have adapted themselves to life in certain parts of the body (tabl. 1, 2, 3). Besides, there are some microbes which make up a temporary (casual) microflora.

With the development of virology and the improvement of virological technique, our concepts on the microflora of the human body were increased. It has been established that not only the open cavities, but the tissues of the human organism are inhabited by numerous persisting viruses which are excreted into the environment with milk, saliva, sputum, perspiration, urine, and faeces.

There are different methods of human microbiocenosis studying. Among them, biopsy, pad method, impression method, sticky film method, swab-washing method, scrub-washing technique etc. After receiving of tested material it should be inoculated on different nutrient media.

Microflora of the skin. Staphylococci, streptococci, moulds and yeasts, diphtheroids, and also certain pathogenic and conditionally pathogenic bacteria live on the surface of the skin. They receive their nutrition from the secretions of the sebaceous and sweat glands, dead cells, and waste products.

The total number of microbes on the skin of one person varies from 85000000 to 1212000000.

When the human body comes into contact with the soil, the clothes and skin are seeded with spores of different species of microbes (organisms responsible for tetanus, anaerobic infections, etc.).

Most frequently the exposed parts of the human body are infected, e. g. the hands, on the surface of which colibacilli, staphylococci, streptococci, enterococci, moulds, yeasts, fungi imperfecti, and spores of aerobic and anaerobic bacilli are found.

Pustular and fungal infections of the skin and gastrointestinal diseases often occur owing to violation of sanitary-hygienic conditions and normal conditions of the work and life of people.

Microbes of the mouth cavity. In the mouth cavity there are more than 100 species of microbes. There are the natural inhabitants (acidophilic bacillus, Treponema microdentium, diplococci, Streptococcus salivarius, Entamoeba gingivalis, etc.). Besides, in the mouth cavity there are foreign microbes or those which have been carried in from the environment together with food, water, and air.

Pathogenic and conditionally pathogenic microbes (staphylococci, streptococci, diphtheria bacilli, diphtheroids, Borrelia organisms, spindle-shaped bacteria), protozoa (amoebae and trichomonads) are found on the mucous membrane of the mouth.

The mouth cavity is a favourable medium for many microbes; it has an optimal temperature, a sufficient amount of food substances, and has a weakly alkaline reaction.

The greatest amount of microbes can be found at the necks of the teeth and in the spaces between teeth. Streptococci and diplococci are found on the tonsils. There are many microbes in other parts of the mouth cavity, which are inaccessible to the bathing action of saliva and the action of lysozyme (an enzyme found in the saliva, lacrimal fluid and sputum). The presence of carious teeth is a condition for increasing the microflora in the mouth cavity, for the appearance of decaying processes and unpleasant odours.

The microflora of the gastrointestinal tract (Table 3, 4). When the stomach functions normally, it is almost devoid of microflora due to the marked bactericidal properties of gastric juice.

The gastric Juice is considered to be a reliable defense barrier against the penetration of  pathogenic and conditionally pathogenic microbes into the intestine. However, the degree of acidity of the gastric juice is not always constant. It varies according to the character of the food and the amount of water consumed.

 

Table 3.

BACTERIA CONSIDERED TO BE NORMAL FLORA OF HUMANS

AND THEIR ANATOMICAL LOCATIONS

 

“++” – prominent; “+” – common; “+/-” –“irregular, occasional or transient.

 

Together with food, lactic acid bacteria, Sarcina ventriculi. hay bacillus, yeasts, etc., enter the stomach from the mouth. In some cases, dysentery, enteric fever, and paratyphoid bacilli and other pathogenic microbes are capable of penetrating into the stomach and then the intestine.

Table 3

EXAMPLES OF TISSUE SPECIFICITIES OF SOME BACTERIA ASSOCIATED WITH HUMANS

 

 

Enterococci, fungi, and various other microbes are relatively rarely found in the duodenum. There are few microbes in the small intestine. Enterococci are found more often than others – In the large intestine there are large amounts of micro-organisms. Almost one-third of the dry weight of the faeces of certain animal species is made up of microbes. Daily, an adult human excretes about 17 million billion micro-organisms with the excrements (Table 4, 5).

Table 4.

EXAMPLES OF SPECIFIC ATTACHMENTS OF BACTERIA

TO HOST CELL OR TISSUE SURFACES

 

 

Table 5.

BACTERIA FOUND IN THE LARGE INTESTINE OF HUMANS

 

The intestinal microflora undergoes essential changes with the age of man. The intestinal tract of the newly-born baby in the first hours of life is sterile. During the first days it becomes inhabited by temporary microflora from the environment, mainly from breast milk. Later on, in the intestine of the newly-born baby a specific bacterial flora is established consisting of lactic acid bacteria (biphidobacteria, acidophilic bacillus), which is retained during the year. It has antagonistic properties in relation to many microbes capable of causing intestinal disorders in breastfed children, and remains during the whole period of breast feeding. However, on the 3rd-5th day of life in the intestine of breast-fed children E. coli and enterococci can be found, the amount of which sharply increases with the change to mixed feeding. After breast feeding is stopped the microflora of the child’s intestine is completely replaced by a microflora typical of adults (E. coli, Clostridium perfringens, Clostridium sporogenes, Streptococcus faecalis. Proteus vulgaris, etc.).

At present it has been established that such a constant inhabitant of the intestine of man as Clostridium perfringens is capable of secreting digestive enzymes. The colibacillus and other species of microbes in the intestine produce the vitamins essential for the human body (B₁, B₂, B₁₂, K). Microbe antagonists (acidophilic, Lactobacillus bulgaricus etc.) are beneficial to the organism as they hinder the development of pathogenic bacteria which, together with infected food and water, may enter the intestine.

The pathogenic serotypes of E. coli which are capable of causing severe diseases (colienteritis) mostly in children have been found to be present in the human intestine together with the on pathogenic species.

Anaerobic bacteria which do not produce spores, the so-called bacteroids, inhabit mainly the lower part of the large intestine in human. They are found during acute appendicitis, postpartum infection, pul-
monary abscesses, septicaemia of different aetiology, postoperative infectious complications in the peritoneal cavity, inflammatory processes of the gastrointestinal tract, respiratory tract, and on the skin

Metchnikoff considered some species of intestinal bacteria to be harmful, causing chronic intoxications. He suggested the method (if combating them by introducing lactic acid bacilli (Lactohacillus bulgaricus) bearing antagonistic properties into the intestine. Besides, Metchnikoff recommended a diet of vegetables and fruit, rich in sugar, and considered it advisable to build one’s life according to the principles of orthobiosis (normal work, healthy relaxation, hygienic conditions, and prophylaxis of diseases).

Enteroviruses live in large quantities in the intestine. They may be found for a long time in healthy persons without causing diseases. In unfavourable conditions associated with some species of bacteria they cause the most varied clinical forms of disease.

Microflora of the respiratory tract. People breathe in a large number of dust particles and adsorbed micro-organisms. Experimentally, it has been established that the amount of microbes in inspired air is 200-500 times greater than in expired air Most of them are trapped in the nasal cavity and only a small amount enters the bronchi, The pulmonary alveoli and the terminal branches of bronchi are usually sterile. The upper respiratory tract (nasopharynx, pharynx) contains relatively constant species (Staphylococcus epidermidis, streptococci, diphtheroids, Gaffkya tetragena, etc.).

When the defense mechanisms of the body are weakened as a result of cooling, starvation, vitamin deficiency, or traumas, the constant inhabitants of the respiratory tract become capable of causing different diseases (acute catarrhs of the respiratory tract, tonsillitis, pneumonia, bronchitis, etc.).

The nasal cavity contains a small amount of microbes. The mucous membrane of the nose produces mucin and lysozyme which have a bactericidal action. However, in spite of this, the nasal cavity has a relatively constant microflora (haemolytic or nasal micrococcus, diphtheroids, non-haemolytic staphylococci, haemolytic staphylococci, saprophytic Gram-negative diplococci, capsular Gram-negative bacteria, haemoglobinophilic bacteria of influenza, Proteus, etc). In the respiratory tract, besides the bacterial microflora. many viruses, in particular adenoviruses. can remain viable for long periods without causing pathological processes.

Microflora of the vagina. In the first 2 days after birth the baby’s vagina is sterile. Sometimes it contains a small amount of Gram-positive bacteria and cocci. After 2-5 days of life the coccal microflora becomes fixed and remains constant until puberty, when it is replaced by Dodderlein’s lactic acid bacilli.

During the menstrual cycle the contents of the vagina become alkaline which is favourable for the development of coccal microflora. During sexual life the microflora of the vagina changes, and many microbes appear which are introduced from outside.

The microflora of the vagina undergoes profound changes during gynaecological diseases (endometritis, metritis, ovaritis, etc.), and after abortions.

The vaginal contents of the healthy woman have a relatively high concentration of sugar and glycogen, and a low content of the diastatic enzyme and proteins. The pH is 4.7 during which all other microbes except for Doderlein’s lactic acid bacilli, cannot develop.

As has been established, the acid medium of the vagina depends on the presence of glycogen which under the influence of vaginal bacteria is transformed into mono- and disaccharides and then into lactic acid. The amount of glycogen depends on the function of the ovaries and the condition of the whole body.

Vaginal bacteria have antagonistic properties; because of this, normal microflora should be protected and should not be exposed to the harmful effect of medicines (antibiotics, sulphonamide preparations, rivanol, osarsol. potassium permanganate, etc.) to which Doderlein’s lactic acid bacilli are more sensitive than the bacteria against which these substances are employed.

Microflora of the urinary tract. In men in the anterior part of the urethra there are Staphylococcus epidermidis. diphtheroids and Gram-negative non-pathogenic bacteria. Mycobacterium smegmatis and mycoplasmas are found on the external parts of the genitalia, and also in the urine of men and women.

The female urethra is usually sterile, in some cases it contains a small amount of non-pathogenic cocci.

The bacteria of the mucous membranes of the eyes include Staphylococcus epidermidis, Corynebacterium xerosis, mycoplasmas, etc. When the organism is weakened or when there are visual disturbances and vitamin deficiency, the normal inhabitants of the mucous membranes may become relatively pathogenic and may cause different diseases of the mucous membrane, such as conjunctivitis, blepharitis and other suppurative processes.

The normal microflora is not constant but depends on the age, nutrition and general condition of the macro-organism. The microflora of the human body undergoes profound changes, especially during various diseases.

Disturbances in the species composition of the normal microflora occurring under the influence of infectious and somatic diseases, and long-term and irrational use of antibiotics bring about the state of dysbacteriosis. This is characterized by disturbances in the assimilation of products of digestion, by changes in the enzymatic processes, and by the cleavage of ready-made physiological secretions. The territorial deviations of microflora cause a whole series of complications: intestinal dyspepsia, toxinfections, suppurative processes, catarrhs of the respiratory tract, pneumonia, candidiasis, etc. In dysbacteriosis the number of lactic acid bacteria is diminished, the number of anaerobes increased, Gram-positive bacteria change to Gram-negative and Gram-negative to Gram-positive, etc.

The question arises whether animal life is possible without microbes, It was already known in the last century that micro-organisms were very rarely found in birds and animals of the arctic regions. There were cases with absolutely no microflora found in the body of some birds. Pasteur made an attempt to raise amicrobic animals, but with the level of techniques of that time the problem could not be solved.

A new branch of biology, gnotobiology, is now developing. It studies the microbe-free organisms. Amicrobic chicks, rats, mice, guinea pigs, sucking pigs. and other animals have been reared.

Amicrobic animals, or gnotobiotes, are subdivided into several groups, monobiotes. (absolutely microbe-free animals), dibiotes (animals infected with one microbial species), polyobiotes (animals harbouring more than one species of microbes in their body).

Germ-free (gnotobiotic) animals are good experimental models for investigating the interactions of animals and microorganisms. To determine the role of the normal microbiota, animals can be delivered by aseptic Caesarean section (the surgical removal of the fetus from the uterus via the abdomen) so they will not be contaminated by the normal microbiota of the vagina and birth canal during vaginal delivery. Germ-free animals can been be raised in the absence of microorganisms by being kept in a sterile environment. They are fed sterile food and water and given sterile air to breathe. Comparing animals possessing normal associated microbiota with germ-free animals permits the exploration of the complex relationships between microorganisms and host animals.

Germ-free animals develop abnormalities of the gastrointestinal tract. They are more susceptible to disease than animals with normal associated microbiota. Germ-free animals are more susceptible to bacterial infection. Organisms such as Bacillus subtilis and Micrococcus luteus, which are harmless to other animals, cause disease in germ-free animals. More exotic pathogenic microorganisms such as Vibrio cholerae and Shigella dysenteriae are far more readily able to establish infections where there are no normal microbiota. They do not have to compete for survival within the intestinal tract. At the same time, though, germ-free animals are resistant to Entamoeba histolytica, the causative organism of amebic dysentery. This is because this protozoan requires normal intestinal bacteria as a food source. Likewise, tooth decay is no problem to germ-free animals, even those on high sugar diets, because they do not have lactic acid bacteria — the bacteria that cause tooth decay — in their oral cavities.

Scientists focused their attention at gnotobiotes because it was necessary to study deeply the role of normal microflora in the mechanisms of infectious pathology and immunity. As compared to the commonly encountered animals, gnotobiotes have a larger caecum, underdeveloped lymphoid tissue, internal organs of lesser weight, smaller blood volume, a reduced content of water in the tissue and of antibodies in blood serum.

Gnotobiology provides the means for revealing more precisely the role of normal microflora in the synthesis of vitamins and amino acids, in the production of congenital and acquired immunity, and the relationship of bacteria and viruses. Much importance is attached to gnotobiology in the study of space and the life conditions of man and animals during space flight.

 

Microflora at the pathological processes of oral cavity. fusospirochetosis. streptococcus mutans that his role in etiology of caries. principles of microbiological diagnosis

 

Міcroflora of mouth

Oral bacteria include streptococci, lactobacilli, staphylococci, corynebacteria, and various anaerobes in particular bacteroides. The oral cavity of the new-born baby does not contain bacteria but rapidly becomes colonized with bacteria such as Streptococcus salivarius. With the appearance of the teeth during the first year colonization by Streptococcus mutans and Streptococcus sanguis occurs as these organisms colonise the dental surface and gingiva. Other strains of streptococci adhere strongly to the gums and cheeks but not to the teeth. The gingival crevice area (supporting structures of the teeth) provides a habitat for a variety of anaerobic species. Bacteroides and spirochetes colonize the mouth around puberty

In the oral cavity  bacteria are found on the tooth surface (above all in subgingival plaque), in the saliva, on the tongue surface and in the tonsillar crypts.

Genera commonly found in the oral cavity are Actinomyces, Arachnia, Bacteroides, Bifidobacterium, Eubacterium, Fusobacterium, Lactobacillus, Leptotrichia, Peptococcus, Streptococcus, Propionibacterium, Selenomonas, Treponema, and Veillonella.

Their role in dental caries, periodontal disease, root canal infections, infections of the hard and soft oral tissue, as well as their importance as foci for disseminated infectious disease is well established. Poor oral hygiene and periodontal disease may promote oropharyngeal colonization by potential respiratory pathogens (Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, etc.).

Anaerobic genera or genera that include anaerobic members found in the oral cavity are Actinomyces, Arachnia, Bacteroides, Bifidobacterium, Eubacterium, Fusobacterium, Lactobacillus, Leptotrichia, Peptococcus, Peptostreptococcus, Propionibacterium, Selenomonas, Treponema, and Veillonella. The incidence of anaerobes varies with age of the individual and with specific sites sampled. In edentulous infants, the incidence of anaerobes is relatively low. In adults, anaerobes are invariably present but are more prevalent in samples from the gingival sulcus than they are in samples from the gingival margin, tooth surfaces, buccal mucosa, tongue, or saliva. In samples from the healthy gingival sulcus, anaerobic, gram-positive bacilli are found in the range of 5%-14%; gram-negative bacilli in the range of 13%-29%; Veillonella in the range of 2%-8%; and gram-positive cocci in the range of 1%-15% of the cultivable flora. From marginal plaque and plaque from the tooth surface, gram-positive bacilli, gram-positive cocci, and Veillonella appear to be the predominant anaerobes. In saliva, Veillonella are the most numerous anaerobes.

It’s estimated that 500-600 different kinds of bacteria thrive on mucus and food remnants in the mouth. A predominant member of this community is the Gram positive bacterium Streptococcus mutans. It grows on biofilms on the surface of teeth (plaque) where it consumes sugar and converts it to lactic acid. Lactic acid erodes the enamel on the surface of teeth, which leads to the formation of cavities. Interestingly, a group of researchers have developed a strategy to combat dental decay by using a genetically modified strain of bacteria that produces a toxin that specifically kills S. mutans. The trick is that this genetically modified strain of bacteria will only survive in your mouth if you provide it with specific nutrients. Basically, you brush the new strain of bacteria onto your teeth and they produce a toxin that prevents the growth of S. mutans thereby reducing the production of lactic acid. To maintain the strain of bacteria in your mouth you provide the essential nutrient by swishing daily with a mouthwash—just remember to feed your bacteria!
        Anaerobic bacteria colonize at the gumline and between the teeth. More anaerobic than aerobic bacteria are found in the mouth, and especially high numbers of Fusobacterium and spirochetes have been implicated in periodontal disease. Both Streptococci and Lactobacilli play a major role in dental caries by converting oligosaccharides from the diet into dextran and levan, which cause plaque formation on the teeth. Fluoride and vitamin B6 reduce the numbers of these bacteria in the mouth, reducing the potential for cavities.¹

 

I. Dental Plaque: Structural, Microbiological and Developmental Characteristics

Dental plaque is a soft deposit that accumulates on the teeth. Plaque can be defined as a complex microbial community, with greater than 10¹⁰ bacteria per milligram. It has been estimated that as many as 400 distinct bacterial species may be found in plaque. In addition to the bacterial cells, plaque contains a small number of epithelial cells, leukocytes, and macrophages. The cells are contained within an extracellular matrix, which is formed from bacterial products and saliva. The extracellular matrix contains protein, polysaccharide and lipids. Figure 1 shows a microscopic view of a smear of mature supragingival plaque. Note the presence of a variety of different bacterial forms (cocci, rods, filaments, as well as a eukaryotic cell.

Inorganic components are also found in dental plaque; largely calcium and phosphorus which are primarily derived from saliva. The inorganic content of plaque is greatly increased with the development of calculus. The process of calculus formation involves the calcification of dental plaque. The practical consequences of calculus formation are that the deposit is significantly more difficult to remove once calcified, and it leaves a rough surface on the root which is easily colonized by plaque. The calculus on the root surface of an extracted tooth is seen in Figure 2. Note the brown to black coloration of the subgingival calculus that extends to the apex of the distobuccal root, in contrast to the whitish color of the supragingival calculus.

Dental plaque can be classified in several different ways. Plaque is classified as supragingival or subgingival based on its relationship to the gingival margin. Supragingival plaque is evident on the tooth above the gingival margin (Figure 3). Plaque can also be classified by its relationship to the tooth surface, as either attached or unattached plaque. The unattached subgingival plaque is more closely associated with the wall of the subgingival tissues than is the attached plaque. Lastly, plaque has been classified by association with disease state as “health-associated” or “disease-associated”. The latter classification is related to differences in the microbial composition of dental plaque in health versus disease.

The development of dental plaque has been studied in humans as well as non-human animal model systems. One of the most commonly used models of plaque development is referred to as the “experimental gingivitis” model (Loe, et al., 1965). This protocol involves the examination of subjects (usually dental students!) who abstain from any oral hygiene measures for a period of three weeks. These studies have provided much information on the structural and microbiological characteristics of dental plaque.

The pellicle is evident as lightly stained material on a tooth surface when patients use disclosing solution (Figure 4). A newly cleaned tooth surface is rapidly covered with a glycoprotein deposit referred to as “pellicle”. The pellicle is derived from salivary constituents which are selectively adsorbed onto the tooth surface. Components of the dental pellicle include albumin, lysozyme, amylase, immunoglobulin A, proline-rich proteins and mucins. The formation of pellicle is the first step in plaque formation.

The pellicle-coated tooth surface is colonized by Gram-positive bacteria such as Streptococcus sanguis, Streptococcus mutans, and Actinomyces viscosus. These organisms are examples of the “primary colonizers” of dental plaque. Bacterial surface molecules interact with components of the dental pellicle to enable the bacteria to attach or adhere to the pellicle-coated tooth surface. For example, specific protein molecules found as part of the bacterial fimbria (hair-like protein extensions from the bacterial cell surface) on both Streptococcus sanguis and Actinomyces viscosus interact with specific proteins of the pellicle (the proline-rich proteins) with a “lock and key” mechanism that results in the bacteria firmly sticking to the pellicle-coating on the tooth surface (Mergenhagen et al. 1987). Within a short time after cleaning a tooth, these Gram-positive species may be found on the tooth surface.

After the initial colonization of the tooth surface, plaque increases by two distinct mechanisms: 1) the multiplication of bacteria already attached to the tooth surface, and 2) the subsequent attachment and multiplication of new bacterial species to cells of bacteria already present in the plaque mass. The secondary colonizers include Gram-negative species such as Fusobacterium nucleatum, Prevotella intermedia, and Capnocytophaga species. A key property of these microorganisms appears to be the ability to adhere to Gram-positive species already present in the existing plaque mass. These organisms would typically be found in plaque after 1 to 3 days of accumulation. Figure 5 shows the accumulation of plaque on a tooth surface one day after prophylaxis. When the structure of dental plaque from this time period is observed, the presence of a complex array of bacterial cocci, rods and filaments is apparent.

After one week of plaque accumulation, other Gram-negative species may also be present in plaque. These species represent what is considered to be the “tertiary colonizers”, and include Porphyromonas gingivalis, Campylobacter rectus, Eikenella corrodens, Actinobacillus actinomycetemcomitans, and the oral spirochetes (Treponema species). The structural characteristics of dental plaque in this time period reveal complex patterns of bacterial cells of cocci, rods, fusiform, filaments, and spirochetes. In particular, specific associations of different bacterial forms have been observed. For example, the adherence of cocci to filaments results in a typical form referred to as “test-tube brushes” or “corn-cob” arrays and these structures can be seen in Figure 6. The structural interactions of the bacteria probably are a reflection of the complex metabolic interactions that are known to occur between different plaque microorganisms. One example of this is the production of succinic acid from Campylobacter species that is known to be used as a growth factor by Porphyromonas gingivalis. Streptococcus and Actinomyces species produce formate, which may then be used by Campylobacter species. Fusobacterium species produce both thiamine and isobutyrate that may be used by spirochetes to support their growth. The metabolic and structural interactions between different plaque microorganisms are a reflection of the incredible complexity of this ecological niche.

The overall pattern observed in dental plaque development is a very characteristic shift from the early predominance of Gram-positive facultative microorganisms to the later predominance of Gram-negative anaerobic microorganisms, as the plaque mass accumulates and matures. This developmental progression is also reflected in the shifts in predominant microorganisms that are observed in the transition from health to disease. Studies of plaque taken from sites of health or disease and examined either microscopically or by culturing have demonstrated distinct differences in health versus disease-associated microbial populations.

Microscopic studies of plaque have examined the presence of different morphological types (“morphotypes”) of bacteria. Figure 7 is a Gram-stained smear of mature subgingival plaque illustrating different morphotypes (cocci, rods, filaments, spirochetes). These studies reveal an increase in the presence of motile rods and spirochetal organisms in gingivitis and periodontitis as compared to gingival health. A major limitation of studies of bacterial morphotypes is that many “health-associated” microorganisms are indistinguishable from “disease-associated” microorganisms (for example, Streptococcus species and Porphyromonas gingivalis, respectively). However, cultural studies also reveal characteristic distinctions between health- and disease-associated plaque. The percentage of Gram-positive rods and cocci decrease in gingivitis- and periodontitis-associated plaque as compared to health-associated plaque. Similarly, the percentage of microbiota compised of Gram-negative anaerobic species is greatly increased in gingivitis (approximately 25) and periodontitis (approximately 75) as compared to health (approximately 13, Slots, 1979). Specific microbial species that are important in plaque development and disease development are outlined below based on their categorization by cell wall morphology (Gram-positive, Gram-negative, or spirochetal) and their physiological status (facultative or anaerobic).

 

Dental Plaque: Relationship of Specific Microorganisms to Periodontal Diseases

Our understanding of the relationship between the microorganisms found in dental plaque and the common dental disease of periodontitis has undergone numerous phases historically (Figure 8, Concepts of Bacterial Etiology, adapted from Socransky, 1992). Early in the 19th century, it was felt that, like the situation with diseases such as tuberculosis, a specific bacterial species was responsible for the disease processes. The criteria by which a given bacterial species was associated with disease historically has been through the application of Koch’s Postulates. These criteria were developed by Robert Koch in the late 1800’s. The criteria are as follows:

1. A specific organism can always be found in association with a given disease.

2. The organism can be isolated and grown in pure culture in the laboratory.

3. The pure culture will produce the disease when inoculated into a susceptible animal.

4. It is possible to recover the organism in pure culture from the experimentally infected animal.

However, the concept that a specific bacterial species was responsible for periodontal diseases fell out of favor for several reasons. First, dispite numerous attempts, a specific bacterial agent was not isolated from diseased individuals. Rather, the organisms found associated with disease were also found associated with health. Good experimental animal model systems of periodontal disease were not available to test the pathogenicity of specific microorganisms (this, in fact, remains problematic today). Further, in the mid 1900’s, epidemiological studies indicated that the older an individual was, the more likely they were to have periodontal disease. This led to the concept that the bacterial plaque itself, irrespective of the specific bacteria found in plaque, was associated with disease. This concept, known as the Non-Specific Plaque Hypothesis (Loesche, 1976), held that all bacteria were equally effective in causing disease.

Several important developments caused a change in this thinking. First, it was realized that organisms that are found as part of the “normal” bacterial flora (i.e., found in health), may function as pathogens under certain conditions. These organisms may be altered, or increase significantly iumbers relative to other non-pathogenic species, to function as pathogens. This type of bacterial pathogen is referred to as an endogenous pathogen, in contrast to an organism that is not normally found in healthy states which is termed an exogenous pathogen. Secondly, tremendous advances were made in the 1960’s and 1970’s in techniques used to culture anaerobic microorganisms (bacterial species that cannot grow in the presence of oxygen). These advances were related to the anaerobic culturing conditions as well as the nutrients required in media to grow anaerobic species, which are typically very fastidious in their nutrient requirements. The growth of anaerobic microorganisms, and examination of their properties using in vitro and in vivo model systems, has now led us back to the understanding that different microorganisms have varying potential to cause disease. Thus, the current concept of the processes involved in the development of periodontal diseases fall under the Specific Plaque Hypothesis (Loesche,1976). The Specific Plaque Hypothesis states that disease results from the action of one or several specific pathogenic species and is often associated with a relative increase in the numbers of these organism found in plaque.

A form of Koch’s Postulates specifically oriented to the situation in periodontal diseases has been proposed by a microbiologist by the name of Socransky. Socransky’s criteria for periodontal pathogens are as follows:

1. ASSOCIATION: A pathogen should be found more frequently and in higher numbers in disease states than in healthy states

2. ELIMINATION: Elimination of the pathogen should be accompanied by elimination or remission of the disease.

3. HOST RESPONSE: There should be evidence of a host response to a specific pathogen which is causing tissue damage.

4. VIRULENCE FACTORS: Properties of a putative pathogen that may function to damage the host tissues should be demonstrated.

5. ANIMAL STUDIES: The ability of a putative pathogen to function in producing disease should be demonstrated in an animal model system.

The two periodontal pathogens that have most thoroughly fulfilled Socransky’s criteria are Actinobacillus actinomycetemcomitans in the form of periodontal disease known as Localized Juvenile periodontitis (LJP), and Porphyromonas gingivalis in the form of periodontal disease known as adult periodontitis. Selected properties of these microorganisms that have been associated with disease are summarized in the following tables.

 

Other species that have been implicated as pathogens, including Fusobacterium nucleatum, Prevotella intermedia, Eikenella corrodens, Campylobacter rectus, Bacteroides forsythus, and the oral spirochetes of the genus Treponema. It is important to note that the disease processes involve not only pathogenic microorganisms, but also a susceptible host. Further, many microorganisms function to the benefit of the host, by inhibiting the growth of potential pathogenic species. One example of such an interaction is Streptococcus sanguis, which produces hydrogen peroxide that is lethal for Actinobacillus actinomycetemcomitans.

Despite tremendous increases in our understanding of the pathogenic properties of specific plaque microorganisms and the role of specific microorganisms in the disease process, current therapy in periodontics is largely non-specific. The treatments that we utilize (e.g., oral hygiene measures, debridement by scaling and root planning, or even the currently available mouthwashes) are oriented towards reducing the accumulation of plaque on the teeth. Future developments in periodontics will involve the development of therapies which prevent the colonization or growth of specific microorganisms that are known to function as pathogens in this environment.

Classification of Periodontal Microorganisms

Porphyromonas gingivalis

Porphyromonas
endodontalis

Prevotella intermedia

Prevotella melaninogenica

Prevotella denticola

Prevotella loescheii

Campylobacter rectus

Camplyobacter curvus

 

 

methods of examination of antibiotic sensitivity of bacteria. the main principles of rational antibiotic therapy of diseases

 

Various chemical substances comparatively harmless for the macroorganism but with a lethal action on pathogenic micro-organisms are widely used in medical practice for treating patients with infectious diseases and in some cases for prophylaxis.

This method was known long ago to ancient people, and was used for treating certain diseases. The Peruvian Indians discovered the therapeutic action of cinchona bark, and in the 18th century cinchona bark was brought to Europe. The inhabitants of Brazil successfully employed the root of the ipecacuanha for treating amoebiasis. Mercury has been extensively employed in the therapy of syphilis. In the middle of the 16^th century this method became known to the people of Europe.

The basis of modem chemotherapy was founded by P. Ehrlich and D. Romanowsky, who formulated the main scientific principles and the essence of chemotherapy. They showed that in the treatment of each infection a substance should be found which, during injection into the diseased body, will bring the least harm to it and cause the most destructive action to the pathogenic (causative) agent. P. Ehrlich devised the principles of synthesis of medicinal substances by chemical variations: methylene blue, derivatives of arsenic–salvarsan (‘”606″), neosalvarsan (“914”). By the further development of chemistry new medicinal preparations could be obtained.

Extensive experimental and clinical tests of chemopreparations were carried out by E  Metchnikoff.

Chemopreparations should have a specific action, a maximal therapeutic effectiveness, and a minimal toxicity for the body.

As a characteristic of the quality of a medicinal preparation, P. Ehrlich introduced the chemotherapeutic index which is the ratio of the maximal tolerated dose to the minimal curative dose:

 

maximal tolerated dose (DT—Dosis tolerata)

                —————————————————————-         > 3

minimal curative dose (DC—Dosis curativa)

 

The chemotherapeutic index should not be less than 3 Chemotherapeutic preparations include a number of compounds used in medicine

Arsenic preparations (novarsenol, myarsenol, aminarsone, osarsol, etc.) are administered in syphilis, relapsing fever, trypanosomiasis, amoebiasis, balantidiasis, anthrax, sodoku, and other diseases.

Bismuth preparations (basic bismuth nitrate, xeroform, basic bismuth salicylate, bioquinol, bismoverol, bithiurol, pentabismol, etc.) are used against enterocolitis and syphilis

Antimony compounds (tartaric antimony potassium salt, stibenil, stibozan, surmine, solusurmine, etc.) are used for treating patients with leishmaniasis and venereal lymphogranulomatosis.

Mercury preparations (mercury salicylate, mercuric iodide, mercury cyanide, calomel, unguentum hydrargyri cinereum containing metallic mercury, etc.) are prescribed for treating patients with syphilis and are used as antiseptics in pyogenic diseases.

Acridine preparations (rivanol, tripaflavine, acriflavine, acricide, flavicide, etc.) are recommended for pyogenic diseases and inflammatory processes of the pharynx and nasopharynx

Antimalarial substances include more than 30 preparations, e g , chinine hydrochloride, quinine sulphate, mepacrine (acrichine), rodochin (plasmocide), proguanyl (bigumal), pyrimethamine (chloridine), resochine, quinocide sulphones and sulphonamides, sulphadiamine, etc.

Alkaloid preparations (emetine, etc.) are used for treating patients with amoebiasis.

Sulphonamide preparations. The introduction into practice of compounds of the sulphonamide group (streptocid, ethasole, norsulphazol, sulphazine, methylsulphazine, sulphadimezin, urosulphan, phthalazole, sulgine, sulphacyi, soluble sulphacyl, disulphormin, etc.) marked a revolution in the chemotherapy of bacterial infections.

Sulphonamide preparations are used for treating pyogenic diseases, tonsillitis, scarlet fever, erysipelas, pneumonia, dysentery, anaerobic infections, gonorrhoea, cystitis, venereal  lymphogranulomatosis, psittacosis, ornithosis, trachoma, blennorrhoea in the newborn, etc.

There are several points of view concerning the mechanism of action of sulphonamides on microbes.

 

Antibiotics (Fr. anti against, bios life) are chemical substances excreted by some micro-organisms which inhibit the growth and development of other microbes (in recent years several antibiotics have been obtained semisynthetically).

 

 

 

 

 

Antibiotics are obtained by special methods employed m the medical industry For the production of antibiotics strains of fungi, actmomycetes, and bacteria are used, which are seeded in a nutrient substrate After a definite growth period the antibiotic is extracted, purified and concentrated, checked for inocuousness and potency of action In composition a number of antibiotics (penicillin, streptomycin, gramicidin, etc ) have optically distorted molecules The antibacterial properties of some antibiotics are associated with optical inversion of their molecules which have the same physicochemical properties as normal molecules and can easily be bound to the enzyme Since they lack the ability to participate in biochemical reactions, this binding is accompanied by a blockade of enzymes, and consequently, a growth inhibition followed by death of the microorganism.

According to the character of action, antibiotics are subdivided into bacteriostatic (tetracyclines, chloramphenicol, and others) and bactericidal (penicillines, ristomycin, and others). Each antibiotic is characterized by a specific antimicrobial spectrum of action. Some antibiotics are inactivated in the presence of animal and plant proteins. Only a few antibiotics have a powerful antibacterial action, which does not decrease in the presence of protein matter of animal tissues and at the same time is not toxic (in certain concentrations) for the human being.

The mechanism of action of antibiotics varies. Penicillin inhibits the synthesis of polymers of the bacterial cell wall (it hinders the use of muramic acid by bacteria), which leads to an increase of cells  incapable of multiplication. Sometimes the action of penicillin leads to the formation of L-forms in the shape of pleomorphic protoplasmic structures. Thus, penicillin has a lethal effect not on the given population, but on its off-spring. The selective action of penicillin on microbes hinders the penetration of glutamic and other amino acids through the cytoplasmic membrane of pathogenic cocci unable to synthesize amino acids which are vitally important for the existence of these bacteria. Penicillin inhibits the ability of the bacterial cell to absorb protein components — aminoacids, and it inhibits the synthesis of the enzyme system and also of adaptive enzymes.

 

 

 

 

Streptomycin inhibits the incorporation of some amino acids in protein synthesis and attacks the bacterial enzyme with the participation of  which the introduction of pyruvic acid into the tricarbonic acid cycle by its union with oxalacetic acid takes place. This antibiotic inhibits the activity of biotin-containing enzymes catalysing the union of carbon dioxide with carbonic acids; it disturbs reading of the genetic code and synthesizes leucine instead of alanine.

Of special interest is the mechanism of action of streptomycin on tubercle bacilli. This preparation does not have a sterilizing action, but inhibits the respiration of tubercle bacilli, which leads to the inhibition of cell reproduction and toxin formation. At the same time stimulation of tissue respiration occurs in the patient as well as an increase in the ability of the macro-organism to destroy tubercle bacilli and their toxins.

The selective action of streptomycin on the tubercle bacillus is due to the fact that the permeability of cell membranes in the bacilli and in the tissue cells  of animals and man differs due to the dissimilar chemical structure of the cytoplasm of these organisms.

There are data showing that streptomycin inhibits the capacity of bacterial cells of the colibacillus to oxidize fumaric and glutamic acids. This leads to an inhibition of adaptive enzyme production.

Chloramphenicol is a specific inhibitor of the biosynthesis of bacterial protein. It comes into action with the peptidyl transferase area of 50S ribosome. Competing with the aminoacyl end of the aminoacyl tRNA, chloramphenicol blocks the formation of the peptide bond.

Tetracyclines, lincomycin, erythromycin, kanamycin, neomycin, spectinomycin, sparsomycin, fucidine and others belong to the group of antibiotics which inhibit protein biosynthesis in bacteria at the ribosome level. The antibiotic rifampicin suppresses protein biosynthesis by inhibiting the activity of RNA polymerase.

Antifungal antibiotics impair the intactness of the cytoplasmic membrane in fungi; antineoplastic antibiotics suppress the synthesis of nucleic acids in bacterial and animal cells and bind with DNA which serves as the matrix for RNA synthesis; bruneomycin leads to sharp inhibition of the synthesis of DNA or to its destruction.

 

There are various hypotheses and theories which have not entirely revealed the mechanism of action of antibiotics, and this question has not been completely solved.

 

The activity of antibiotics is expressed in international units (IU). Thus, for example, 1 IU of penicillin (Oxford unit) is the smallest amount of preparation inhibiting the growth of a standard Staphylococcus aureus strain. Recently the method of determining the activity of antibiotics according to the weight of the preparation has received wide application.

One unit of activity (AU) corresponds to the activity of 0.6 micrograms (ug) of. the chemically pure crystalline sodium salt of benzylpenicillin. Consequently, in 1 mg of sodium salt of benzylpenicillin there may  be 1667 AU, and in 1 mg of potassium salt — 1600 AU. For practical purposes both preparations are manufactured with an activity not less than 1550 AU.

The concentration of dry preparations as well as of solutions is expressed as the number of micrograms of active substance in 1 g of preparation or in 1 mg of solution.

 

Antibiotics are classified according to the chemical structure of the dmg, the molecular mechanism, and the spectrum of activity exerted on the cells.

 

According to origin, antibiotics are subdivided into the following groups.

 

Antibiotics produced by fungi. Penicillin is produced by the fungi Penicillium notatum, Penicillium chrysogenum, etc. Penicillin is produced as sodium and potassium salts. It dissolves readily in water, but its solutions are not stable. It is a dipeptide consisting of dimethylcysteine and acetylserine.

Penicillin is used in staphylococcal, streptococcal, and meningococcal infections, anaerobic infections, gonorrhoea, syphilis, leptospirosis, anthrax and other diseases.

Penicillin preparations include ecmonovocillin which is a form of long-acting penicillin, maintaining the necessary therapeutic concentration of penicillin in the blood. It is used only for intramuscular injections. The indications are the same as for the application of penicillin.

Semisynthetic penicillins (methicillin, oxacillin) are used in infection with penicillin-resistant staphylococci; ampicillin is prescribed in mixed infections. Resistance, however, develops faster to semisynthetic preparations than to natural ones. Novobiocin and ristomycin cause a favourable therapeutic effect.

Antibiotics produced by actinomycetes. 1. Streptomycin is obtained from Streptomyces griseus. Chemically it consists of two components: the nitrous base of streptidin and streptobiosamine. Streptomycin is a base and forms salts with acids, which readily dissolve in water and are insoluble in organic solvents. It has a bacteriostatic property in relation to Gram-negative as well as to Gram-positive pathogenic microbes.

Streptomycin has a good therapeutic action on tuberculosis, tuberculous meningitis, plague, brucellosis, tularaemia, whooping cough, etc.

2. Chloramphenicol is obtained from the cultural fluid of a strain of Streptomyces venezuelae, isolated from the soil in tropical South America. It has a good therapeutic effect during dysentery, enteric fever, typhus fever and other rickettsioses.

3. Chlortetracycline (biomycin, aureomycin) is produced by Streptomyces aureofaciens. It is employed during staphylococcal infections, pneumonia, subacute septic endocarditis, rickettsioses, amoebiasis, dysentery, whooping cough, gonorrhoea, brucellosis, tularaemia, trachoma, psittacosis, peritonitis, surgical sepsis and other diseases.

4. Tetracycline is a derivative of chlortetracycline. It is obtained by reductive dechlorination of chlortetracycline. Tetracycline has a wide spectrum of action, it inhibits many species of Gram-positive, Gram-negative and acid-fast microbes. It also inhibits the development of many rickettsiae and some protozoa. It is used in treating patients with cholera, pneumonia, subacute septic endocarditis, amoebiasis, dysentery, whooping cough, gonorrhoea, in diseases of the urogenital tract, typhus fever and other rickettsioses, and for the prevention of suppurative processes in surgery. Tetracycline hydrochloride is manufactured in the form of pills with pure tetracycline or in combination with nystatin.

5. Oxytetracycline (terramycin) is obtained from Streptomyces rimosus. In spectrum and mode of action it is close to chlortetracycline. Rondomycin (6-methyl-5-hydroxytetracycline) is a homologue of oxytetracycline. It is absorbed rapidly. Rondomycin possesses a broad spectrum of action (suppresses Gram-positive and Gram-negative bacteria, i. e. cocci, Salmonella organisms, Shigella organisms, pathogenic E. coli serotypes) and is administered per os.

6. Erythromycin is obtained from Streptomyces erythraeus. It is administered in streptococcal diseases. In experiments on animals it has proved to be effective in diseases caused by Gram-positive and Gram-negative bacteria, rickettsiae, chlamydias, intestinal amoebae and trichomonads. Diphtheria bacilli are quite sensitive to erythromycin

7. Neomycin has been isolated from Streptomyces fradiae. It has a bacteriostatic action against Gram-negative and Gram-positive bacteria. The preparation is slightly toxic. It is prescribed mainly for the local treatment of suppurative processes, caused by staphylococci which are resistant to penicillin and to other antibiotics, and also during colienteritis, the causative agents of which are the pathogenic serotypes of E. coli.

8. Nystatin has been extracted from the cultural fluid of Streptomyces noursei. It inhibits many pathogenic fungi and some pathogenic protozoa. It is non-toxic when used per os. It has received wide application in treatment of candidiasis.

9. Kanamycin is an antibiotic produced by Streptomyces kanamycetius. In mode of action it resembles streptomycin and neomycin. It inhibits the growth of Gram-positive and Gram-negative bacteria. Kanamycin is used for treating patients with tuberculosis in whom the causative agent became resistant to antituberculous chemopreparations and antibiotics. It is prescribed for treating anthrax, gonorrhoea and for acute and chronic forms of infections of the urinary tract, and diseases caused by resistant strains of staphylococci.

Cycloserine obtained from Streptomyces lavendulae and other actinomycetes belongs to this group of antibiotics. It produces a beneficial therapeutic effect in tuberculosis; it disturbs the synthesis of the cell wall of mycobacteria and other Gram-positive micro-organisms. Oleandomycin obtained from Streptomyces antibioticus culture fluid inhibits the vital activity of Gram-positive bacteria, Mycobactenum tuberculosis, rickettsia, and Chlamidobacteriales organisms. Levorin produced by Actinomyces levoris is employed for treating superficial and deep candidiases.

Amphotericin (A and B) are antimycotic antibiotics obtained from Streptomyces nodosum. They are effective against yeast-like fungi, pathogens of deep and systemic mycosis, particularly, histoplasmosis, chromomycosis, sporotrichosis.

                Chemical structure  of differens antibiotics

 

Antibiotics produced by bacteria. 1. Gramicidin isolated from a culture of B. brevis has a bacteriostatic and bactericidal action on some pyogenic cocci.

2. Polymyxins  are produced by Bac. polymyxa. They are prescribed in diseases caused by Gram-negative bacteria.

 

Semisynthetic antibiotics. This group includes some penicillins obtained on the basis of6-aminopenicillmic acid, the nucleus of penicillin (methicillin, oxacillin, dioxacillm, ampicillin, etc.).

 

Semisynthetic penicillins

 

 Semisynthetic penicillins

 

 

 

           Semisynthetic cephalosporins

 

 The antibiotic levomycetin (an analogue of natural chloramphemcol) is obtained  by synthesis. Combined preparations have also been produced on a mass scale, e. g. vitacycline (tetracycline with vitamins C, b(and B, and some others). New medicinal forms of tetracyclines having weaker side effects have been devised.

 

Resistance of microbes to antibiotics. With the extensive use of antibiotics in medical practice, many species of pathogenic micro-organisms became resistant to them.

Resistance may develop to one or simultaneously to more antibiotics (multiple resistance).

The molecular mechanism of the production of resistance to antibiotics is determined by genes localized in the bacterial nucleoids or in the plasmids, the cytoplasmic transmissible genetic structures.

Resistance to antibiotics occurs as the result of disturbed translation of genetic information and altered synthesis of the polypeptide chain, diminished permeability of the cytoplasmic membrane and cell wall, and the formation, due to the effect of R-plasmids, of enzymes inactivating antibiotics (ampicillin, chloramphenicol, kanamycin, streptomycin, tetracycline, etc.).

Mutations according to the nucleoid genes, leading to antibiotic resistance, form with a frequency of 10^–6 to 10^–12. Owing to this, the occurrence of simultaneous mutations to two and more antibiotics is excluded; they may develop, however, as the result of independent mutation in a strain primarily resistant to one of the antibiotics.

Resistance to penicillin is linked with penicillinase (B-lactamase) synthesis controlled by one of the genes of the R-factor. Penicillinases are synthesized under the effect of not only the R-factor genes but also the nucleoid genes. Resistance to chloramphenicol is determined by the action of the enzyme — chloramphenicol acetyl-phenicolacetyl transferase coded by the gene of the R-factor. Five enzymes are responsible for the resistance to antibiotics of the aminoglycoside group. Inactivation of antibiotics in the R+ strains with multiple resistance is accomplished by three types of reactions, phosphorylation, acetylation, and adenylation. It has been established that a bacterial cell may be resistant to more than one antibiotic by one gene.(Fig.).

 

 

 

                          Resistance plasmids (R-factors)

 

 

 

 

                               

 

                                      R plasmid and genes of  resistance

 

 

Due account is given in medical practice to cross-resistance to antibacterial agents which have the same chemical structure. It has been found to exist between preparations of the tetracycline series and new semisynthetic antibiotics (morphocycline, glycocycline, dibiomycin, and ditetracycline), preparations of penicillin (benzylpenicillin, phenoxymethylpenicillin, ephycillin), compounds of the nitrofuran group, and between sulphanilamides.

 

 

 

 

                                    Origin of resistent forms

 

 

Result of medical treatment of angina by antibiotics

 

Due to the wide distribution of staphylococci resistant to antibiotics a search for new preparations became necessary. At present a semisynthetic staphylococcal penicillin has been obtained which has a distinct bacteriostatic action on resistant strains of pathogenic staphylococci With the isolation of the penicilliucleus, 6-aminopenicillinic acid (6APA), it became possible to obtain various derivatives of penicillin.

Dimethylchlorotetracycline from the group of tetracyclines is used for the treatment of many infectious diseases and in doses half as strong as tetracycline. A good result has been obtained in treatment of inflammatory processes of the urinary tract.

With the discovery of the antibiotic griseofulvin dermatology was enriched with an effective preparation with the help of which diseases of the skin, hair and nails caused by fungi imperfecti could be treated.

Some antibiotics have a poisonous effect on rats, insects and mites. They are used for exterminating rodents and arthropods, the vectors of infectious diseases.

Antibiotics (kormogrisin, chlortetracycline, etc.) stimulate the growth of animals and fowl, and are therefore widely used in agriculture.

Of interest is the very difficult problem of chemotherapy of viral diseases. At present there are no effective drugs against viral infections. This is due to the biological peculiarities of viruses as obligatory intracellular parasites, which must be acted upon by other means than those used in microbial diseases. In recent years many new antibiotics have been obtained which have a good effect in the treatment of murine leucoses. Some of them are employed successfully in agriculture for treating fowl leucoses. Antitumour antibiotics include actinomycins C, D, K, F, etc., carcinophilin, mytomycin, actinoxanthin, chrysomalin, aurantin, sarcomycin.

 

Side effects of antibiotics. It has been established that large doses of penicillin and streptomycin have a neurotoxic action, tetracyclines affect the liver, chloromycetin has a toxic effect on the haematopoietic organs, and chlortetracycline and oxytetracycline upon intravenous injection may lead to collapse with a lethal outcome.

 

 

Effect of tetracyclin on decolorization dental enamel

 

 Upon injection of penicillin and streptomycin a rash, contact dermatitis, angioneurotic oedema, anaphylactic reactions or allergic asthma may occur. Quite frequently allergic reactions arise during local application of antibiotics. Of the most practical importance is their indirect action which is mainly due to the development of resistant strains of micro-organisms, sometimes causing furuncles or severe generalized diseases which develop vigorously, in some cases with a lethal outcome. In case of the application of antibiotics with a wide spectrum of action infections may develop which are caused by resistant strains of Proteus and fungi. Staphylococcal colitis proceeds very severely, and is characterized by profuse diarrhoea, dehydration of the body, toxic phenomena, shock and collapse.

 

 

 

                                      Candidosis

 

Of great hazard is the formation of resistant staphylococci which cause various postoperative complications — persistent furunculosis and staphylococcal septicaemias.

A severe complication is anaphylactic shock from the use of penicillin in which a rapid drop in blood pressure, cyanosis, superficial breathing, loss of consciousness, and convulsions are observed, and in some cases death occurs. Complications caused by penicillin are characterized by allergic reactions and proceed according to the serum sickness type.

In prolonged use of penicillin or levomycetin (in syphilis and enteric fever) collapse is one of the severe side effects.

Contact dermatitis is an allergic reaction of a medicinal origin. This disease is caused by the action of streptomycin in medical personnel and patients using this preparation over long periods. Quite often allergic manifestations are recorded in the mucous membranes such as hyperaemia and oedema of the pharynx and tongue. In children antibiotics with a wide spectrum of action cause perianal skin hyperaemia, and hyperaemia of the rectal mucosa.

 

 

           

 

 

Side effect after the reception of rifampin

 

In some countries the number of cases of infection with staphylococcal pneumonia in children has increased. It has been suggested that this can be explained partly by the origin of penicillin-resistant strains of staphylococci. The disease has the tendency of becoming complicated with abscesses, empyema, pneumothorax and the formation of cysts.

Antimicrobial agents cause the formation of numerous variants of microbes with weak pathogenicity (atypical strains, filterable forms, L-forms) which lead to the formation of latent forms of infections marked by recurrences and exacerbations.

Antibacterial agents may induce disorders of the genetic apparatus of the macro-organism’s cells and cause chromosomal aberrations; some of them possess a teratogenic effect leading to the development of foetal monstrosities if they are taken in the first days of pregnancy.

 

Due to the wide distribution of antibiotic-resistant pathogenic bacteria, combined treatment is recommended with the use of new antibiotics to which the causative agents of infectious diseases have yet not developed resistance. To prevent the development of resistant forms of microbes, combined preparations are prescribed: penicillin and streptomycin, erythromycin and oxytetracycline, etc.

 

 

                                            Combined action of antibiotics

 

Tetracycline with nystatine are applied for the prevention of candidiasis.

The use of preparations which block selectively R-plasmid replication and those which promote the elimination of antibiotic modifying enzymes is believed to be promising in the control of multiple antibiotic-resistance.

 

 

The doctrine about infection. Pathogenicity and virulence of bacteria. An experimental infection.

The term infection (L. infectio to infect) signifies the sum of biological processes which take place in the macro-organism upon the penetration of pathogenic micro-organisms into it. independent of whether the penetration will entail the development of an obvious or a latent disease or whether the macro-organism will only become a temporary carrier of the causative agent.

The historically developed interaction of the susceptible human organism and the pathogenic micro-organism in certain conditions of the external and social environment which gives rise to an obvious or latent pathological process is called an infectious process.

From the biological point of view, the infectious process is a kind of parasitism in which two live organisms adapted to different environmental effects enter into combat.

Infectious disease designates one of the extreme degrees of manifestation of the infectious process. Infectious diseases are considered to be phenomena including biological and social factors. Thus, for example, the mechanisms of transmitting infectious diseases, their severity and outcome are provided for mainly by social conditions.

Infectious diseases differ from other diseases in that they are caused by live causative agents of a plant and animal origin and are characterized by contagiousness, the presence of a latent period, specific reactions of the body to the causative agent and production of immunity.

With the development of genetics the conception of the infectious agent has now become considerably extended. In many species of pathogenic agents the infectious properties are posses sed by high-molecular DNA containing cytoplasmic structures (plasmids) as well as by the nucleic acids of tumour (oncogenic) viruses which are not organisms but are capable of accomp lishing genetic information inherent in the corresponding viruses. It has thus been proved that besides diseases in which the infectious process is caused by living agents, there are infections occurring on a molecular level which are characterized by the ability to be transmitted not only through the external environment, but from the parents to the offspring.

 

Main Features of Pathogenic Microorganisms. Pathogenicity. This is the potential capacity of certain species of microbes to cause an infectious process. Pathogenicity is characterized by a complex of pathogenic properties in the microbe formed in the process of the historical development of the struggle for existence and , adaptation to parasitic life in plant, animal and human organisms. Pathogenicity is a specific character of pathogenic microbes. Pathogenic microbes, for the most part, are characterized by a specific action. Each species is capable of giving rise to a definite infectious process.

The specificity of the infectious process is quite an important feature which becomes evident in the localization of the causative agent, selectivity of tissue and organ affection, clinical picture of the disease, mechanisms of isolating microbes from the organism and in production of immunity. The peculiarities of each causative agent as an extreme stimulant are taken into account when devising methods of clinical and laboratory diagnosis, of therapy and prevention of infectious diseases.

Historically developed ecological factors play an essential role in the development of the specificity of pathogenic micro-organisms and their ability to cause diseases in certain species of hosts. These factors ensure a definite and regular nature of the transmission of the causative agent from one individual to another.

Virulence. Virulence signifies the degree of pathogenicity of the given culture (strain) Virulence, therefore, is an index of the qualitative individual nature of the pathogenic micro-organism. Virulence in pathogenic microbes changes under the influence of natural conditions.

It can be increased by a sequence of passages through susceptible laboratory animals, and also by transformation, transduction and lysogenic conversion. Virulence can be weakened by the action of different factors on the micro-organism, e. g. the defense forces of the organism, antimicrobial preparations, high temperatures, immune sera, disinfectants, seeding from one nutrient medium to another, etc. Artificial reduction of the virulence of pathogenic microbes is widely used in the preparation of live vaccines, applied for the specific prophylaxis of a number of infectious diseases.

The virulence of microorganisms is closely linked with the genetic function, the auxotrophic property in particular; in the presence of a deficit in two growth factors in mutant strains virulence is lost and cannot be restored while the immunogenic property is maintained Infection and infectious process.

In characterizing pathogenic microbes a unit of virulence has been established — Dlm (Dosis letalis minima), representing the minimum amount of live microbes which in a certain period of time bring about death of the corresponding laboratory animals. Since animals have an individual sensitivity to a pathogenic microbe, then the absolute lethal dose Dcl (Dosis certa letalis) which will kill 100 per cent of the experimental animals has been established. This provides for a more accurate characteristic. At present LD₅₀ (the dose which is lethal to one half of the infected animals) is considered to be the most suitable, the use of which allows for a minimal correction in evaluating the virulence in pathogenic bacteria, and may serve as an objective criterion for comparison with other units of virulence. That number of pathogenic bacteria which is capable of giving rise to an infectious disease is known as the infectious dose of a pathogenic micro-organism.

In tests on volunteers it was established, for instance, that the infectious dose is 10⁸ cells for enteropathogenic 0124 E. coli, 10⁵-10¹⁰ for Salmonella organisms, 10⁵ for Salmonella typhi (a dose of 10⁷ caused the disease in 50 per cent of infected volunteers, a dose of 10⁸–10¹⁰ in 89 to 95 per cent), 10⁶ -10¹¹ for the El Tor cholerae vibrio, and 10 to 100 bacterial cells for Shigella dysenteriae. The action of small and large doses of microbes is of great significance in the development of the infectious process, in the length of the incubation (latent) period, and in the severity and outcome of the disease.

Under favourable conditions one microbial cell with a cell division rate of 20 minutes can give a progeny of 250000 individuals in six hours, and in several hours the amount of microbes may attain many thousand millions which create a large physiological burden on the tissues and organs of the infected organism. The virulence of pathogenic micro-organisms is associated with toxin production, invasiveness, capsule production, aggressiveness and other factors.

 

The virulence of pathogenic microorganisms is associated with adherence, invasiveness, capsule production, toxin production, aggressiveness and other factors.

 

 

 

 

Adherence of bacteria to host cells

 

Adherence of vibrio cholera on the mucose

 

Microbial toxins. According to the nature of production, microbial toxins are subdivided into exotoxins and endotoxins. Exotoxins include toxins produced by the causative agents of botulism, tetanus, anaerobic infections, diphtheria, and by some species of Shigella and haemolytic streptococci, and staphylococci, by the causative agents of diphtheria, whooping cough, plague, cholera, anthrax, by the parahaemolytic vibrio, etc.

The mechanism of toxin production has been recently studied in more detail It was established that the genes of toxigenicity (tox⁺ genes) are located in the temperate phage DNA in some bacterial species (Cor. diphtheriae, S. aureus, etc.), in the plasmids in others (E. coli etc.); in Cl. histolyticum and Cl. novyi toxin production is associated with genes located in the DNA of the temperate phage and with genes responsible for sporulation.

Toxigenycity is not a compulsory species property since all known toxigenic bacteria may exist without producing a toxin. The activity of the tox⁺ genes is controlled by repressers of the bacterial cell itself, the production of which is induced by certain substances contained in the nutrient medium.

The activity of exotoxins is stipulated by certain parts of the protein molecule, active centres which are amine groups of toxins; the blocking of these groups with formaldehyde results in the loss of toxicity.

More than 50 protein exotoxins of bacteria are known to date. They are subdivided into three classes.

 

Class A includes exotoxins secreted into the external environment:  cholerogen (V. cholerae); haemolysin (V. parahaemolyticus); alpha, beta, delta and gamma haemolysins (S. aureus); histotoxin, dermonecrotoxin and haemolysin (Cor. diphtheriae), alpha– and delta–haemolysin and beta and epsilon toxin (C/. perfringens); oedema and lethal toxin (Bac. anthracis), and others.

Class B includes exotoxins which are partly secreted, partly bound with the microbial cell: labile toxin {Bord. pertussis); alpha toxin (C/. novyi); tetanospasmin (Cl. tetani); neurotoxin (C/. botulinum).

Class C consists of exotoxins bound with the microbial cell; Sh. dysenteriae exotoxin; Y. pestis mouse toxin; Cl. perfringens enterotoxin; Bord, pertussis histamine-sensitizing and lymphocytosis-stimulating factors.

 

Exotoxins easily diffuse from the cell into the surrounding nutrient medium. They are characterized by a markedly distinct toxicity, and act on the susceptible organism in very small doses. Exotoxins have the properties of enzymes hydrolysing vitally important components of the cells of tissues and organs.

 

In chemical structure exotoxins belong to substances of a protein nature. They are weakly stable to the action of light, oxygen, and temperature (they decompose at 60-80°C within 10-60 minutes, and on boiling they break down immediately). In a dried condition they are more stable to high temperature, light, and oxygen. The addition of saccharose to the toxins also increases their resistance to heat.

 

Under the effect of 0.3-0.4 per cent formalin and 38-40°C temperature, diphtheria toxin within 30 days loses its toxic properties and changes into an anatoxin.

 

Some exotoxins (diphtheria, tetanus and anaerobic infections) break Infection and infectious process down under the influence of digestive enzymes as a result of which they become harmless when administered orally. Other exotoxins (of Clostridium botulinum, Clostridium perfringens and pathogenic staphylococci) do not break down in the stomach and intestine and cause intoxication of the organism during oral administration.

The potency of toxins is determined on sensitive laboratory animals according to Dlm and LD₅₀. For example. 1 Dlm of the diphtheria toxin represents the minimal amount which during subcutaneous injection into 250 g guinea pigs kills them on the fourth day.

The minimal lethal dose of the native diphtheria toxin for the guinea pig is within the range of 0.002 ml, tetanus toxin for white mouse — 0.000005 ml, and botulinus toxin for the guinea pig — from 0.00001 to 0.000001 ml.

In recent years pure tetanus, botulinus and diphtheria toxins have been obtained. They are purified by different methods: coagulation at the isoelectric point, repeated precipitation by trichloroacetic acid at a low temperature and a pH of 4.0, salting out by ammonium sulphate and adsorption by various substances.

Purified toxins have a characteristically high toxicity for sensitive laboratory animals. Thus, for example, 1 mg of the diphtheria toxin contains 40 000 000 Dim for the guinea pig, and 1 mg of botulinus toxin contains 1000000000 Dim for the white mouse. Crystalline toxins are even more toxic.

The causative agents of enteric fever, paratyphoids, dysentery, gonorrhoea, meningitis and many other Gram-negative bacteria do not  produce exotoxins they contain endotoxins. Endotoxins are more  firmly bound with the body of the bacterial cell, are less toxic and act on the organism in large doses; their latent period is usually estimated in hours, and the selective action is poorly expressed. According to chemical structure, endotoxins are related to glucoside-lipid and polysaccharide compounds or phospholipid-protein complexes. They are thermostable. Some endotoxins withstand boiling and autoclaving at 120°C for 30 minutes. Under the effect of formalin and a high temperature they are partially rendered harmless.

 

Comparative Characteristics of Toxins

 

The majority of protein bacterial toxins catalyse certain chemical processes, break down vitally important compounds, are active in extremely small doses, have a latent period and inhibit the defensive functions of tissues. Some bacterial toxins have the properties of lecithinase. Thus, for example, Clostridium perfringens produces exotoxin (lecithinase C) which is able to cleave lecithin into phosphorylcholine and a diglyceride.

Necrosis of the muscular tissue is caused as a result of the combined action of lecithinase, collagenase and mucinase (hyaluronidase). Collagenase and mucinase decompose the connective tissue of the muscles, and lecithinase dissolves the lecithin of the membrane of muscle fibres. Haemolysis during anaerobic infections takes place due to lysis of lecithin of the stroma in erythrocytes. Bacterial toxins are characterized by organotropy (monotropy and polytropy) due to which the toxigenic micro-organisms bring about tissue necrosis in localized foci of the causative agent.  The necrotic manifestation of toxins has a great adaptive significance for the causative agent. Firstly, the toxins change live and reactive tissue into a substrate harmless for the pathogenic microbes. Secondly, the necrotic tissue protects the parasite from the effects of the defense reactions of the macro-organism.

Toxins are regarded as enzymatic poisons, which are capable of arresting metabolic processes. This view point is considered to be the most probable. It has been suggested that in the process of development of saprophytic bacteria entering into long symbiosis with animal organisms, the ability to produce enzymes, facilitating symbiosis with tissues, and increasing their life activities at the expense of the host, was gradually raised. Eventually, due to the establishment of the parasitic mode of life, the enzymes of these bacteria became more specialized as a result of which adaptive enzymes transformed into enzymatic toxins — exotoxins. Therefore, it can be considered, that toxic infections were formed in a later period, preceded by a simple parasitism and a disease of a septic type.

Exotoxins are capable of causing potentiation when, under the effect of a mixture of toxins, their action in the organism is more marked. An especially distinct potential action is shown by toxins of organisms responsible for anaerobic infections and tetanus and of staphylococci, and diphtheria bacilli.

Some protein toxins cause haemolysis of erythrocytes (staphylococci, streptococci, etc.). The streptococcal haemolysin is freed from the cells during autolysis. Intravenous injection into guinea pigs is lethal. During subcutaneous injection in small doses it brings about the production of antibodies in the organism. The toxin is inactivated by cholesterol, pepsin, papain and trypsin. The haemolytic activity of the streptococcal  toxin is determined by the degree of haemolysis of a 1 per cent suspension of erythrocytes.

Microbes producing alpha-haemolysin cause the production of green or dark-green colonies of microbes on blood agar, as a result of the haematometamorphosis of the iron in erythrocytes. Beta-haemolysin dissolves erythrocytes, and upon the cultivation of bacteria producing beta-haemolysin, transparent zones of haemolysis are formed around the colonies.

Besides, a number of pathogenic microbes produce gamma-haemolysin which causes the haemolysis of erythrocytes in rabbits, humans, guinea pigs and is characterized by poor resistance to heating. Also, delta-haemolysin has been discovered which destroys the erythrocytes of man and some animals. It is excreted, for example, by pathogenic strains of staphylococci. Staphylococci and streptococci produce leucocidins which destroy pleomorphonuclear leucocytes.

 

 

Action of the hemolysin on red blood cells

 

Pathogenic strains of bacteria produce coagulase which causes coagulation of human, horse and rabbit plasma. Coagulase does not clot the plasma of guinea pigs, rats and chickens. Some bacterial enzymes have toxic properties.

 

 

 

 

Coagulase activity of bacteria

 

Thus, for example, more than 200 species of microbes (bacteria of pneumonia, ozaena and rhinoscleroma, Proteus, continental strains of causative agents of plague, etc.) produce urease, which proved to be a toxic enzyme. Many amino acid decarboxylases produced by causative agents of anaerobic infections and other microbes have toxic properties.

Lecithinases subdivided into A, B and C are typical enzymatic toxins. Lecithinase A is found in snake, bee and scorpion venom, lecithinase B — in plants, and lecithinase C — in many pathogenic microbes, especially in some causative agents of anaerobic infections.

Clostridium perfringens produces a typical alpha-toxin, which is considered to be a specific bacterial enzyme.

The enzyme neuraminidase (a protein) splits the alpha-ketoside bond formed by neuraminic acid in oligosaccharides, polysaccharides, and carbohydrate components of complex proteins. Neuraminidase is produced by cholera vibrios, the causative agents of anaerobic infection, streptococci, and Corynebacterium diphtheriae, and is found in the membranes of the influenza virus. It splits sialic acids from the surface of the cells, which results in changes in the three-dimensional configurations of the surfaces, diminished firmness of their structure, and reduced resistance.

Some microbes produce toxic substances: methylamine, dimethyl-amine, histamine, choline, neurine etc. Toxic amines are products of the decomposition of bacterial protein, and may accumulate in spoiled foodstuffs and serve as a source of food poisonings.

A number of micro-organisms produce ammonia and cause ammonia intoxication (Clostridium histolyticum, etc.). Ammonia is produced by deamination of amino acids.                                  

Rickettsial toxins are relatively labile substances, closely bound with the cells of the rickettsiae themselves. They comparatively quickly disintegrate after the death of rickettsiae from the action of formalin or heating at 56-60 C for 30 minutes.

Viral toxins are thermolabile, sensitive to the action of formalin and other substances. Viral toxins are easily neutralized by specific immune sera.

Human pathogenic viruses also contain toxic components. They have been found in causative agents of influenza, parotitis, etc. The influenza virus, for instance, contains five protein compounds: transcriptase, nucleoid protein, neuraminidase, haemagglutinin, and protein of the inner membrane; they are all heterogenous in relation to the human organism and, therefore, toxic.

Toxins cause distortions in metabolism, causing changes in the adrenalin and ascorbic acid levels. Under the influence of toxins, profound inhibition of such an important ‘ink in metabolism as the Krebs  oxidation cycle of tricarboxylic acids occurs.

Local as well as general manifestations of intoxications are accompanied by morphological changes in the formed elements of the blood. in the composition of proteins, enzymes, in the serological (production of antibodies), general clinical (temperature and neuropsychic) reactions, in disturbances in the respiratory organs, cardiovascular system, etc. Anatomical changes are characterized by inflammatory processes in the lymph nodes or by affection of certain organs and tissues.

 

 

Invasive properties of pathogenic bacteria. Virulent microbes are characterized by the ability to penetrate tissues of the infected organism. By chemical analysis it has been established that the greater part of the main substance of connective tissue contains polysaccharide – hyaluronic acid which is capable of resisting penetration into the tissue of different foreign Substances, including pathogenic microbes.

This protective barrier of connective tissue can be overcome due to the disintegration of hyaluronic acid by toxic substances of animal, plant and microbial origin. In 1928 F. Duran-Reynals established that upon infection of a rabbit with the vaccinia virus (cowpox), the infectious process increased considerably, if together with the virus aqueous extracts of rabbit, guinea pig or mouse testes were injected intracu- taneously. Later on, it was established that factors capable of increasing the permeability of tissues are found in some bacteria, snake venom and in different tissues and organs of animals. Substances causing this change in the permeability of tissues are known as spreading factors.

A certain enzyme was isolated from different tissues which hydrolysed hyaluronic acid. Some spreading factors are similar to this enzyme which is known as hyaluronidase.

Spreading factors are characterized by an extremely high activity. They act in very small doses, disintegrate at 60°C within 30 minutes, and have enzymatic properties. Spreading factors are not confined to hyaluronidase. They include substances differing iature. They include fibrinolysin produced by haemolytic streptococci of the A group, pathogenic staphylococci, and Infection and infectious process organisms involved in anaerobic infections, etc. Spreading factors increase the local primary action of pathogenic microbes affecting the connective tissue, and enhance the development of a general infection. They were found in many pathogenic micro-organisms (staphylococci, streptococci, causative agents of anaerobic infections, tetanus, diphtheria, etc.)

The effect of spreading factors on the course of infectious diseases is related to the virulence of the causative agent. In weakly virulent microbes such as staphylococci, colibacilli, and Proteus, the spreading factor increases the infectious process, if a large quantity of the microbes are injected. In highly virulent causative agents (Mycobacterium tuberculosis, type I S. pneumoniae) spreading factors increase the infectious disease with a minimal number of bacteria, sometimes with only several individuals.

 

The invasion of cells by bacteria

 

 

The role of capsular material in bacterial virulence. Some pathogenic micro-organisms (bacilli of anthrax, Clostridium perfringens, S. pneumoniae, causative agents of plague and tularaemia) are capable of producing a capsule in animal and human bodies. Certain micro-organisms produce capsules in the organism as well as iutrient media (causative agents of rhinoscleroma, ozaena, pneumonia).

Capsule production makes the microbes resistant to phagocytosis and antibodies, and increases their invasive properties. Thus, for example, capsular anthrax bacilli are not subject to phagocytosis, while noncapsular variants are easily phagocytized.

 

 

The high virulence of capsular microbes is associated with the toxic substances contained in the capsule. In chemical composition, the capsular material in some microbes consists of complex polysaccharides, and in others it consists of proteins. It can be different in separate strains of the same species, and on the other hand may be similar in different bacteria. There are nitrogenous and nitrogen free compounds in the capsular polysaccharides. They give the micro-organisms type specificity. In types II and III S. pneumoniae, the capsule is a glucoside of cellobiuronic acid in a highly polymerized state. In types I and IV the capsule contains highly polymerized compounds of aminosaccharides and organic substances. In some bacilli the capsule consists of polypeptides of d-glutamic acid, while in bacteria of Friedlander’s pneumonia it is a polymer carbohydrate, and in anthrax bacilli it consists of glycoprotein.

Bacterial aggressins. Besides toxigenicity, invasiveness and capsule production, pathogenic microbes are capable of producing substances which inhibit the defense mechanisms of the organism, and increase the pathogenic action of many causative agents of infectious diseases. 0. Bail named them aggressins. They were found in peritoneal and pleural exudates of laboratory animals infected with anthrax bacilli, S. pneumoniae, and other microbes. Aggressins alone, separated from bacteria and exudate cells by filtration, upon injection into the animal are harmless, but upon their addition to a non-lethal dose of the microbes, a severe         infectious process develops, often ending in death of the animal.

Aggressins were found in causative agents of enteric fever, paratyphoids, cholera, anthrax, diphtheria, plague, tuberculosis and pyogenic diseases. Aggressins are probably not one substance but several different substances occurring in the processes of vital activity of pathogenic microorganisms (some compounds of the surface structures of the microbial cells, products of DNA and RNA splitting).

 

Virulence is a dynamic property which is controlled by the mutation process occurring constantly both in the causative agent and in the host and which provides the continuity of the selection of changes advantageous for both. The ranges of virulence vary from absolute parasitism to the level of saprophytism. Pathogenic species may coexist with the macro-organism as latent forms for long periods of time.

 

Role of the Macro-Organism, Environment and Social Conditions in the Origin and Development of the Infectious Process.    The origin of the infectious disease depends on the reactivity of the human body, the quality and quantity of the causative agent, and the influence of the external environment and social conditions. Depending on the relationship of these factors, the infectious process may terminate in the death of the causative agent, the death of the host or the establishment of mutual adaptation between the host and the parasite.

The penetration of the causative agent into the body does not always entail disease but in many cases it is limited by a short-term infection without any manifestation of the disease or by a comparatively long carrying state (streptococci, adenoviruses, enteroviruses, herpes virus, malarial plasmodium, Entamoeba histolytica).

The reactivity of the human body with its immunobiological readiness to render the pathogenic micro-organism harmless is closely related with the environment, with conditions of life, nature of work and nutrition, hygienic and ageneral cultural level and many other factors.

The condition of the macro-organism and its resistance have a decisive significance in the origin, course and outcome of the infectious disease.

Susceptibility depends to a certain extent on age and sex due to certain physiological peculiarities. For example, during menstruation, pregnancy and labour the female organism becomes more sensitive, particularly to streptococcal diseases.

Children are more susceptible to some infectious diseases, and less susceptible to others than adults. Resistance to many infectious diseases in children up to the age of 6 months is associated with a poorly developed central nervous system, and also with the presence of maternal immunity. Besides, it has been established that in relation to some diseases (dysentery, staphylococcal and streptococcal diseases, colienteritis and infections caused by Coxsackie virus) children are more susceptible than adults. The varied age resistance to infectious diseases depends on the nature of metabolism, the function of the organs of internal secretion, and on peculiarities of immunity.

 Such factors as nature of nutrition (general starvation, deficiency of proteins, fats, carbohydrates, vitamins, and trace elements), overstrain, Infection and infectious process cooling, sanitary-hygienic conditions of work and life, also various somatic diseases, chronic poisonings and disturbances in the normal activity of the central nervous system have the effect of increasing the susceptibility to infectious diseases.

        General starvation is accompanied by an aggravation of tuberculosis, dysentery, furunculosis and other diseases. As a result of starvatioot only individual, but specific immunity is lost. For example, during starvation, pigeons become susceptible to anthrax to which they are resistant in a normal state. Lowering of the resistance in animals is not only due to general starvation, but also due to a deficiency of individual components of food, e.g., proteins, fats, carbohydrates. Starvation is accompanied by a disturbance in the protein metabolism, which leads to a decrease in the synthesis of immune globulins (antibodies), and a lowering of the activity of phagocytes.

Vitamin deficiencies have a great influence on the susceptibility to infectious diseases. A deficiency of vitamin A provides for the appearance of catarrhs of the mucous membranes of the eye and leads to xerophthalmia, enhances the development of skin affections, bronchopneumonia, influenza and acute catarrhs of the upper respiratory tract. A deficiency of vitamin b) , causes an increased susceptibility to leprosy and to a number of pathogenic and conditionally pathogenic microbes Vitamin C deficiency causes a decline in the resistance to tuberculosis, diphtheria, streptococcus, staphylococcus, pneumococcus and other diseases.

Quite important is the fact that during many infectious diseases as a result of the lethal action of drugs on the normal intestinal microflora which supply the organism with vitamins of the B group, vitamin deficiencies develop.

In the past years great heed has been paid to the problems of the study of mineral metabolism. A deficiency of iron, calcium, magnesium, copper, zinc, iodine, manganese, boron, cobalt and molybdenum leads to a disturbance in metabolism, a decrease in the resistance of the organism and an increase in the susceptibility to infectious diseases. Small amounts of trace elements are capable of increasing the defence mechanisms of the macro-organism, in particular, the phagocytic activity of leukocytes. They restore the previously impaired biochemical functions.

Physical and mental overstrain associated with an irregular organization of working hours and a disturbance of conditions of life causes a weakening of the defence mechanisms to many infectious diseases. Cooling lowers the resistance of the organism in relation to pathogenic and conditionally pathogenic microbes, enhances the development of pneumonia, catarrhs of the upper respiratory tract and other diseases. Pasteur proved that cooling in chickens causes a disturbance of specific immunity to anthrax. When the environmental temperature increases, penguins die from auto-infections caused by aspergilli.

Cooling as well as overheating of the body of animals leads to disturbances in biocatalytic  reactions, a weakening of the organism and lowering of immunity to infectious diseases. It is known, for example, that acute catarrhs are observed in the autumn-winter period, while colienteritis and infections caused by Coxsackie and ECHO viruses develop in the summer.

The effect of ultraviolet rays and sunlight on the organism depends on the wave length, intensity and duration of application. Observations have shown that sunlight has a favourable effect on the organism, and to a certain degree increases the resistance to infectious diseases. However, in a number of cases, lengthy and intense irradiation is accompanied by a decrease in the resistance of the human organism to a number of pathogenic microbes. For example, spring relapses of malaria are observed in people infected by plasmodia and exposed to intense solar radiation.

Of great theoretical and practical importance is the action of ionising radiation. As has been established small doses of X-rays increase the resistance of animals to various diseases, while increased doses lower it and enhance the activity of normal microflora and development of bacteremia and septicaemia. At the same time the permeability of mucous membranes is disturbed, their barrier capacity is reduced, and the function of the reticuloendothelial system and defence properties of the blood are sharply lowered. Especially dangerous to man are increasing doses of ionising radiation as a result of the testing of nuclear weapons. Radioactive strontium accumulates in the atmosphere. It causes deep changes in the hemopoietic function of the bone marrow, the formation of tumours and impairs reproductive ability.

Poor sanitary hygienic conditions of work and life have an unfavourable effect on the human body. Despite the fact that there are 2.5 million tons of air per each person, due to its pollution in large cities and industrial centres the incidence of respiratory diseases among the people is growing lately. This results in the spread of chronic diseases (cancer of the lungs, emphysema, asthma, etc.). The polluted air has a detrimental effect on animals and the vegetable kingdom. A deficiency of oxygen in the building, an excess of carbon dioxide and other harmful gases cause chronic toxicosis and are favourable for the development of tuberculosis. The presence in the air of dust containing a large amount of silicates disturbs the integrity of the mucous membranes of the respiratory tract, increases the possibility of infection by different micro-organisms, and leads to such diseases as tuberculosis, actinomycosis, aspergillosis, etc. Limited insolation also causes various disturbances in the activity of the body and enhances the development of diseases. Besides these harmful external factors, a great influence on the susceptibility to infectious diseases is caused by various somatic diseases (diabetes and other disturbances of the endocrine organs, diseases of the cardiovascular system, liver, kidneys, chronic poisonings by alcohol, nicotine and other poisons).

The hypophysis-adrenal system is of great importance in maintaining stability of the internal medium of the organism. The system is stimulated by the action of different stimulants, e. g. mechanical traumas, cold, heat, ultraviolet and ionising radiation, micro-organisms, etc. As a result of an excess deficiency or abnormal combination of hormones such as STH (somatotrophic hormone), ACTH (adrenocorticotrophic hormone), various disturbances in the functions of the organism may occur. Thus, for example, cortisone inhibits the inflammatory reaction, and therefore enhances the development of the infectious process. The somatotrophic hormone, on the other hand, activates the inflammatory process and causes an anti-infectious action.

Disturbances of the normal activity of the central nervous system deserve special attention. As is known, the causative agents of infectious diseases are extraordinary biological stimulants. With experimental infections, principally by neurotropic stimulants, it had been observed long ago that the injection of the infected material into the brain is accompanied by the greatest number of deaths.

Mental disturbances also lower the regulating function of the central nervous system. The mental patients in psychiatric hospitals more often contract infectious diseases.

Under the influence of various national disasters (hunger, war, earthquakes, floods) infectious diseases attain a mass distribution and are accompanied by a high death-rate and disability.

Thus, the infectious process reveals itself in the unity of biological and social factors. The disease incidence, severity of the clinical course and death-rate depend closely on the activity of the main economic laws of social formations.

 

BIOLOGICAL EXAMINATION. Biological study consists of infecting animals for the purpose of isolating the culture of the causative agents and their subsequent examination for pathogenicity and virulence.

Choice of experimental animals depends on the aim of the study. Most frequently used are rabbits, guinea pigs, albino mice, and albino rats. This is explained by the fact that they are susceptible to the causative agents of various infections diseases in man, easy to handle, and propagate readily. Hamsters, polecats, cotton rats, monkeys, birds, etc. may also be occasionally infected.

Specialized, particularly virological, laboratories, make use of genetically standardized, so-called inbred animals (mice, rabbits, guinea pigs, and others).

Working with experimental animals, one should keep it in mind that they may have spontaneous bacterial and viral diseases and latent infections activated as a result of additional artificial in­oculation. This hinders the isolation of pure culture of the causative agent and determination of its aetiological role. Gnotobiotes (without microflora) and animals free of pathogenic microorganisms have no such drawback. Currently they include chickens, rats, mice, guinea pigs, pigs, etc.

Laboratory animals are distinguished by their species, age, and individual sensitivity toward microorganisms. Thus, in selecting animals for study it is necessary to take into account their species and age. For instance, sensitivity in mongrel animals may show con­siderable individual variations. The use of inbred animals with a definite constant susceptibility toward microorganisms excludes individual variations in sensitivity and allows for reproducible re­sults.

Animals are infected for .isolating pure culture of the causative agent in cases where it is impossible to obtain it by any other method (for example, in contamination of the studied objects by extra­neous microflora which inhibits growth of the causative agent and in case of insignificant amounts of microorganisms or their trans­formation into filtering forms). Thus, in studying decayed corpses of rodents for the presence of plague causative agents, one inoculates (with suspension of the organs or blood) guinea pigs which die 3-7 days later with manifestations of septicaemia. Pure culture of the causative agent is readily isolated from the blood of internal organs.

Contamination of susceptible animals for reproducing the infec­tious process is used in diseases caused by Rickettsia and viruses.

Injection to mice of material from a patient with tickborne enceph­alitis brings about paralysis and death in these animals. To de­termine pathogenicity and virulence of the causative agents of plague, tularaemia, botulism, anthrax, and some viral diseases, cultures obtained from patients arc inoculated into albino mice, guinea pigs. rats, or suckling mice.

 

Methods of inoculation. In experimental inoculation of animals the studied material is administered via different routes: epicutaneously, subcutaneously, intracutaneously, intramusculariy, intravenously, per os, and in various organs and tissues such as the brain, mucosa, respiratory tract, etc. The method of material inoculation depends on affinity of the causative agent to definite tissues of the organism (tropism), while the volume of inoculum depends on the method of its administration and the species of animals .

 

Examination OF MICROORGANISM VIRULENCE. In studying characteristics of pathogenic microorganisms, it may be occasionally necessary to determine their virulence (degree of pathogenicity). It may be done for the purpose of characterizing the infectious causal organisms isolated from patients, carriers or the environment, for estimating the residual virulence of live vaccines, and for determining the immunity tension in animals, etc.

Virulence is defined in Dlm (Dosis letalis minima), i.e., the minimal amount of microorganisms causing death of the animal of a definite species, and LD₅₀, i.e., the minimal dose inducing death of50 per cent of the infected animals. The LD₅₀ is a more reliable indicator of virulence, being less dependent on individual sensitivity of animals.

In determining the LD₅₀, one should strictly standardize such variables as a species, sex, and weight of animals and conditions of their keeping and feeding. Ten-fold dilutions are prepared from aculture of bacteria, viruses or toxin, each dilution being injected to 4-6 animals. After a definite period of time count the number of animals that 4 have died and calculate the LD₅₀ using the method proposed by Herd and Muench. The method  is based on a logical postulation that the tested animals, that have sussumbed following inoculation with some dilution of the infective material studied would have died following inoculation with any lower dilution The data presented in Table 2 demonstrate that 50 per cent of the dose (LD₅₀) is between 10^–5 and 10^–6 dilutions of the bacteria containing material.

 

Determination of the LD₅₀ by the Reed and Muench Technique

 

For its accurate expression, one should determine the value X which is added to the logarithm of the dilution below 50 per cent of the dose (5 in our example):

     A – 50

X = ———- ,

     A – B

where A is the proportion of dead animals receiving the dilution below 50 per cent of the dose (80 per cent in our case); B is the percentage of dead animals receiving the dilution above 50 per cent of the dose (20 per cent in our example). Putting actual values in the formula, one will have:

    80 – 50

X = ———- .

   80 – 20

Hence, in the aforementioned case the LD₅₀ corresponds to 10^-5,5 dilution of the bacterial suspension Since in obtaining serial ten–fold dilutions 0 1 ml of the infective material is transferred sequentially (adding it to 0 9 ml of solvent), 1 ml of the initial bacterial suspension contains 10^6,5 of the LD₅₀.

 

The dynamics of the development of the infectious process consists of the incubation and prodromal periods, the height of the disease and period of recovery      ( convalescence). A certain period of time elapses from the moment of penetration of the pathogenic microbe to the onset of the first sings of the disease, which has beeamed the incubation period of the disease. It varies from several hours ( in cholera, toxinfections and plague) to several months and years ( in leishmaniasis, leprosy).

The duration of the incubation period depends on the degree of the general and specific immunity of the human body, its reactivity, sensitization (increased sensitivity), influence of harmful environmental factors and social conditions of life, and on the dose and virulence of the causative agent .

Incubation Periods of Diseases

 

 

One of the forms of interrelationship which occurs between the pathogenic micro-organism and a human or animal body without manifesting an obvious disease is carrier state. The ability of the causative agent to carry infectious diseases has been confirmed only in a relatively immune organism. Regarding specificity of action, carrier state has much in common with the infectious process. In some infectious diseases an intense and prolonged post-infectious immunity is produced which excludes carrier state (measles, smallpox, chickenpox, etc.). In other diseases during the period of convalescence a carrier state may be prominent which is different in frequency and duration (cholera, enteric fever, paratyphoid, dysentery, amoebiasis, scarlet fever, diphtheria. meningitis, malaria, encephalitis, poliomyelitis, etc.).

Carrier state may be found in healthy persons who have come into contact with diphtheria, meningitis, enteric fever, cholera, amoebiasis, encephalitis and poliomyelitis patients. Carrier state with a duration of 3 months is considered acute, while carrier state for longer periods is Infection and infectious process considered chronic. Prolonged carrier state (years and decades) has been described in enteric fever.

When infection occurs not with one species of causative agent, but with two or more, one speaks of mixed infection (measles and scarlet fever, measles and tuberculosis). If the infectious process is caused by micro-organisms changed under the influence of one or several comembers of the parasite coenosis, then this state is known as parainfection.

In some cases infection causes a weakening of the body which then becomes susceptible to other diseases. Thus, for example, after influenza or measles pneumonia occurs. This is known as secondary infection.

There are also focal and generalized infections. For example, during infection with staphylococcus, the infectious process causes furunculosis, and if the causative agent penetrates into the blood sepsis will develop. An alternate occurrence of focal and generalized infections is observed during tuberculosis and syphilis.

Reinfection is a repeated infection by the same species of microbe responsible for the disease which terminated in convalescence (gonorrhoea, syphilis, etc.).

Superinfection is a fresh infection of the body in which the main disease has not ended. Superinfection occurs in many infectious diseases in their acute and chronic forms.

Relapse is a return of the symptoms of the same disease (relapsing fever, paratyphoid fevers, etc.). Of certain significance in the occurrences of relapses is the low level of immunolo gical activity of the organism during illness and convalescence.

 

TRANSMISSON   OF   INFECTIOUS   AGENTS. Infectious diseases that can be spread from one host to another are said to be communicable or contagious. The term communicable disease implies direct transmission from one person to another. Preventing such diseases often is accomplished by avoiding contact with infected individuals. Measures such as quarantine were devised to avoid exposure. Measles, German measles, influenza, gonorrhoea, and genital herpes are all highly communicable. This means that the pathogens causing these diseases are readily transmitted with high frequency from an infected individual to a susceptible host.

Some diseases are not caused by agents that are communicable from one human to another. Tetanus, rabies, and Lyme disease are examples of noncommunicable infectious diseases. This means that they are acquired from the environment and are not spread directly from one person to another. Some noncommunicable diseases are caused not by the effects of invading microorganisms on host tissues, but rather by the ingestion of toxins made by the invading microorganisms. Such diseases are called intoxications rather than infections. For example, staphylococcal food poisoning is an intoxication that results from the ingestion of enterotoxin rather than the growth of Staphylococcus bacteria in the body.

The source of an infectious agent is known as the reservoir. Humans are the principal reservoirs for microorganisms that cause human diseases. Individuals infected with a pathogen act as the source of infection for others. The pathogens that cause contagious diseases move from one infected individual to the next People who come in contact with someone suffering from a contagious disease are at risk of contracking that disease unless they are immune. If they are immune, their host defences protect them against that particular pathogen

In some cases, infected individuals do not develop disease symptoms Such individuals are called asymptomatic carriers or simply carriers. Although they do not become sick, carriers are important reservoirs of infectious agents. The classic case of disease transmission by such a carrier occurred in the earl 1900s when a cook, Mary Mallon, known as “Typhoid Mary,” spread typhoid fever from one community to another.

Some diseases can be transmitted to humans by direct contact with infected animals, by ingesting contaminated meat, or, more frequently, by vector. Vectors are organisms that carry the disease agent to the host. The vector need not develop disease It only transmits the disease agent from a reservoir to a susceptible individual. Arthropods, such as mosquitoes, are frequently the vectors of human disease

Pathogens also can be transmitted from infected mother to her fetus or infant. Syphilis and rubella can be transmitted across the placenta. Hepatitis, gonorrhoea, and chlamydial infection can be acquired as the newborn passes through the birth canal. Transmission between individuals can also be by sexual intercourse, touching, breathing aerosols (airborne, minute droplets of water that contain microorganisms), blood transfusions, or contaminated hypodermic needles.

The reservoirs of human pathogens can be nonliving sources such as soil and water For example,tetanus is generally acquired when spores of Clostridium tetani, which are widely distributed in soil, contaminate a wound. Often diseases acquired from such sources are noncommunicable. Such diseases are singular events and are not normally transmitted from one infected individual to the next

Thus, a reservoir is a source of an infectious agent, which may be air, water, soil, animals, or people.

Routes of diseases transmission. There are various modes which pathogens are transmitted from a source to a susceptible individual. 

Pathogenic microorganisms gain access to the body through a limited number of routes. These specific routes are known as portals of entry. The routes of entry are the respiratory tract, gastrotestinal tract, genitourinary tract, skin, and wounds. The invasive properties of specific pathogens permit them to penetrate the body’s defense mechanisms through a specific portal of entry. Most pathogenic microorganisms will cause disease only if they enter the body via this specific route. For example, depositing Clostridium tetani on the intact skin surface does not  result in disease, while deposition of C. tetani into deep wounds results in the deadly disease tetanus.

 

Portals of Entry for Some Specific Disease-causing Microorganisms

 

 

Portal of entry	Microorganism	Type of micro-organism	Disease
Skin	Staphylococcus aureus Papilloma virus Trichophyton and Epidermophyton species	Bacterium Virus Fungus	Impetigo Warts Athlete’s foot; tinea
Gastrointestinal tract	Salmonella typhi	Bacterium	Typhoid fever
	Poliovirus	Virus	Poliomyelitis
	Giardia lamblia	Protozoan	Giardiasis
Genitourinary tract	Treponema pallidum	Bacterium	Syphilis
	Herpes simplex virus	Virus	Genital herpes
Respiratory tract	Bordetella pertussis	Bacterium	Whooping cough
	Influenza virus	Virus	Influenza
	Histoplasma capsulatum	Fungus	Histoplasmosis
Wound	Clostrtdium perfringens	Bacterium	Gas gangrene
	Rabies virus	Virus	Rabies
	Sporotrix schenckii	Fungus	Rose’s disease

 

 

nosocomial (hospital-acquired) infections. Medical procedures are designed to cure disease Some procedures used in the treatment of diseases, however, can inadvertently introduce pathogenic microorganisms into the body and initiate an infectious process. Even a puncture wound with a sterile hypodermic syringe can pick up microorganisms from the vicinity of the puncture and carry them through the skin surface. The routine cleansing of wounds and use of topical antiseptics after minor skin punctures and abrasions are accepted prophylactic measures to prevent the establishment of infections

Patients in hospitals often are m a debilitated state of health. Their body defences are weak. They are therefore, susceptible to various infectious disease. The term nosocomial infections is used to describe hospital-acquired infection. Nosocomial infections include pneumonia acquired m hospital urinary tract infections that develop as a result of the insertion of a catheter, and infections of the genital tract that develop from gynaecological procedure. Nosocomial infections affect approximately 2 million  patients hospitalized annually m the United States alone. The numbers of nosocomial infections have been reduced in the United States during the past decade This has been accomplished by increasing epidemiological standards and procedures such as educational seminars, the infection control nurse,  and hospital committees designed to identify and break the routes of transmission of pathogens to patients.

Surgical procedures often expose deep body tissues to potentially pathogenic microorganisms. A surgical incision circumvents normal body defense mechanisms. Great care is taken in modern surgical practices, therefore, to minimize microbial contamination of exposed tissues. These practices include the use of clean operating rooms with minimal numbers of airborne microorganisms, sterile instruments, masks, and gowns. All of these prevent the spread of microorganisms from the surgical staff to the patient. The application of topical antiseptics before making incisions also prevents accidental contamination of the wound with the indigenous skin microbiota of the patient. After many surgical procedures, antibiotics are given for several days as a prophylactic measure.

Despite all of these precautions of maintaining aseptic practices, infections still sometimes occur after surgery. Infections after surgery can be serious because the patient is already in a debilitated state. The onset of such infections is generally marked by fever. A purulent lesion may develop around the wound. Serious complications may follow open heart surgery if the patient develops endocarditis, caused by Staphylococcus or Streptococcus species. In surgical procedures involving cutting the intestines, the normal gut microbiota may contaminate other body tissues unless great care is taken to minimize such contamination. Antibiotics are also used in such cases to prevent microbial growth. The specific microorganisms causing infections of surgical wounds and the specific tissues that may be involved depend on the nature of the surgery and the tissues that are exposed to potential contamination with pathogens.

Surgical practices use elaborate aseptic procedures to minimize potential infection, but nevertheless, infections sometimes occur after surgery, attesting to the vulnerability of the body to microbial infection when the skin barrier is disrupted and host defense mechanisms are impaired.

 

respiratory tract AND airborne transmission. We inhale 10,000 to 20,000 litres of air per day. This volume of air usually contains between 10,000 and 1,000,000 microorganisms. It should not be surprising, therefore, that the respiratory tract provides a portal of entry for many human pathogens. Potential pathogens freely enter the respiratory tract through the normal inhalation of air. Various viruses, bacteria, and fungi are able to multiply within the tissues of the respiratory tract. Sometimes they cause localized infections. At other times they enter the circulatory system through the numerous blood vessels associated with the respiratory tract and spread through the bloodstream to other sites in the body  (FIG 2). To establish an infection via the respiratory tract, a pathogen must overcome the natural defense mechanisms that are particularly extensive in the lower respiratory tract There are numerous phagocytic cells in this area. While the potential for respiratory infection is great, fortunately, the actual rate of disease is low.

Airborne transmission occurs when pathogenic microorganisms are transferred from an infected to a susceptible individual via the air Droplets regularly become airborne during normal breathing, but the coughing and sneezing associated with respiratory tract infections are primarily responsible for the spread of pathogens in aerosols and thus the airborne transmission of disease. Airborne pathogens often become suspended in aerosols. Aerosols are clouds of tiny water droplets suspended in air The incidence of these diseases can be reduced by covering one’s nose and mouth while coughing and sneezing and avoiding contact with contagious individuals. These are practices we are taught to follow at an early age.

 

Transmission through the air is undoubtedly the main route of transmission of pathogens that enter via the respiratory tract. In spite of conditions of dryness, extreme temperatures, and ultraviolet radiation that characterize the air and which prevent them from growing in that environment, microorganisms still reach new hosts through the air. Some bacteria, particularly Gram-positive bacteria, can survive for several months in dust particles. Bacterial and fungal spores and naked viruses can live even longer. The incidence of airborne infections has increased in recent years because so many new buildings are sealed and have self-contained recirculating air systems for temperature control.

 

gastrointestinal tract — water AND food borne transmission. Microorganisms routinely enter the gastrointestinal tract in association with ingested food and water Waterborne and foodborne pathogens can infect the digestive system and cause gastrointestinal symptoms The large resident microbiota that develops in the human intestinal tract after birth is important for the maintenance of good health This population is usually not involved m disease processes and is normally noninvasive Waterborne and foodborne transmission generally involves transmission of pathogens that enter via the mouth and exit via the anus Generally, the establishment of infection through the gastrointestinal tract requires a relatively large infectious dose This means that a relatively large number of pathogenic microorganisms are required to successfully overcome the inherent defense mechanisms of the gastrointestinal tract. High infectious doses often are encountered m waters contaminated by sewage or other sources of human fecal matter.

Genitourinary tract—sexual transmission. The genitourinary tract provides the portal of entry for pathogens that are directly transmitted during sexual intercourse. Such infections are known as venereal or sexually transmitted diseases. The physiological properties of the pathogens causing these diseases restrict their transmission, for the most part, to direct physical contact. They have very limited natural survival times outside infected tissues. The overall control of sexually transmitted diseases rests with breaking the network of transmission This necessitates public health practices that seek to identify and treat all sexual partners of any one diagnosed as having one of the sexually transmitted diseases.

superficial body tissues – direct contact transmission. In some cases the deposition of pathogenic microorganisms on the skin surface can lead to an infectious disease. Since they require direct contact between skin and microorganisms for transmission to occur, these diseases are called contact diseases. Some diseases transmitted in this manner are superficial skin infections. On others, the pathogens are able to enter the body and spread systemically. Relatively few microorganisms possess the enzymatic capability to establish infections through the skin surface. Some microorganisms, however, are able to enter the subcutaneous layers through the channels provided by hair follicles. Transmission of some contact diseases may follow minor abrasions that allow the pathogens to circumvent the normal skin barrier.

parenteral route.  Punctures, injections, bites, cuts, wounds, surgical incisions, and cracking skin due to swelling or drying establish portals of entry to a host for a potential pathogen. Such access is called the parenteral route (from Greek para [beside] and enterik [intestinal tract]. Microorganisms thus gain entry to the body by being deposited directly into the tissues beneath the skin or into the mucous membrane.

Animal Bites and Disease Transmission. Many animals are carriers of microorganisms and transmit pathogens to humans through bites. Animals that transmit pathogens are called vectors. In some cases the nonhuman animal also suffers disease, but in many cases such animals are only carriers. Animal bites simultaneously disrupt the skin barrier and inoculate the wound with microorganisms whose pathogens potentially may be life threatening. Arthropods, particularly insects, commonly act as vectors of some very dangerous human pathogens. Their bites can establish serious infections.

Many human infections that are transmitted via animal bites involve animals that act as reservoirs for the pathogens. The animal populations that maintain the pathogen and act as reservoirs may themselves suffer from a disease caused by that pathogen. These diseases, termed zoonoses, are defined as infectious diseases of nonhuman animals transmissible to humans.

 

 

 

 

 

http://interactive.usask.ca/Ski/agriculture/soils/soilliv/soilliv_micfl.html

http://www.google.com/search?hl=en&q=soil+microflora

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=647419&dopt=Abstract

http://jac.oxfordjournals.org/cgi/content/abstract/46/suppl_1/41

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3103209&dopt=Abstract

www.gsbs.utmb.edu/microbook/ch006.htm

www.pedresearch.org/cgi/content/full/54/5/739

http://www.online-medical-dictionary.org/Microbial+Antagonism.asp?q=Microbial+Antagonism

http://www.mansfield.ohio-state.edu/~sabedon/biol2035.htm

http://en.wikipedia.org/wiki/Antibiotic

http://www.nlm.nih.gov/medlineplus/antibiotics.html 

http://www.intmed.mcw.edu/AntibioticGuide.html

http://whyfiles.org/038badbugs/

http://www.niaid.nih.gov/factsheets/antimicro.htm

http://textbookofbacteriology.net/BSRP.html

http://www.bacteriamuseum.org/niches/pbacteria/pathogenicity.shtml

http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap20.html#Symbioses