SANITARY MICROBIOLOGY. ECOLOGY OF MICROORGANISMS. MICROFLORA OF A SOIL, WATER, AIR. METHODS OF THEIR EXAMINATION.
HUMAN MICROFLORA AND METHODS OF ITS STUDYING. DYSBACTERIOSIS, CAUSES OF ITS APPEARANCE.
Microflora of the environment
Microbes are distributed everywhere in the environment surrounding us. They are found in the soil, water, air, in plants, animals, food products, various utensils, in the human body, and on the surface of the human body.
The relationship of microorganisms with the environment has beeamed ecology (Gr. oikos home, native land, logos idea, science). This is an-adaptive relationship. Micro-organisms have a remarkable ability to adapt themselves to certain environmental conditions. Individual ecology, population ecology, and association community ecology are distinguished. The study of micro-organism ecology forms the basis for understanding parasitism, zoonotic diseases, and particularly diseases with natural nidality, as well as for elaborating measures for the control of various infectious diseases.
Soil Microflora
Types of Microorganisms in Soil
Living organisms both plants and animals, constitute an important component of soil. The pioneering investigations of a number of early microbiologists showed for the first time that the soil was not an inert static material but a medium pulsating with life. The soil is now believed to be a dynamic or rather a living system, containing a dynamic population of organisms/microorganisms. Cultivated soil has relatively more population of microorganisms than the fallow land, and the soils rich in organic matter contain much more population than sandy and eroded soils. Microbes in the soil are important to us in maintaining soil fertility / productivity, cycling of nutrient elements in the biosphere and sources of industrial products such as enzymes, antibiotics, vitamins, hormones, organic acids etc. At the same time certain soil microbes are the causal agents of human and plant diseases.
The soil organisms are broadly classified in to two groups viz soil flora and soil fauna, the detailed classification of which is as follows.
Soil Organisms
A) Microflora: 1. Bacteria 2. Fungi, Molds, Yeast, Mushroom 3. Actinomycetes, Stretomyces 4. Algae eg. BGA, Yellow Green Algae, Golden Brown Algae.
1. Bacteria is again classified in I) Heterotrophic eg. symbiotic & non – symbiotic N2 fixers, Ammonifier, Cellulose Decomposers, Denitrifiers II) Autrotrophic eg. Nitrosomonas, Nitrobacter,
b) Macroflora: Roots of higher plants
B. Soil Fauna
a) Microfauna: Protozoa, Nematodes
b) Macrofauna: Earthworms. moles, ants & others.
Soil Microorganisms: Actinomycetes
1. Transitional group between bacteria and fungi
2. Active in degrading more resistant organic compounds
3. Optimal growth at alkaline pH
4. 2 important products:
– produce antibodies (streptomycin is produced by an actino)
– produce geosmin
5. Negative impact – potato scab (Streptomyces scabies)
Soil Microorganisms: Fungi
1. Dominate the soil biomass
2. Obligate aerobes
3. Can survive desiccation
4. Dominate in acid soils
5. Negative impacts:
– Apple replant disease (Rhizoctonia, Pythium, Fusarium, and Phytophtora)
– Powdery mildew is caused by a fungus
6. Beneficials:– Penicillium
Microbiological investigation of soil. For this purpose it is necessary to select most typical area not more then 25 m2. The samples are taken from different places of the are field along the diagonal, the angles and the center 10 — 20 cms deep. The weight of each sample must be 100 – 200 g. The total weight of the soil 0,5 – 1 kg.
After careful mixing take an average sample of weight 100 – 200 g. Put the samples of soil in the sterile banks, mark and deliver to the laboratory. The soil specimens for plating are grinded in sterile mortar, make serial dilutions in an isotonic solution of sodium chloride 1: 10, 1:100, 1: 1000 etc. Plate 0,1 – 1 ml of specimens into special media for aerobic and anaerobic microbes. After incubation at optimal temperature count the colonies on the plates.
Microflora of the Water
Water microbiology is concerned with the microorganisms that live in water, or can be transported from one habitat to another by water.
Water can support the growth of many types of microorganisms. This can be advantageous. For example, the chemical activities of certain strains of yeasts provide us with beer and bread. As well, the growth of some bacteria in contaminated water can help digest the poisons from the water.
However, the presence of other disease causing microbes in water is unhealthy and even life threatening. For example, bacteria that live in the intestinal tracts of humans and other warm blooded animals, such as Escherichia coli, Salmonella, Shigella, and Vibrio, can contaminate water if feces enters the water. Contamination of drinking water with a type of Escherichia coli known as O157:H7 can be fatal. The contamination of the municipal water supply of
The intestinal tract of warm-blooded animals also contains viruses that can contaminate water and cause disease. Examples include rotavirus, enteroviruses, and coxsackievirus.
Another group of microbes of concern in water microbiology are protozoa. The two protozoa of the most concern are Giardia and Cryptosporidium. They live normally in the intestinal tract of animals such as beaver and deer. Giardia and Cryptosporidium form dormant and hardy forms called cysts during their life cycles. The cyst forms are resistant to chlorine, which is the most popular form of drinking water disinfection, and can pass through the filters used in many water treatment plants. If ingested in drinking water they can cause debilitating and prolonged diarrhea in humans, and can be life threatening to those people with impaired immune systems. Cryptosporidium contamination of the drinking water of
Many microorganisms are found naturally in fresh and saltwater. These include bacteria, cyanobacteria, protozoa, algae, and tiny animals such as rotifers. These can be important in the food chain that forms the basis of life in the water. For example, the microbes called cyanobacteria can convert the energy of the sun into the energy it needs to live. The plentiful numbers of these organisms in turn are used as food for other life. The algae that thrive in water is also an important food source for other forms of life.
A variety of microorganisms live in fresh water. The region of a water body near the shoreline (the littoral zone) is well lighted, shallow, and warmer than other regions of the water. Photosynthetic algae and bacteria that use light as energy thrive in this zone. Further away from the shore is the limnitic zone. Photosynthetic microbes also live here. As the water deepens, temperatures become colder and the oxygen concentration and light in the water decrease. Now, microbes that require oxygen do not thrive. Instead, purple and green sulfur bacteria, which can grow without oxygen, dominate. Finally, at the bottom of fresh waters (the benthic zone), few microbes survive. Bacteria that can survive in the absence of oxygen and sunlight, such as methane producing bacteria, thrive.
Saltwater presents a different environment to microorganisms. The higher salt concentration, higher pH, and lower nutrients, relative to freshwater, are lethal to many microorganisms. But, salt loving (halophilic) bacteria abound near the surface, and some bacteria that also live in freshwater are plentiful (i.e., Pseudomonas and Vibrio). Also, in 2001, researchers demonstrated that the ancient form of microbial life known as archaebacteria is one of the dominant forms of life in the ocean. The role of archaebacteria in the ocean food chain is not yet known, but must be of vital importance
Another microorganism found in saltwater are a type of algae known as dinoflagellelates. The rapid growth and multiplication of dinoflagellates can turn the water red. This “red tide” depletes the water of nutrients and oxygen, which can cause many fish to die. As well, humans can become ill by eating contaminated fish.
Water can also be an ideal means of transporting microorganisms from one place to another. For example, the water that is carried in the hulls of ships to stabilize the vessels during their ocean voyages is now known to be a means of transporting microorganisms around the globe. One of these organisms, a bacterium called Vibrio cholerae, causes life threatening diarrhea in humans.
Drinking water is usually treated to minimize the risk of microbial contamination. The importance of drinking water treatment has been known for centuries. For example, in pre-Christian times the storage of drinking water in jugs made of metal was practiced. Now, the anti-bacterial effect of some metals is known. Similarly, the boiling of drinking water, as a means of protection of water has long been known.
Chemicals such as chlorine or chlorine derivatives has been a popular means of killing bacteria such as Escherichia coli in water since the early decades of the twentieth century. Other bacteria-killing treatments that are increasingly becoming popular include the use of a gas called ozone and the disabling of the microbe’s genetic material by the use of ultraviolet light. Microbes can also be physically excluded form the water by passing the water through a filter. Modern filters have holes in them that are so tiny that even particles as miniscule as viruses can be trapped.
An important aspect of water microbiology, particularly for drinking water, is the testing of the water to ensure that it is safe to drink. Water quality testing can de done in several ways. One popular test measures the turbidity of the water. Turbidity gives an indication of the amount of suspended material in the water. Typically, if material such as soil is present in the water then microorganisms will also be present. The presence of particles even as small as bacteria and viruses can decrease the clarity of the water. Turbidity is a quick way of indicating if water quality is deteriorating, and so if action should be taken to correct the water problem.
In many countries, water microbiology is also the subject of legislation. Regulations specify how often water sources are sampled, how the sampling is done, how the analysis will be performed, what microbes are detected, and the acceptable limits for the target microorganisms in the water sample. Testing for microbes that cause disease (i.e., Salmonella typhymurium and Vibrio cholerae) can be expensive and, if the bacteria are present in low numbers, they may escape detection. Instead, other more numerous bacteria provide an indication of fecal pollution of the water. Escherichia coli has been used as an indicator of fecal pollution for decades. The bacterium is present in the intestinal tract in huge numbers, and is more numerous than the disease-causing bacteria and viruses. The chances of detecting Escherichia coli is better than detecting the actual disease causing microorganisms. Escherichia coli also had the advantage of not being capable of growing and reproducing in the water (except in the warm and food-laden waters of tropical countries). Thus, the presence of the bacterium in water is indicative of recent fecal pollution. Finally, Escherichia coli can be detected easily and inexpensively.
Pseudomonas fluorescens, Micrococcus roseus, etc., are among the specific aquatic aerobic microorganisms. Anaerobic bacteria are very rarely found in water.
The microflora of rivers depends on the degree of pollution and the quality of purification of sewage waters flowing into river beds. Micro-organisms are widespread in the waters of the seas and oceans. They have been found at different depths (3700-
The degree of contamination of the water with organisms is expressed as saprobity which designates the total of all living matter in water containing accumulations of animal and plant remains. Water is subdivided into three zones. Polysaprobic zone is strongly polluted water, poor in oxygen and rich in organic compounds. The number of bacteria in 1 ml reaches 1 000000 and more. Colibacilli and anaerobic bacteria predominate which bring about the processes of putrefaction and fermentation. In the mesosaprobic zone (zone of moderate pollution) the mineralization of organic substances with intense oxidation and marked nitrification takes place. The number of bacteria in 1 ml of water amounts to hundreds of thousands, and there is a marked decrease in the number of colibacilli. The ohgosaprobic zone is characteristic of pure water. The number of microbes is low, and in 1 ml there are a few tens or hundreds; this zone is devoid of the colibacillus.
Depending on the degree of pollution pathogenic bacteria can survive in reservoirs and for a certain time can remain viable. Thus, for example, in tap water, river, or well water salmonellae of enteric fever can live from 2 days to 3 months, shigellae — 5-9 days, leptospirae — from 7 to 150 days. The cholera, vibrio El Tor lives in water for many months, the causative agent of tularaemia — from a few days to 3 months.
Tap water is considered clean if it contains a total amount of 100 microbes per ml, doubtful if there are 100-150 microbes, and polluted if 500 and more are present. In well water and in open reservoirs the amount of microbes in 1 ml should not exceed 1000. Besides, the quality of the water is determined by the presence of E. coli and its variants.
The degree of faecal pollution of water is estimated by the colititre or coli-index. The colititre is the smallest amount of water in millilitres in which one E. coli bacillus is found. The coli-index is the number of individuals off. coli found in 1 litre of water. Tap water is considered good if the colititre is within the limits of 300-500. Water is considered to be good quality if the coli-index is 2-3.
Due to the fact that Str. faecalis (enterococci) are constant inhabitants only of the intestine in man and warm-blooded animals, and are highly resistant to temperature variations and other environmental factors, they are taken into account with the colititre and coli-index for the determination of the degree of faecal pollution of water, sewage waters, soil, and other objects. At present new standards of enterococcus indices are being worked out.
Water is an important factor for the transmission of a number of infectious diseases (enteric fever, paratyphoids, cholera, dysentery, leptospiroses, etc.).
Due to the enormous sanitary-epidemiological role of water in relation to the intestinal group of diseases, it became necessary to work out rapid indicator methods for revealing colibacillus and pathogenic bacteria in water.
These include the methods of luminescent microscopy for the investigation of water for the presence of pathogenic microbes and the determination of the increase of the titre of the phage. Upon the addition of specific phages to liquids containing a homologous microbe in 6-10 hours a considerable increase in the amount of phage particles can be observed.
For a more complete and profound study of the microflora of the soil and water capillary microscopy is used. The principle is that very thin capillary tubes are placed in the soil or water reservoirs after which their contents are exposed to microscopic investigations. This method reveals those species of micro-organisms which do not grow in ordinary nutrient media, and which for many years were unknown to microbiologists.
Microbiological investigation of water. The sanitary – bacteriological investigation of water includes determination of total number of microbes in 1 ml of water, determination of a coli-index or coli-titer, and detection of pathogenic microbes, their toxins and Е. соli bacteriophages.
There are quantitative parameters of faecal pollution: a coli-titre and coli-index. Coli-titre is an index, which characterise an amount of water in millilitres which contain one E.coli. Coli-index characterises a number of E. coli in one litre of water.
The important part of investigation is taking of water samples. This procedure can do only by special persons (sanitary doctor or his assistant).
The samples of water for investigation are taken in 0,5 L sterile bottles.
Quantitative Analysis. Bacteria cannot be accurately counted by microscopic examination unless there are at least 100 million (108) cells per mililiter Natural bodies of water, however, rarely contain more than 105 cells per millilitre. The method employed is therefore the plate count. A measured volume of water is serially diluted (see below), following which 1 mL from each dilution tube is plated in nutrient agar and the resulting colonies counted. Since only cells able to form colonies are counted, the method is also known as the “viable count”.
A typical example of serial dilution would be the following. One millilitre of the water sample is aseptically transferred by pipette to 9 mL of sterile water. The mixture is thoroughly shaken, yielding a 1:10 dilution (For obvious reasons, this is also known as the “10–1” dilution). The process is repeated serially until a dilution is reached that contains between 30 and 300 colony-forming cells per millilitre, at which point several 1-mL samples are plated in a nutrient medium. Since the original sample may have contained up to 1 million (106) viable bacteria, it is necessary to dilute all the way to 10–5, plate 1-mL samples from each dilution tube, and then count the colonies only on those plates containing 30-300 colonies. The reasons for these numerical limits are that with over 300 colonies the plate becomes too crowded to permit each cell to form a visible colony, whereas with below 30 colonies the percent counting error becomes too great. (The statistical error of sampling can be calculated as follows’ The standard deviation of the count equals the square root of N, where N equals the average of many samples. Ninety-five percent of all samples will give counts within 2 standard deviations of the average. For example, if the average count is 36, then 95% of all samples will lie between 24 and 48 [36 ± 12]. In other words, within 95% confidence limits a sample count of 36 has an error of plus or minus 33%.) Assume that the above procedure has been carried out with the results shown in Table 3. The 10–3 dilution has a suitable number of colonies, the others being either too high or too low for accuracy. The original water sample is calculated to have contained 72,000 (72 x 103) viable cells per millilitre.
Qualitative Analysis. The methods of plating and enrichment culture are used to obtain a picture of the aquatic bacterial population. Although such methods are satisfactory for general biologic studies, they are inadequate for the purpose of sanitary water analysis; this involves the detection of intestinal bacteria in water, since their presence indicates sewage pollution and the consequent danger of the spread of enteric diseases. Since any enteric bacteria would be greatly outnumbered by other types present in the water samples, a selective technique is necessary in order to detect them. Two widely used procedures for sanitary water analysis are as follows:
1.Tube method. Dilutions of a water sample are inoculated into tubes of a medium which is elective for coliform bacteria and in which all coliform bacteria but few noncoliform bacteria will form acid and gas. Such media include MacConkey’s medium, which contains bile salts as inhibitors of noncoliform bacteria; lactose-containing media; and glutamatecontaining media. Cultures showing both acid and gas may then be subjected to further tests to confirm the presence of Escherichia coli or closely related enteric gram-negative rods. Such tests include streaking cultures on a lactose-peptone agar containing eosin and methylene blue (EMB agar), on which E coli forms characteristic blue-black colonies with a metallic sheen; subculturing at 44 °C; and a series of diagnostic biochemical tests
2. Membrane filtration method. A large measured volume of water is filtered through a sterilized membrane of a type that retains bacteria on its surface while permitting the rapid passage of smaller particles and water (fig.1). The membrane is then transferred to the surface of an agar plate containing a selective differential medium for coliform bacteria (fig. 2). Upon incubation, coliform bacteria give rise to typical colonies on the surface of the membrane. The advantages of this method are speed (the complete test takes less than 24 hours) and quantitation, the number of coliform cells being determined for a given volume of water.
The drinking water should not have more than 100 microbes in 1 ml. The microbic number in water of wells and open reservoirs can be up 1000.
During determination of a coli-index and coli-titre of water it is necessary to take into consideration the ability of Е. coli of the man and animal to grow at 43 C
Microflora of the Air
Of all environments, air is the simplest one and it occurs in a single phase gas. The relative quantities of various gases in air, by volume percentage are nitrogen 78%, oxygen 21 %, argon 0.9%, carbon dioxide 0.03%, hydrogen 0.01 % and other gases in trace amounts. In addition to various gases, dust and condensed vapor may also be found in air
Various layers can be recognized in the atmosphere upto a height of about 1000km. The layer nearest to the earth is called as troposphere. In temperate regions, troposphere extends upto about
Microbes Found in Air– In addition to gases, dust particles and water vapour, air also contains microorganisms. There are vegetative cells and spores of bacteria, fungi and algae, viruses and protozoan cysts. Since air is often exposed to sunlight, it has a higher temperature and less moisture. So, if not protected from desiccation, most of these microbial forms will die.Air is mainly it transport or dispersal medium for microorganisms. They occur in relatively small numbers in air when compared with soil or water. The microflora of air can be studied under two headings outdoor and indoor microflora.
Sources of Microorganisms in Air – Although a number of microorganisms are present in air, it doesn’t have an indigenous flora. Air is not a natural environment for microorganisms as it doesn’t contain enough moisture and nutrients to support their growth and reproduction.
Quite a number of sources have been studied in this connection and almost all of them have been found to be responsible for the air microflora. One of the most common sources of air microflora is the soil.
Soil microorganisms when disturbed by the wind blow, liberated into the air and remain suspended there for a long period of time. Man made actions like digging or plaguing the soil may also release soil borne microbes into the air. Similarly microorganisms found in water may also be released into the air in the form of water droplets or aerosols. Splashing of water by wind action or tidal action may also produce droplets or aerosols. Air currents may bring the microorganisms from plant or animal surfaces into air. These organisms may be either commensals or plant or animal pathogens. Studies show that plant pathogenic microorganisms are spread over very long distances through air. For example, spores of Puccinia graminis travel over a thousand kilometers. However, the transmission of animal diseases is not usually important in outside air.
The main source of airborne microorganisms is human beings. Their surface flora may be shed at times and may be disseminated into the air. Similarly, the commensal as well as pathogenic flora of the upper respiratory tract and the mouth are constantly discharged into the air by activities like coughing, sneezing, talking and laughing.
The microorganisms are discharged out in three different forms which are grouped on the basis of their relative size and moisture content. They are droplets, droplet nuclei and infectious dust. It was Wells, who described the formation of droplet nuclei. This initiated the studies on the significance of airborne transmission. A brief description of these agents is given below
Significance of Air Microflora – Although, when compared with the microorganisms of other environments, air microflora are very low iumber, they playa very significant role. This is due to the fact that the air is in contact with almost all animate and inanimate objects.
The significance of air flora has been studied since
He microscopically demonstrated the presence of microorganisms in the cotton. In his famous swaecked flask experiment, he showed that growth could not occur in sterile media unless airborne contamination had occurred.
Distribution of Microbes in Air – No microbes are indigenous to the atmosphere rather they represent allochthonous populations transported from aquatic and terrestrial habitats into the atmosphere. Microbes of air within 300-1,000 or more feet of the earth’s surface are the organisms of soil that have become attached to fragments of dried leaves, straw or dust particles, being blown away by the wind. Species vary greatly in their sensitivity to a given value of relative humidity, temperature and radiation exposures.
More microbes are found in air over land masses than far at sea. Spores of fungi, especially Alternaria, Cladosporium, Penicillium and Aspergillus are more numerous than other forms over sea within about
Microbes found in air over populated land areas below altitude of
In the dust and air of schools and hospital wards or the rooms of persons suffering from infectious diseases, microbes such as tubercle bacilli, streptococci, pneumococci and staphylococci have been demonstrated.
These respiratory bacteria are dispersed in air in the droplets of saliva and mucus produced by coughing, sneezing, talking and laughing. Viruses of respiratory tract and some enteric tract are also transmitted by dust and air. Pathogens in dust are primarily derived from the objects contaminated with infectious secretions that after drying become infectious dust.
Droplets are usually formed by sneezing, coughing and talking. Each droplet consists of saliva and mucus and each may contain thousands of microbes. It has been estimated that the number of bacteria in a single sneeze may be between 10,000 and 100,000. Small droplets in a warm, dry atmosphere are dry before they reach the floor and thus quickly become droplet nuclei.
Many plant pathogens are also transported from one field to another through air and the spread of many fungal diseases of plants can be predicted by measuring the concentration of airborne fungal spores. Human bacterial pathogens which cause important airborne diseases such as diphtheria, meningitis, pneumonia, tuberculosis and whooping cough are described in the chapter “Bacterial Diseases of Man”.
Air Microflora Significance in Hospitals – Although hospitals are the war fields for combating against diseases, there are certain occasions in which additional new infectious diseases can be acquired during hospitalization. Air within the hospital may act as a reservoir of pathogenic microorganisms which are transmitted by the patients.Infection acquired during the hospitalization are called nosocomial infections and the pathogens involved are called as nosocomial pathogens. Infections, manifested by the corresponding symptoms, after three days of hospitalization can be regarded as nosocomial infection (Gleckman & Hibert, 1982 and Bonten& Stobberingh, 1995). Nosocomial infection may arise in a hospital unit or may be brought in by the staff or patients admitted to the hospital.The common microorganisms associated with hospital infection are Haemophilus influenzae, Streptococcus pneumoniae, Staphylococcus aureus, Pseudomonas aeruginosa, members of Enterobacteriaceae and respiratory viruses. Development of high antibiotic resistance is a potential problem among nosocomial pathogens. For example, Methicillin Resistant Staphylococcus aureus (MRSA) and gentamicin resistant Gram-negative bacilli are of common occurrence. Even antiseptic liquids used would contain bacteria, for example Pseudomonas, due to their natural resistance to certain disinfectants and antiseptics and to many antibiotics.
Nosocomial pathogens may cause or spread hospital outbreaks. Nosocomial pneumonia is becoming a serious problem nowadays and a number of pathogens have been associated with it. (Bonten & Stobberingh, 1995). Frequent agents are Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, Enterobacter, Klebsiella, Escherichia coli and Haemophilus influenzae. Other less frequent agents are enterococci, streptococci other than S. pneumoniae, Serratia marcescens, Citrobacter freundii, Acinetobacter sp. and Xanthomonas sp.
In addition Legionella, Chlamydia pneumoniae and Mycobacterium tuberculosis have also been reported. Nosocomial transmissions of tuberculosis from patients to patients and from patients to health care workers have also been well documented (Wenger et a/., 1995). There are two main routes of transmission for nosocomial pathogens, contact (either direct or indirect) and airborne spread. Airborne spread is less common than the spread by direct or indirect contact. It occurs by the following mechanisms. The source may be either from persons or from inanimate objects.
In case of spread from persons the droplets from mouth, skin scales from nose, skin exudates and infected lesion transmit diseases such as measles, tuberculosis, pneumonia, staphylococcal sepsis and streptococcal sepsis. Talking, coughing and sneezing produce droplets. Skin scales are shed during wound dressing or bed making.
In case of inanimate sources particles from respiratory equipment and air-conditioning plant may transmit diseases. These include Gram-negative respiratory infection, Legionnaire’s disease and fungal infections.
Air Microflora Significance in Human Health – The significance of air microflora in human health relies on the fact that air acts as a medium for the transmission of infectious agents. An adult man inhales about ‘5m3 of air per day. Although most of the microorganisms present in air are harmless saprophytes and commensals, less than I % of the airborne bacteria are pathogens.
Eventhough the contamination level is very low, the probability of a person becoming infected will be greatest if he is exposed to a high concentration of airborne pathogens. Carriers, either with the manifestation of corresponding symptoms or without any apparent symptoms, may continuously release respiratory pathogens in the exhaled air.
Staphylococcus aureus is the most commonly found pathogen in air since the carriers are commonly present. The number of S. aureus in air may vary between 0-l/m3 and 50/m3.
Practically speaking, outdoor air doesn’t contain disease causing pathogen in a significant number to cause any infection. The purity of outdoor air, however, is an essential part of man’s environment. Dispersion and dilution by large volume of air is an inherent mechanism of air sanitation in outside air.
In the case of indoor air chance for the spread of infectious disease is more, especially in areas where people gather in large numbers. For example, in theatres, schools etc.
Air-Borne Microorganisms and Human Diseases
Air-borne microorganisms cause dangerous diseases in human beings. A detailed study of these diseases falls under the preview of a text book of medical microbiology. A chart representing air-borne diseases is given below for ready reference :
The composition of the microbes of the air is quite variable. It depends on many factors: on the extent to which air is contaminated with mineral and organic suspensions, on the temperature, rainfall, locality, humidity, and other factors. The more dust, smoke, and soot in the air, the greater the number of microbes. Each particle of dust or smoke is able to adsorb on its surface numerous microbes. Microbes are rarely found on the surfaces of mountains, in the seas of Arctic lands covered with snow, in oceans, and in snow.
The microflora of the air consists of very different species which enter it from the soil, plants, and animal organisms. Pigmented saprophytic bacteria (micrococci, various sarcinae), cryptogams (hay bacillus, B. cereus, B. megaterium), actinomycetes, moulds, yeasts, etc., are often found in the air.
The number of microbes in the air vanes from a few specimens to many tens of thousands per 1 cu m. Thus, for example, the air of the
According to the investigations of E. Mishustin, 1 cu m of air in
At present Streptococcus viridans serves as sanitary indices for the air of closed buildings, and haemolytic streptococci and pathogenic staphylococci are a direct epidemiological hazard.
Depending on the time of the year, the composition and the amount of microflora change. If the total amount of microbes in winter is accepted as 1, then in spring it will be 1.7, in summer— 2 and in autumn — 1.2.
The total amount of microbes in an operating room before operation should not exceed 500 per 1 cu m of air, and after the operatioot more than 1000. There should be no pathogenic staphylococci and streptococci in 250 litres of air. In operating rooms of maternity hospitals before work the number of saprophyte microflora colonies isolated from the air by precipitating microbes on meat-peptone agar within 30 minutes should not exceed 20. In 1 gram of dust in hospitals, there are up to 200000 pyogenic (haemolytic) streptococci.
The number of microbes in factories and homes is associated closely with the sanitary hygienic conditions of the building. In overcrowding, poor ventilation and natural lighting and if the premises are not properly cleaned, the number of microbes increases. Dry cleaning processes, infrequent floor washing, the use of dirty rags and brushes, and drying them in the same room make the conditions favourable for the accumulation of microbes in air. The causative agents of influenza, measles, scarlet fever, diphtheria, whooping cough, meningococcal infections, tonsillitis, acute catarrhs of the respiratory tract, tuberculosis, smallpox, pneumatic plague, and other diseases can be transmitted through the air together with droplets of mucus and sputum during sneezing, coughing, and talking.
Microbes can be spread by air currents, by aerial dust and aerial droplets. During sneezing, coughing and talking, a sick person can expel pathogenic bacteria together with droplets of mucus and sputum into the surrounding environment within a radius of 1.0-1.5 m or more. Microorganisms contained in air can remain in three phases of the bacterial aerosol — droplet, droplet-nuclei, and dust. An aerosol is the physical system of solid or liquid particles suspended in a gaseous medium.
On the average a person breathes about 12000-14000 litres of air daily, while 99.8 per cent of the microbes contained in air are held back in the respiratory tract. The bacterial aerosol produced naturally in the nasopharynx, during sneezing and coughing is thrown into the air — up to 60000 droplets of different size. Among them almost 60 per cent consist of large droplets (100 mcm), 30 per cent — of average sized droplets (50 mcm), and 10 per cent — of small (5 mcm) droplets.
The greatest amount of bacteria is discharged during sneezing, less — during coughing, and still less — during talking. With each sneeze a man expels from 10000 to 1 000000 droplets. In one cough from 10 to 1000 droplets containing bacteria are discharged into the environment, and when a person utters 10-20 words — up to 80 droplets are expelled. The nature of the bacterial aerosol depends on the viscosity of the secretion excreted from the respiratory tract. A liquid secretion is dispersed into more minute droplets more easily than a viscous one. Near the person expelling the bacteria a more concentrated aerosol of bacterial droplets from 1 to 2000 mcm in size is produced. Most of the droplets are from 2 to 100 mcm in size. Large droplets from 100 to 2000 mcm in size are thrown out to a distance of 2-3 m and more and quickly settle on the ground. Small drops of the bacterial aerosol (1-10 mcm) can remain in a suspended condition for a long time (for hours or days).
The air is an unfavourable medium for microbes. The absence of nutrient substances, the presence of moisture, optimal temperature, the lethal activity of sunlight, and desiccation do not create conditions for keeping microbes viable and most of them perish. However, the relatively short period during which the microbes are in air is quite enough to bring about the transmission of pathogenic bacteria and viruses from sick to healthy persons, and to cause extensive epidemics of diseases such as influenza.
For the purpose of prophylaxis various methods are used in protecting humans from infection via air-borne dust. Thus, the sputum of tuberculosis patients is burned or decontaminated, the room is frequently ventilated, and cleaned by mopping, the streets are sprayed, drainage and absorbers are used, and masks are used during sorting of wool and rags, etc. The air of operating rooms, isolating rooms, wards, and bacteriological laboratories is decontaminated by ultraviolet radiation (mercury-quartz, uviol lamps, etc.).
The laboratory investigation of air is carried out to determine the qualitative and quantitative composition of its microflora. This is achieved by using simple and complex methods. For a more accurate investigation of microbial contents of the air special apparatus are used (Rechmensky’s bacterial absorber, Krotov’s apparatus, Kiktenko’s apparatus, and others).
At present rapid methods for the indication of microbes in the environment are being devised which will allow quick determinations of the presence of micro-organisms in the soil, the water and air.
Microbiological investigation of the air. The sanitary – hygienic investigation of the microflora of the air includes determination both the total number of microbes in 1 m3 of the air and revealing of pathogenic staphylococci and streptococci. For taking the samples sedimentation and aspiration methods are used.
Plate method (sedimentation method). The Petry’s dishes with meat-peptone agar or another special nutrient media for staphylococci and streptococci, for example blood agar, yolk-salt agar are used. They stay in open form at various height from a floor. It is recommend to take one sample on every 20 m2 of a premises. Term of exposition depends on prospective quantity of microbes in the air. With a plenty of microorganisms a cup is opened for 5 – 10 minutes, with a little – for 20 — 40 minutes.
Place the dishes into 37 C incubator for 24 hrs and then incubate for 48 hrs at room temperature (18-20 C). Study colonies, count them, and isolate pure culture of different microbes.
According to Omeliansky data on a surface of medium by 100 cm2 sedimentate in 5 minutes as so many microbes, as they present in 10 L of air. For example, on the dish surface with MPA after 5 minute exposure 32 colonies have grown. It is necessary to calculate amount of microbes which are present in 1 m3 of the air, applying the Omeliansky’s formula. The plate has 78,5 cm2(S = r2 =3,14 • 52 = 78,5 см2). Thus, it is possible to determine, what quantity of microbes (х) would grow at the given exposure on a surface of medium in 100 см2,
x = (32 • 100) : 78,5 = 40
This quantity of microbes contains in 10 L of the air, and in 1 m3 (
For determination of microbial dissemination degree quantity of the colonies on the dish surface which have been counted should be multiplied with one of multiplier.
If on a Petry’s dish (78,5 cm2) at a 10-minute exposure 40 colonies have grown, the quantity of microbes in 1 m3 of the air will be equal 40 • 60 = 2400.
Aspiration method. It is based on a shock action of an air jet about a surface of a medium. Krotov’s apparatus is used for this purpose. It give us the possibility to let pass 50 –100 L of air with a speed of 25 L per minute through clinoid chink in the special glass above the open dish MPA. The rotation of Petry’s dish (1 rotation/sec) provides uniform dispersion of microorganisms on all surface of a medium. Then dish is incubated in a thermostat at 37 C for 18-24 hrs.
For example, 250 colonies are revealed on the surface of dish after 2-minutes exposure with a 25 l/min speed. Thus the number of microbes (x) in 1 l of the air is: x = (250 • 1000) : 50 = 5000.
There are temporary standards of a sanitary – hygienic state of the air: in operating room the total number of microbes prior to the beginning of the operation must be no more than 500 in 1 m3, after the operation – 1000.
In preoperative and dressing rooms limiting number of microbes prior before the beginning of work – 750 microbes in 1 m3, after work – 1500. In birth wards the total number of microbes is about 2000 in 1 m3 of the air, and staphylococci and streptococci are not higher then 24 in 1 m3, and iewborn rooms – about 44 in 1 m3.
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 acquireconsiderably 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 aspseudomembranous 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.
Normal Flora of Skin
Skin provides good examples of various microenvironments. Skin regions have been compared to geographic regions of Earth: the desert of the forearm, the cool woods of the scalp, and the tropical forest of the armpit. The composition of the dermal microflora varies from site to site according to the character of the microenvironment. A different bacterial flora characterizes each of three regions of skin: (1) axilla, perineum, and toe webs; (2) hand, face and trunk; and (3) upper arms and legs. Skin sites with partial occlusion (axilla, perineum, and toe webs) harbor more microorganisms than do less occluded areas (legs, arms, and trunk). These quantitative differences may relate to increased amount of moisture, higher body temperature, and greater concentrations of skin surface lipids. The axilla, perineum, and toe webs are more frequently colonized by Gram-negative bacilli than are drier areas of the skin.
The number of bacteria on an individual’s skin remains relatively constant; bacterial survival and the extent of colonization probably depend partly on the exposure of skin to a particular environment and partly on the innate and species-specific bactericidal activity in skin. Also, a high degree of specificity is involved in the adherence of bacteria to epithelial surfaces. Not all bacteria attach to skin; staphylococci, which are the major element of the nasal flora, possess a distinct advantage over viridans streptococci in colonizing the nasal mucosa. Conversely, viridans streptococci are not seen in large numbers on the skin or in the nose but dominate the oral flora.
The microbiology literature is inconsistent about the density of bacteria on the skin; one reason for this is the variety of methods used to collect skin bacteria. The scrub method yields the highest and most accurate counts for a given skin area. Most microorganisms live in the superficial layers of the stratum corneum and in the upper parts of the hair follicles. Some bacteria, however, reside in the deeper areas of the hair follicles and are beyond the reach of ordinary disinfection procedures. These bacteria are a reservoir for recolonization after the surface bacteria are removed.
Staphylococcus epidermidis
S. epidermidis is a major inhabitant of the skin, and in some areas it makes up more than 90 percent of the resident aerobic flora.
Staphylococcus aureus
The nose and perineum are the most common sites for S. aureus colonization, which is present in 10 percent to more than 40 percent of normal adults. S. aureus is prevalent (67 percent) on vulvar skin. Its occurrence in the nasal passages varies with age, being greater in the newborn, less in adults. S. aureus is extremely common (80 to 100 percent) on the skin of patients with certain dermatologic diseases such as atopic dermatitis, but the reason for this finding is unclear.
Micrococci
Micrococci are not as common as staphylococci and diphtheroids; however, they are frequently present on normal skin. Micrococcus luteus, the predominant species, usually accounts for 20 to 80 percent of the micrococci isolated from the skin.
Diphtheroids (Coryneforms)
The term diphtheroid denotes a wide range of bacteria belonging to the genus Corynebacterium. Classification of diphtheroids remains unsatisfactory; for convenience, cutaneous diphtheroids have been categorized into the following four groups: lipophilic or nonlipophilic diphtheroids; anaerobic diphtheroids; diphtheroids producing porphyrins (coral red fluorescence when viewed under ultraviolet light); and those that possess some keratinolytic enzymes and are associated with trichomycosis axillaris (infection of axillary hair). Lipophilic diphtheroids are extremely common in the axilla, whereas nonlipophilic strains are found more commonly on glabrous skin.
Anaerobic diphtheroids are most common in areas rich in sebaceous glands. Although the name Corynebacterium acnes was originally used to describe skin anaerobic diphtheroids, these are now classified as Propionibacterium acnes and as P. granulosum. P. acnes is seen eight times more frequently than P. granulosum in acne lesions and is probably involved in acne pathogenesis. Children younger than 10 years are rarely colonized with P. acnes. The appearance of this organism on the skin is probably related to the onset of secretion of sebum (a semi-fluid substance composed of fatty acids and epithelial debris secreted from sebaceous glands) at puberty. P. avidum, the third species of cutaneous anaerobic diphtheroids, is rare in acne lesions and is more often isolated from the axilla.
Streptococci
Streptococci, especially β-hemolytic streptococci, are rarely seen oormal skin. The paucity of β-hemolytic streptococci on the skin is attributed at least in part to the presence of lipids on the skin, as these lipids are lethal to streptococci. Other groups of streptococci, such as α-hemolytic streptococci, exist primarily in the mouth, from where they may, in rare instances, spread to the skin.
Gram-Negative Bacilli
Gram-negative bacteria make up a small proportion of the skin flora. In view of their extraordinary numbers in the gut and in the natural environment, their scarcity on skin is striking. They are seen in moist intertriginous areas, such as the toe webs and axilla, and not on dry skin. Desiccation is the major factor preventing the multiplication of Gram-negative bacteria on intact skin.Enterobacter, Klebsiella, Escherichia coli, and Proteus spp. are the predominant Gram-negative organisms found on the skin. Acinetobacter spp also occurs on the skin of normal individuals and, like other Gram-negative bacteria, is more common in the moist intertriginous areas.
Nail Flora
The microbiology of a normal nail is generally similar to that of the skin. Dust particles and other extraneous materials may get trapped under the nail, depending on what the nail contacts. In addition to resident skin flora, these dust particles may carry fungi and bacilli. Aspergillus, Penicillium, Cladosporium, and Mucor are the major types of fungi found under the nails.
Oral and Upper Respiratory Tract Flora
The oral flora is involved in dental caries and periodontal disease, which affect about 80 percent. of the population in the Western world. Anaerobes in the oral flora are responsible for many of the brain, face, and lung infections that are frequently manifested by abscess formation.
The pharynx and trachea contain primarily those bacterial genera found in the normal oral cavity (for example, α-and β-hemolytic streptococci); however, anaerobes, staphylococci, neisseriae, diphtheroids, and others are also present. Potentially pathogenic organisms such as Haemophilus, mycoplasmas, and pneumococci may also be found in the pharynx. Anaerobic organisms also are reported frequently. The upper respiratory tract is so often the site of initial colonization by pathogens (Neisseria meningitides, C. diphtheriae, Bordetella pertussis, and many others) and could be considered the first region of attack for such organisms. In contrast, the lower respiratory tract (small bronchi and alveoli) is usually sterile, because particles the size of bacteria do not readily reach it. If bacteria do reach these regions, they encounter host defense mechanisms, such as alveolar macrophages, that are not present in the pharynx.
Gastrointestinal Tract Flora
The stomach is a relatively hostile environment for bacteria. It contains bacteria swallowed with the food and those dislodged from the mouth. Acidity lowers the bacterial count, which is highest (approximately 103 to 106 organisms/g of contents) after meals and lowest (frequently undetectable) after digestion. Some Helicobacter species can colonize the stomach and are associated with type B gastritis and peptic ulcer disease. Aspirates of duodenal or jejunal fluid contain approximately 103 organisms/ml in most individuals. Most of the bacteria cultured (streptococci, lactobacilli, Bacteroides) are thought to be transients. Levels of 105 to about 107 bacteria/ml in such aspirates usually indicate an abnormality in the digestive system (for example, achlorhydria or malabsorption syndrome). Rapid peristalsis and the presence of bile may explain in part the paucity of organisms in the upper gastrointestinal tract. Further along the jejunum and into the ileum, bacterial populations begin to increase, and at the ileocecal junction they reach levels of 106 to 108 organisms/ml, with streptococci, lactobacilli, Bacteroides, and bifidobacteria predominating.
Concentrations of 109 to 1011 bacteria/g of contents are frequently found in human colon and feces. This flora includes a bewildering array of bacteria (more than 400 species have been identified); nonetheless, 95 to 99 percent belong to anaerobic genera such as Bacteroides, Bifidobacterium, Eubacterium, Peptostreptococcus, and Clostridium. In this highly anaerobic region of the intestine, these genera proliferate, occupy most available niches, and produce metabolic waste products such as acetic, butyric, and lactic acids. The strict anaerobic conditions, physical exclusion (as is shown in many animal studies), and bacterial waste products are factors that inhibit the growth of other bacteria in the large bowel.
Although the normal flora can inhibit pathogens, many of its members can produce disease in humans. Anaerobes in the intestinal tract are the primary agents of intra-abdominal abscesses and peritonitis. Bowel perforations produced by appendicitis, cancer, infarction, surgery, or gunshot wounds almost always seed the peritoneal cavity and adjacent organs with the normal flora. Anaerobes can also cause problems within the gastrointestinal lumen. Treatment with antibiotics may allow certain anaerobic species to become predominant and cause disease. For example,Clostridium difficile, which can remain viable in a patient undergoing antimicrobial therapy, may produce pseudomembranous colitis. Other intestinal pathologic conditions or surgery can cause bacterial overgrowth in the upper small intestine. Anaerobic bacteria can then deconjugate bile acids in this region and bind available vitamin B12 so that the vitamin and fats are malabsorbed. In these situations, the patient usually has been compromised in some way; therefore, the infection caused by the normal intestinal flora is secondary to another problem.
More information is available on the animal than the human microflora. Research on animals has revealed that unusual filamentous microorganisms attach to ileal epithelial cells and modify host membranes with few or no harmful effects. Microorganisms have been observed in thick layers on gastrointestinal surfaces and in the crypts of Lieberkuhn. Other studies indicate that the immune response can be modulated by the intestinal flora. Studies of the role of the intestinal flora in biosynthesis of vitamin K and other host-utilizable products, conversion of bile acids (perhaps to cocarcinogens), and ammonia production (which can play a role in hepatic coma) show the dual role of the microbial flora in influencing the health of the host. More basic studies of the human bowel flora are necessary to define their effect on humans.
Urogenital Flora
The type of bacterial flora found in the vagina depends on the age, pH, and hormonal levels of the host. Lactobacillus spp. predominate in female infants (vaginal pH, approximately 5) during the first month of life. Glycogen secretion seems to cease from about I month of age to puberty. During this time, diphtheroids, S. epidermidis, streptococci, and E. coli predominate at a higher pH (approximately pH 7). At puberty, glycogen secretion resumes, the pH drops, and women acquire an adult flora in which L. acidophilus, corynebacteria, peptostreptococci, staphylococci, streptococci, and Bacteroides predominate. After menopause, pH again rises, less glycogen is secreted, and the flora returns to that found in prepubescent females. Yeasts (Torulopsis andCandida) are occasionally found in the vagina (10 to 30 percent of women); these sometimes increase and cause vaginitis.
In the anterior urethra of humans, S. epidermidis, enterococci, and diphtheroids are found frequently; E. coli, Proteus, and Neisseria (nonpathogenic species) are reported occasionally (10 to 30 percent). Because of the normal flora residing in the urethra, care must be taken in clinically interpreting urine cultures; urine samples may contain these organisms at a level of 104/ml if a midstream (clean-catch) specimen is not obtained.
Conjunctival Flora
The conjunctival flora is sparse. Approximately 17 to 49 percent of culture samples are negative. Lysozyme, secreted in tears, may play a role in controlling the bacteria by interfering with their cell wall formation. When positive samples show bacteria, corynebacteria, Neisseriae, and Moraxellae are cultured. Staphylococci and streptococci are also present, and recent reports indicate that Haemophilus parainfluenzae is present in 25 percent of conjunctival samples.
Significance of the
The fact that the normal flora substantially influences the well-being of the host was not well understood until germ-free animals became available. Germ-free animals were obtained by cesarean section and maintained in special isolators; this allowed the investigator to raise them in an environment free from detectable viruses, bacteria, and other organisms. Two interesting observations were made about animals raised under germ-free conditions. First, the germ-free animals lived almost twice as long as their conventionally maintained counterparts, and second, the major causes of death were different in the two groups. Infection often caused death in conventional animals, but intestinal atonia frequently killed germ-free animals. Other investigations showed that germ-free animals have anatomic, physiologic, and immunologic features not shared with conventional animals. For example, in germ-free animals, the alimentary lamina propria is underdeveloped, little or no immunoglobulin is present in sera or secretions, intestinal motility is reduced, and the intestinal epithelial cell renewal rate is approximately one-half that of normal animals (4 rather than 2 days).
Although the foregoing indicates that bacterial flora may be undesirable, studies with antibiotic treated animals suggest that the flora protects individuals from pathogens. Investigators have used streptomycin to reduce the normal flora and have then infected animals with streptomycin-resistant Salmonella. Normally, about 106 organisms are needed to establish a gastrointestinal infection, but in streptomycin-treated animals whose flora is altered, fewer than 10 organisms were needed to cause infectious disease. Further studies suggested that fermentation products (acetic and butyric acids) produced by the normal flora inhibited Salmonella growth in the gastrointestinal tract.
The normal flora in humans usually develops in an orderly sequence, or succession, after birth, leading to the stable populations of bacteria that make up the normal adult flora. The main factor determining the composition of the normal flora in a body region is the nature of the local environment, which is determined by pH, temperature, redox potential, and oxygen, water, and nutrient levels. Other factors such as peristalsis, saliva, lysozyme secretion, and secretion of immunoglobulins also play roles in flora control. The local environment is like a concerto in which one principal instrument usually dominates. For example, an infant begins to contact organisms as it moves through the birth canal. A Gram-positive population (bifidobacteria arid lactobacilli) predominates in the gastrointestinal tract early in life if the infant is breast-fed. This bacterial population is reduced and displaced somewhat by a Gram-negative flora (Enterobacteriaceae) when the baby is bottle-fed. The type of liquid diet provided to the infant is the principal instrument of this flora control; immunoglobulins and, perhaps, other elements in breast milk may also be important.
What, then, is the significance of the normal flora? Animal and some human studies suggest that the flora influences human anatomy, physiology, lifespan, and, ultimately, cause of death. Although the causal relationship of flora to death and disease in humans is accepted, of her roles of the human microflora need further study.
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 human nutrition. 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.
