ANTIBIOTICS – 1

An antibacterial is an agent that inhibits bacterial growth or kills bacteria.^[1] The term is often used synonymously with the term antibiotic(s). Today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic(s) has come to denote a broader range of antimicrobial compounds, including anti-fungal and other compounds.^[2] Antibacterials must be distinguished from disinfectants (sanitizing agents), which are less-selective substances used to destroy microorganisms.

The term antibiotic was first used in 1942 by Selman Waksman and his collaborators in journal articles to describe any substance produced by a microorganism that is antagonistic to the growth of other microorganisms in high dilution.^[3] This definition excluded substances that kill bacteria but are not produced by microorganisms (such as gastric juices and hydrogen peroxide). It also excluded synthetic antibacterial compounds such as the sulfonamides. Many antibacterial compounds are relatively small molecules with a molecular weight of less than 2000 atomic mass units.

With advances in medicinal chemistry, most of today's antibacterials chemically are semisynthetic modifications of various natural compounds.^[4] These include, for example, the beta-lactam antibacterials, which include the penicillins (produced by fungi in the genus Penicillium), the cephalosporins, and the carbapenems. Compounds that are still isolated from living organisms are the aminoglycosides, whereas other antibacterials—for example, the sulfonamides, the quinolones, and the oxazolidinones—are produced solely by chemical synthesis. In accordance with this, many antibacterial compounds are classified on the basis of chemical/biosynthetic origin into natural, semisynthetic, and synthetic. Another classification system is based on biological activity; in this classification, antibacterials are divided into two broad groups according to their biological effect on microorganisms: bactericidal agents kill bacteria, and bacteriostatic agents slow down or stall bacterial growth.

History

 

Penicillin, the first natural antibiotic discovered by Alexander Fleming in 1928

Before the early 20th century, treatments for infections were based primarily on medicinal folklore. Mixtures with antimicrobial properties that were used in treatments of infections were described over 2000 years ago.^[5] Many ancient cultures, including the ancient Egyptians and ancient Greeks, used specially selected mold and plant materials and extracts to treat infections.^[6][7] More recent observations made in the laboratory of antibiosis between micro-organisms led to the discovery of natural antibacterials produced by microorganisms. Louis Pasteur observed, "if we could intervene in the antagonism observed between some bacteria, it would offer perhaps the greatest hopes for therapeutics".^[8] The term 'antibiosis', meaning "against life," was introduced by the French bacteriologist Jean Paul Vuillemin as a descriptive name of the phenomenon exhibited by these early antibacterial drugs.^[9][10] Antibiosis was first described in 1877 in bacteria when Louis Pasteur and Robert Koch observed that an airborne bacillus could inhibit the growth of Bacillus anthracis.^[11] These drugs were later renamed antibiotics by Selman Waksman, an American microbiologist, in 1942.^[3][9] John Tyndall first described antagonistic activities by fungi against bacteria in England in 1875.^[8] Synthetic antibiotic chemotherapy as a science and development of antibacterials began in Germany with Paul Ehrlich in the late 1880s.^[9] Ehrlich noted certain dyes would color human, animal, or bacterial cells, while others did not. He then proposed the idea that it might be possible to create chemicals that would act as a selective drug that would bind to and kill bacteria without harming the human host. After screening hundreds of dyes against various organisms, he discovered a medicinally useful drug, the synthetic antibacterial Salvarsan^[9][12][13] now called arsphenamine.

In 1895, Vincenzo Tiberio, physician of the University of Naples discovered that a mold (Penicillium) in a water well had an antibacterial action.^[14][15] After this initial chemotherapeutic compound proved effective, others pursued similar lines of inquiry, but it was not until in 1928 that Alexander Fleming observed antibiosis against bacteria by a fungus of the genus Penicillium. Fleming postulated the effect was mediated by an antibacterial compound named penicillin, and that its antibacterial properties could be exploited for chemotherapy. He initially characterized some of its biological properties, and attempted to use a crude preparation to treat some infections, but he was unable pursue its further development without the aid of trained chemists.^[16][17]

Alexander Fleming

The first sulfonamide and first commercially available antibacterial, Prontosil, was developed by a research team led by Gerhard Domagk in 1932 at the Bayer Laboratories of the IG Farben conglomerate in Germany.^[13] Domagk received the 1939 Nobel Prize for Medicine for his efforts. Prontosil had a relatively broad effect against Gram-positive cocci, but not against enterobacteria. Research was stimulated apace by its success. The discovery and development of this sulfonamide drug opened the era of antibacterial. In 1939, coinciding with the start of World War II, Rene Dubos reported the discovery of the first naturally derived antibiotic, gramicidin from B. brevis. It was one of the first commercially manufactured antibiotics universally and very effectively used to treat wounds and ulcers during World War II.^[18] Gramicidin could not be used systemically because of toxicity. Research results obtained during that period were not shared between the Axis and the Allied powers during the war. Florey and Chain succeeded in purifying the first penicillin, penicillin G in 1942, but it did not become widely available outside the Allied military before 1945. The chemical structure of penicillin was determined by Dorothy Crowfoot Hodgkin in 1945. Purified penicillin displayed potent antibacterial activity against a wide range of bacteria and had low toxicity in humans. Furthermore, its activity was not inhibited by biological constituents such as pus, unlike the synthetic sulfonamides. The discovery of such a powerful antibiotic was unprecedented, and the development of penicillin led to renewed interest in the search for antibiotic compounds with similar efficacy and safety.^[19] For their discovery and development of penicillin as a therapeutic drug, Ernst Chain, Howard Florey, and Alexander Fleming shared the 1945 Nobel Prize in Medicine. Florey credited Dubos with pioneering the approach of deliberately and systematically searching for antibacterial compounds, which had led to the discovery of gramicidin and had revived Florey's research in penicillin.^[18]

Etymology

The term "antibacterial" derives from Greek ἀντί (anti), "against"^[20] + βακτήριον (baktērion), diminutive of βακτηρία (baktēria), "staff, cane",^[21] because the first ones to be discovered were rod-shaped,^[22] and the term "antibiotic" derives from anti + βιωτικός (biōtikos), "fil for life, lively",^[23] which comes from βίωσις (biōsis), "way of life",^[24] and that from βίος (bios), "life".^[25]

Medical uses

• Treatment

• Bacterial infection

• Protozoan infection, e.g., metronidazole is effective against several parasitics

• Immunomodulation, e.g., tetracycline, which is effective in periodontal inflammation, and dapsone, which is effective in autoimmune diseases such as oral mucous membrane pemphigoid^[26]

• Prevention of infection

• Surgical wound

• Dental antibiotic prophylaxis^[27][28]

• Conditions of neutropenia, e.g. cancer-related

Pharmacodynamics

Testing the susceptibility of Staphylococcus aureus to antibiotics by the Kirby-Bauer disk diffusion method - antibiotics diffuse from antibiotic-containing disks and inhibit growth of S. aureus, resulting in a zone of inhibition.

The successful outcome of antimicrobial therapy with antibacterial compounds depends on several factors. These include host defense mechanisms, the location of infection, and the pharmacokinetic and pharmacodynamic properties of the antibacterial.^[29] A bactericidal activity of antibacterials may depend on the bacterial growth phase, and it often requires ongoing metabolic activity and division of bacterial cells.^[30] These findings are based on laboratory studies, and in clinical settings have also been shown to eliminate bacterial infection.^[29][31] Since the activity of antibacterials depends frequently on its concentration,^[32] in vitro characterization of antibacterial activity commonly includes the determination of the minimum inhibitory concentration and minimum bactericidal concentration of an antibacterial.^[29][33] To predict clinical outcome, the antimicrobial activity of an antibacterial is usually combined with its pharmacokinetic profile, and several pharmacological parameters are used as markers of drug efficacy.^[34][35]

Classes

Molecular targets of antibiotics on the bacteria cell

Antibacterial antibiotics are commonly classified based on their mechanism of action, chemical structure, or spectrum of activity. Most target bacterial functions or growth processes.^[9] Those that target the bacterial cell wall (penicillins and cephalosporins) or the cell membrane (polymyxins), or interfere with essential bacterial enzymes (rifamycins, lipiarmycins, quinolones, and sulfonamides) have bactericidal activities. Those that target protein synthesis (macrolides, lincosamides and tetracyclines) are usually bacteriostatic (with the exception of bactericidal aminoglycosides).^[36] Further categorization is based on their target specificity. "Narrow-spectrum" antibacterial antibiotics target specific types of bacteria, such as Gram-negative or Gram-positive bacteria, whereas broad-spectrum antibiotics affect a wide range of bacteria. Following a 40-year hiatus in discovering new classes of antibacterial compounds, four new classes of antibacterial antibiotics have been brought into clinical use: cyclic lipopeptides (such as daptomycin), glycylcyclines (such as tigecycline), oxazolidinones (such as linezolid) and lipiarmycins (such as fidaxomicin).^[37][38]

Production

Main article: Production of antibiotics

Since the first pioneering efforts of Florey and Chain in 1939, the importance of antibiotics, including antibacterials, to medicine has led to intense research into producing antibacterials at large scales. Following screening of antibacterials against a wide range of bacteria, production of the active compounds is carried out using fermentation, usually in strongly aerobic conditions.^[39]

Administration

Oral antibacterials are orally ingested, whereas intravenous administration may be used in more serious cases^{[citation needed]}, such as deep-seated systemic infections. Antibiotics may also sometimes be administered topically, as with eye drops or ointments.

Side-effects

Antibacterials are screened for any negative effects on humans or other mammals before approval for clinical use, and are usually considered safe and most are well tolerated. However, some antibacterials have been associated with a range of adverse effects.^[40] Side-effects range from mild to very serious depending on the antibiotics used, the microbial organisms targeted, and the individual patient.^{[citation needed]} Safety profiles of newer drugs are often not as well established as for those that have a long history of use.^[40] Adverse effects range from fever and nausea to major allergic reactions, including photodermatitis and anaphylaxis.^{[citation needed]} Common side-effects include diarrhea, resulting from disruption of the species composition in the intestinal flora, resulting, for example, in overgrowth of pathogenic bacteria, such as Clostridium difficile.^[41] Antibacterials can also affect the vaginal flora, and may lead to overgrowth of yeast species of the genus Candida in the vulvo-vaginal area.^[42] Additional side-effects can result from interaction with other drugs, such as elevated risk of tendon damage from administration of a quinolone antibiotic with a systemic corticosteroid. Some scientists have hypothesized that the indiscriminate use of antibiotics alter the host microbiota and this has been associated with chronic disease.^[43][44]

Drug-drug interactions

Birth control pills

The majority of studies indicate antibiotics do not interfere with contraceptive pills,^[45] such as clinical studies that suggest the failure rate of contraceptive pills caused by antibiotics is very low (about 1%).^[46] In cases where antibacterials have been suggested to affect the efficiency of birth control pills, such as for the broad-spectrum antibacterial rifampicin, these cases may be due to an increase in the activities of hepatic liver enzymes causing increased breakdown of the pill's active ingredients.^[45] Effects on the intestinal flora, which might result in reduced absorption of estrogens in the colon, have also been suggested, but such suggestions have been inconclusive and controversial.^[47][48] Clinicians have recommended that extra contraceptive measures be applied during therapies using antibacterials that are suspected to interact with oral contraceptives.^[45]

Alcohol

Interactions between alcohol and certain antibacterials may occur and may cause side-effects and decreased effectiveness of antibacterial therapy.^[49][50]

"It is sensible to avoid drinking alcohol when taking medication. However, it is unlikely that drinking alcohol in moderation will cause problems if you are taking most common antibiotics. However, there are specific types of antibiotics with which alcohol should be avoided completely, because of serious side-effects."^[51]

Therefore, potential risks of side-effects and effectiveness depend on the type of antibacterial administered. Despite the lack of a categorical counterindication, the belief that alcohol and antibacterials should never be mixed is widespread.

Antibacterials such as metronidazole, tinidazole, cephamandole, latamoxef, cefoperazone, cefmenoxime, and furazolidone, cause a disulfiram-like chemical reaction with alcohol by inhibiting its breakdown by acetaldehyde dehydrogenase, which may result in vomiting, nausea, and shortness of breath.^[51]

Other effects of alcohol on antibacterial activity include altered activity of the liver enzymes that break down the antibacterial compound.^[52] In addition, serum levels of doxycycline and erythromycin succinate^{[clarification needed]} two bacteriostatic antibacterials (see above) may be reduced by alcohol consumption, resulting in reduced efficacy and diminished pharmacotherapeutic effect.^[53]

Resistance

SEM depicting methicillin-resistant Staphylococcus aureus bacteria

The emergence of resistance of bacteria to antibacterial drugs is a common phenomenon. Emergence of resistance often reflects evolutionary processes that take place during antibacterial drug therapy. The antibacterial treatment may select for bacterial strains with physiologically or genetically enhanced capacity to survive high doses of antibacterials. Under certain conditions, it may result in preferential growth of resistant bacteria, while growth of susceptible bacteria is inhibited by the drug.^[54] For example, antibacterial selection for strains having previously acquired antibacterial-resistance genes was demonstrated in 1943 by the Luria–Delbrück experiment.^[55] Antibacterials such as penicillin and erythromycin, which used to have high efficacy against many bacterial species and strains, have become less effective, because of increased resistance of many bacterial strains.^[56]

The survival of bacteria often results from an inheritable resistance,^[57] but the growth of resistance to antibacterials also occurs through horizontal gene transfer. Horizontal transfer is more likely to happen in locations of frequent antibiotic use.^[58]

Antibacterial resistance may impose a biological cost, thereby reducing fitness of resistant strains, which can limit the spread of antibacterial-resistant bacteria, for example, in the absence of antibacterial compounds. Additional mutations, however, may compensate for this fitness cost and can aid the survival of these bacteria.^[59]

Paleontological data show that both antibiotics and antibiotic resistance are ancient compounds and mechanisms.^[60] Natural antibiotics (produced by microorganisms to compete against other microorganisms), however, are evolutionarily robust, i.e., microorganisms are often unable to develop resistance against them. Molecular data confirm this observation, showing that the evolution of bacterial proteins targeted by antibiotics is highly constrained compared with the evolution of other proteins. For example, mutations in genes coding for antibiotics-targeted proteins tend to be deleterious, making these genes subject to strong purifying selection, which stringently maintains the sequence and structure of their cognate proteins.^[61]

Several molecular mechanisms of antibacterial resistance exist. Intrinsic antibacterial resistance may be part of the genetic makeup of bacterial strains.^[62] For example, an antibiotic target may be absent from the bacterial genome. Acquired resistance results from a mutation in the bacterial chromosome or the acquisition of extra-chromosomal DNA.^[62] Antibacterial-producing bacteria have evolved resistance mechanisms that have been shown to be similar to, and may have been transferred to, antibacterial-resistant strains.^[63][64] The spread of antibacterial resistance often occurs through vertical transmission of mutations during growth and by genetic recombination of DNA by horizontal genetic exchange.^[57] For instance, antibacterial resistance genes can be exchanged between different bacterial strains or species via plasmids that carry these resistance genes.^[57][65] Plasmids that carry several different resistance genes can confer resistance to multiple antibacterials.^[65] Cross-resistance to several antibacterials may also occur when a resistance mechanism encoded by a single gene conveys resistance to more than one antibacterial compound.^[65]

Antibacterial-resistant strains and species, sometimes referred to as "superbugs", now contribute to the emergence of diseases that were for a while well controlled. For example, emergent bacterial strains causing tuberculosis (TB) that are resistant to previously effective antibacterial treatments pose many therapeutic challenges. Every year, nearly half a million new cases of multidrug-resistant tuberculosis (MDR-TB) are estimated to occur worldwide.^[66] For example, NDM-1 is a newly identified enzyme conveying bacterial resistance to a broad range of beta-lactam antibacterials.^[67] The United Kingdom's Health Protection Agency has stated that "most isolates with NDM-1 enzyme are resistant to all standard intravenous antibiotics for treatment of severe infections."^[68]

Alternatives

The increase in bacterial strains that are resistant to conventional antibacterial therapies has prompted the development of bacterial disease treatment strategies which are alternatives to conventional antibacterials.

Resistance-modifying agents

One strategy to address bacterial drug resistance is the discovery and application of compounds that modify resistance to common antibacterials. For example, some resistance-modifying agents may inhibit multidrug resistance mechanisms, such as drug efflux from the cell, thus increasing the susceptibility of bacteria to an antibacterial. Targets include:

• The efflux inhibitor Phe-Arg-β-naphthylamide.^[82]

• Beta-lactamase inhibitors, such as clavulanic acid and sulbactam.

Metabolic stimuli such as sugar can help eradicate a certain type of antibiotic tolerant bacteria by keeping their metabolism active.^[83]

Phage therapy

Phage therapy is the use of viruses that infect bacteria (i.e. phages) for the treatment of bacterial infections.^[84][85] Phages are common in bacterial populations and control the growth of bacteria in many environments, including in the intestine, the ocean, and the soil.^[86] Phage therapy was in use in the 1920s and 1930s in the US, Western Europe, and Eastern Europe. However, success rates of this therapy have not been firmly established, because only a limited number of clinical trials testing the efficacy of phage therapy have been conducted.^[85] These studies were performed mainly in the former Soviet Union, at the Eliava Institute of Bacteriophage, Microbiology and Virology, Republic of Georgia.^[87] The development of antibacterial-resistant bacteria has sparked renewed interest in phage therapy in Western medicine.^[88] Several companies (e.g., Intralytix, Novolytics, and Gangagen), universities, and foundations across the world now focus on phage therapies.^{[88][89][90][91]} One concern with this therapeutic strategy is the use of genetically engineered viruses, which limits certain aspects of phage therapy.^[85][92][93]

Bacteriocins

Bacteriocins are peptides that can be more readily engineered than small molecules,^[94] and are possible alternatives to conventional antibacterial compounds.^[95] Different classes of bacteriocins have different potential as therapeutic agents. Small-molecule bacteriocins (microcins and lantibiotics) are similar to the classic antibiotics; colicin-like bacteriocins possess a narrow spectrum, and require molecular diagnostics prior to therapy.^{[citation needed]} Limitations of large-molecule antibacterials include reduced transport across membranes and within the human body. For this reason, they are usually applied topically or gastrointestinally.^[96]

Chelation

Chelation of micronutrients that are essential for bacterial growth to restrict pathogen spread in vivo might supplement some antibacterials. For example, limiting the iron availability in the human body restricts bacterial proliferation.^[97][98] Many bacteria, however, possess mechanisms (such as siderophores) for scavenging iron within environmental niches in the human body, and experimental developments of iron chelators, therefore, aim to reduce iron availability specifically to bacterial pathogens.^[99]

Vaccines

Vaccines rely on immune modulation or augmentation. Vaccination either excites or reinforces the immune competency of a host to ward off infection, leading to the activation of macrophages, the production of antibodies, inflammation, and other classic immune reactions. Antibacterial vaccines have been responsible for a drastic reduction in global bacterial diseases.^{[citation needed]} Vaccines made from attenuated whole cells or lysates have been replaced largely by less reactogenic, cell-free vaccines consisting of purified components, including capsular polysaccharides and their conjugates, to protein carriers, as well as inactivated toxins (toxoids) and proteins.^[100]

Biotherapy

Biotherapy may employ organisms, such as protozoa,^[101] to consume the bacterial pathogens. Another such approach is maggot therapy.

Probiotics

Probiotics consist of a live culture of bacteria, which may become established as competing symbionts, and inhibit or interfere with colonization by microbial pathogens.^[102]

Silver

Silver mechanism of action includes disruption of multiple bacterial cellular processes (disulfide bond formation, metabolism, and iron homeostasis), which may lead to increased production of reactive oxygen species, and increased cell membrane permeability. In both mouse model and in vitro, evidence suggests that silver may increase the activity and potentially restore antibiotic susceptibility to antibiotic resistant bacterial. In one study, silver sensitized gram-negative bacteria in mouse and in vitro to vancomycin, "thereby expanding the antibacterial spectrum of this drug." Finally, "both in vitro and in a mouse biofilm infection model that silver can enhance antibacterial action against bacteria that produce biofilms."^[103]

Host defense peptides

An additional therapeutic agent is the enhancement of the multifunctional properties of natural anti-infectives, such as cationic host defense (antimicrobial) peptides (HDPs).^[100]

Antimicrobial coatings

Functionalization of antimicrobial surfaces can be used for sterilization, self-cleaning, and surface protection.

Antimicrobial copper alloy surfaces

Main articles: Antimicrobial properties of copper and Antimicrobial copper-alloy touch surfaces

Copper-alloy surfaces have natural intrinsic properties to effectively and quickly destroy bacteria. The United States Environmental Protection Agency has approved the registration of 355 different antibacterial copper alloys that kill E. coli O157:H7, methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus, Enterobacter aerogenes, and Pseudomonas aeruginosa in less than 2 hours of contact. As a public hygienic measure in addition to regular cleaning, antimicrobial copper alloys are being installed in healthcare facilities and in a subway transit system.^{[104][105][106]}

Targeting the trans-translational pathway

Using high-throughput screening, researchers in Pennsylvania State University discovered 46 small molecules capable of disrupting the trans-translational pathway, the mechanism used by bacteria to facilitate protein synthesis and is unique to bacteria only. One of the most potent molecules, KKL-35, was found to cause cell death in Shigella, Bacillus anthracis and Mycobacterium tuberculosis, and no resistant mutant strains were observed in the experiments. This result demonstrated that KKL-35 could be effective in a wide spectrum of bacterial species due to its targeted pathway and may represent a new class of antibiotics.^[107][108]

Status of new antibiotics development (2013)

In a policy report released by the Infectious Disease Society of America (IDSA) on April 2013, IDSA expressed grave concern over the weak pipeline of antibiotics to combat the growing ability of bacteria, especially the Gram-negative bacilli (GNB), to develop resistance to antibiotics. Since 2009, only 2 new antibiotics were approved in United States, and the number of new antibiotics annually approved for marketing continues to decline. The report could identify only seven antibiotics currently in phase 2 or phase 3 clinical trials to treat the GNB which includes E. coli, Salmonella, Shigella and the Enterobacteriaceae bacteria, and these drugs do not address the entire spectrum of the resistance developed by those bacteria.^[109][110] Some of these seven new antibiotics are combination of existent antibiotics, they include:

• Ceftolozane/tazobactam (CXA-201; CXA-101/tazobactam): Antipseudomonal cephalosporin/β-lactamase inhibitor combination (cell wall synthesis inhibitor). In phase 3.

• Ceftazidime/avibactam (ceftazidime/NXL104): Antipseudomonal cephalosporin/β-lactamase inhibitor combination (cell wall synthesis inhibitor). In phase 3.

• Ceftaroline/avibactam (CPT-avibactam; ceftaroline/NXL104): Anti-MRSA cephalosporin/ β-lactamase inhibitor combination (cell wall synthesis inhibitor)

• Imipenem/MK-7655: Carbapenem/ β-lactamase inhibitor combination (cell wall synthesis inhibitor). In phase 2.

• Plazomicin (ACHN-490): Aminoglycoside (protein synthesis inhibitor). In phase 2.

• Eravacycline (TP-434): A synthetic tetracycline derivative / protein synthesis inhibitor targeting the ribosome being developed by Tetraphase. Phase 2 trials complete.^[111]

• Brilacidin (PMX-30063): Peptide defense protein mimetic (cell membrane disruption). In phase 2.

The IDSA’s prognosis for sustainable R&D infrastructure for antibiotics development will depend upon clarification of FDA regulatory clinical trial guidance which would facilitate the speedy approval of new drugs, and the appropriate economic incentives for the pharmaceuticals companies to invest in this endeavor.^[110]

Antibiotic Classification & Mechanism

Bacteria Overview

Antibiotic Resistance Mechanisms

• Bacteria develop ability to hydrolyze these drugs using β lactamase

• confers resistance to penicillin

• e.g. E. coli, Staph epidermidis, Pseudomonas aeruginosa, Klebsiella pneumoniae

• add β lactamase inhibitor e.g. clavulanic acid in amoxicillin-clavulanate (Augmentin)

• Genetic mutation of mecA

• a bacterial gene encoding a penicillin-binding protein. New PBP has reduced affinity for antibiotics

• confers resistance to methicillin, oxacillin, nafcillin

• e.g. MRSA

• Altered cell wall permeability

• confers resistance to tetracyclines, quinolones, trimethoprim and β lactam antibiotics

• Creation of biofilm barrier

• provides an environment where offending bacteria can multiply safe from the hoste immune system

• Salmonella

• Staph epidermidis

• Active efflux pumps

• confers resistance to erythromycin and tetracycline

• e.g. msrA gene in Staph

• Altered peptidoglycan subunit (altered D-alanyl-D-alanine of NAM/NAG-peptide) 

• confers resistance to vancomycin

• e.g. vancomycin resistant enterococcus (VRE) 

• Ribosome alteration

• erm gene confer inducible resistance to MLS (macrolide lincosamide streptogranin) agents via methylation of 23s rRNA

• demonstrate using D zone test

Penicillins

• Mechanism

• interfer with bacterial cell wall synthesis

• Subclassification and tested examples

• natural

• penicillin G

• penicillinase-resistant

• methicillin (Staphcillin)

• aminopenicillins ampicillin (Omnipen, Polycillin)

Cephalosporins

• Overview

• bactericidal

• Mechanism

• disrupts the synthesis of the peptidoglycan layer of bacterial cell walls  

• does so through competitive inhibition on PCB (penicllin binding proteins)

• peptidoglycan layer is important for cell wall structural integrity.

• same mechanicsm of action as beta-lactam antibiotics (such as penicillins)

• Subclassification and tested examples

• first generation

• cefazolin (Ancef, Kefzol)

• second generation

• cefaclor (Ceclor)

• third generation

• cefriazone (Rocephin)

• fourth generation cefepime (Maxipime)

Fluoroquinolones

• Mechanism

• blocks DNA replication via inhibition of DNA gyrase 

• Side effects

• inhibit early fracture healing through toxic effects on chondrocytes 

• increased rates of tendinitis, with special predilection for the Achilles tendon. 

• tenocytes in the Achilles tendon have exhibited degenerative changes when viewed microscopically after fluoroquinolone administration.

• recent clinical studies have shown an increased relative risk of Achilles tendon rupture of 3.7. 

• Subclassification and tested examples

• ciprofloxacin (Cipro) levofloxacin (Levaquin)

Aminoglycosides

• Mechanism

• bactericidal

• inhibition of bacterial protein synthesis 

• work by binding to the 30s ribosome subunit, leading to the misreading of mRNA. This misreading results in the synthesis of abnormal peptides that accumulate intracellularly and eventually lead to cell death. These antibiotics arebactericidal.

• Subclassification and tested examples gentamicin (Garamycin)

Vancomycin

• Coverage

• gram-positive bacteria

• Mechanism

• bactericidal

• an inhibitor of cell wall synthesis 

• Resistance

• increasing emergence of vancomycin-resistant enterococci has resulted in the development of guidelines for use by the  (CDC) 

• indications for vancomycin 

• serious allergies to penicillins or beta-lactam antimicrobials 

• serious infections caused by susceptible organisms resistant to penicillins (MRSA, MRSE) surgical prophylaxis for major procedures involving implantation of prostheses in institutions with a high rate of MRSA or MRSE

Rifampin

Most effective against intracellular phagocytized Staphylococcus aureus in macrophages 

Linezolid

Linezolid binds to the 23S portion of the 50S subunit and acts by preventing the formation of the initiation complex between the the 30S and 50S subunits of the ribosome

Splenectomy

• Splenectomy patients or patients with functional hyposplenism require the following vaccines and/or antibiotics  

• Pneumococcal immunization

• Haemophilus influenza type B vaccine

• Meningococcal group C conjugate vaccine

• Influenza immunization

Lifelong prophylactic antibiotics (oral phenoxymethylpenicillin or erythromycin)

Beta-Lactam Antibiotics & Other Inhibitors of Cell Wall Synthesis

Beta-Lactam Compounds

Penicillins

The penicillins are classified as -lactam drugs because of their unique four-membered lactam ring. They share features of chemistry, mechanism of action, pharmacologic and clinical effects, and immunologic characteristics with cephalosporins, monobactams, carbapenems, and –lactamase inhibitors, which also

are – lactam compounds

.

 

Chemistry

All penicillins have the basic structure shown in Figure 43–1. A thiazolidine ring (A) is attached to a -lactam ring (B) that carries a secondary amino group (RNH–). Substituents (R; examples shown in Figure 43–2) can be attached to the amino group. Structural integrity of the 6-aminopenicillanic acid nucleus is essential for the biologic activity of these compounds. If the -lactam ring is enzymatically cleaved by bacterial -lactamases, the resulting product, penicilloic acid, lacks antibacterial activity.

The attachment of different substitutents to 6-aminopenicillanic acid determines the essential pharmacologic and antibacterial properties of the resulting molecules. Penicillins can be assigned to one of three groups (below). Within each of these groups are compounds that are relatively stable to gastric acid and suitable for oral administration, eg, penicillin V, dicloxacillin, and amoxicillin. The side chains of some representatives of each group are shown in Figure 43–2, with a few distinguishing characteristics. Penicillins (eg, penicillin G).

 

These have the greatest activity against gram-positive organisms, gram-negative cocci, and non- - lactamase-producing anaerobes. However, they have little activity against gram-negative rods.

They are susceptible to hydrolysis by lactamases.

These drugs retain the antibacterial spectrum of penicillin and have improved activity against gramnegative organisms, but they are destroyed by lactamases.

Figure 43–1.

Penicillin Units and Formulations

The activity of penicillin G was originally defined in units. Crystalline sodium penicillin G contains approximately 1600 units/mg (1 unit = 0.6 g; 1 million units of penicillin = 0.6 g).

Semisynthetic penicillins are prescribed by weight rather than units. The minimum inhibitory concentration (MIC) of any penicillin (or other antimicrobial) is usually given in g/mL.

Most penicillins are dispensed as the sodium or potassium salt of the free acid. Potassium penicillin G contains about 1.7 meq of K+ per million units of penicillin (2.8 meq/g). Nafcillin contains Na+, 2.8 meq/g. Procaine salts and benzathine salts of penicillin G provide repository forms for intramuscular injection. In dry crystalline form, penicillin salts are stable for long periods (eg, for years at 4 °C). Solutions lose their activity rapidly (eg, 24 hours at 20 °C) and must be prepared fresh for administration.

Mechanism of Action

Penicillins, like all -lactam antibiotics, inhibit bacterial growth by interfering with a specific step in bacterial cell wall synthesis. The cell wall is a rigid outer layer that is not found in animal cells. It completely surrounds the cytoplasmic membrane (Figure 43–3), maintaining the shape of the cell and preventing cell lysis from high osmotic pressure. The cell wall is composed of a complex crosslinked polymer, peptidoglycan (murein, mucopeptide), consisting of polysaccharides and polypeptides. The polysaccharide contains alternating amino sugars, N-acetylglucosamine and Nacetylmuramic acid (Figure 43–4). A five-amino-acid peptide is linked to the N-acetylmuramic acid sugar. This peptide terminates in D-alanyl-D-alanine. Penicillin-binding proteins (PBPs) catalyze the

transpeptidase reaction that removes the terminal alanine to form a crosslink with a nearby peptide, which gives cell wall its structural rigidity. -Lactam antibiotics are structural analogs of the natural D-Ala-D-Ala substrate and they are covalently bound by PBPs at the active site. After a –lactam antibiotic has attached to the PBP, the transpeptidation reaction is inhibited (Figure 43–5), peptidoglycan synthesis is blocked, and the cell dies. The exact mechanism responsible for cell death is not completely understood, but autolysins, bacterial enzymes that remodel and break down cell wall, are involved.

Penicillins and cephalosporins are bactericidal only if cells are actively growing and synthesizing cell wall.

Cephalosporins & Cephamycins

Cephalosporins and cephamycins are similar to penicillins chemically, in mechanism of action, and in toxicity. Cephalosporins are more stable than penicillins to many bacterial -lactamases and therefore usually have a broader spectrum of activity. Cephalosporins are not active against enterococci and Listeria monocytogenes.

Chemistry

The nucleus of the cephalosporins, 7-aminocephalosporanic acid (Figure 43–6), bears a close resemblance to 6-aminopenicillanic acid (Figure 43–1). The intrinsic antimicrobial activity of natural cephalosporins is low, but the attachment of various R1 and R2 groups has yielded drugs of good therapeutic activity and low toxicity (Figure 43–6). The cephalosporins have molecular weights of 400–450. They are soluble in water and relatively stable to pH and temperature changes. Cephalosporins can be classified into four major groups or generations, depending mainly on the spectrum of antimicrobial activity. As a general rule, first-generation compounds have better activity against gram-positive organisms and the later compounds exhibit improved activity against gram-negative aerobic organisms. Other Beta Lactam Drugs

Monobactams

These are drugs with a monocyclic -lactam ring (Figure 43–1)

They are relatively resistant to lactamases and active against gram-negative rods (including pseudomonas and serratia). They have no activity against gram-positive bacteria or anaerobes.

Aztreonam is the only monobactam available in the USA. It resembles aminoglycosides in its spectrum of activity. Aztreonam is given intravenously every 8 hours in a dose of 1–2 g, providing peak serum levels of

 

100 g/mL. The half-life is 1–2 hours and is greatly prolonged in renal failure.

Penicillin-allergic patients tolerate aztreonam without reaction. Occasional skin rashes and elevations of serum aminotransferases occur during administration of aztreonam, but major toxicity has not yet been reported. The clinical usefulness of aztreonam has not been fully defined.

Beta-Lactamase Inhibitors (Clavulanic Acid, Sulbactam, & Tazobactam)

These substances resemble -lactam molecules (Figure 43–7) but themselves have very weak antibacterial action. They are potent inhibitors of many but not all bacterial lactamases and can protect hydrolyzable penicillins from inactivation by these enzymes. -Lactamase inhibitors are most active against Ambler class A lactamases (plasmid-encoded transposable element [TEM] - lactamases in particular) such as those produced by staphylococci, H influenzae, N gonorrhoeae,

salmonella, shigella, E coli, and K pneumoniae. They are not good inhibitors of class C - lactamases, which typically are chromosomally encoded and inducible, produced by enterobacter, citrobacter, serratia, and pseudomonas, but they do inhibit chromosomal lactamases of legionella, bacteroides, and branhamella. The three inhibitors differ slightly with respect to pharmacology, stability, potency, and activity, but these differences are of little therapeutic significance. -Lactamase inhibitors are available only in fixed combinations with specific penicillins. The antibacterial spectrum of the combination is determined by the companion penicillin, not the -lactamase inhibitor. (The fixed combinations available in the USA are listed in the Preparations Available section.) An inhibitor will extend the spectrum of a penicillin provided that the inactivity of the penicillin is due to destruction by lactamase and that the inhibitor is active against thelactamase that is produced. Thus, ampicillinsulbactam is active against -lactamase-producing S aureus and H influenzae but not serratia, which produces a lactamase that is not inhibited by sulbactam. Similarly, if a strain of P aeruginosa is resistant to piperacillin, it will also be resistant to piperacillin-tazobactam, since tazobactam does not inhibit the chromosomal lactamase.

The indications for penicillin- -lactamase inhibitor combinations are empirical therapy for infections caused by a wide range of potential pathogens in both immunocompromised and immunocompetent patients and treatment of mixed aerobic and anaerobic infections, such as intraabdominal infections.

Figure 43–6.

 

Doses are the same as those used for the single agents except that the recommended dosage of piperacillin in the piperacillin-tazobactam combination is 3 g every 6 hours. This is less than the recommended 3–4 g every 4–6 hours for piperacillin alone, raising concerns about the use of the combination for treatment of suspected pseudomonal infection.

Adjustments for renal insufficiency are made based on the penicillin component.

Carbapenems

The carbapenems are structurally related to -lactam antibiotics (Figure 43–1). Ertapenem, imipenem, and meropenem are licensed for use in the USA. Imipenem has a wide spectrum with good activity against many gram-negative rods, including Pseudomonas aeruginosa, gram-positive organisms, and anaerobes. It is resistant to most lactamases but not metallo- lactamases.

Enterococcus faecium, methicillin-resistant strains of staphylococci, Clostridium difficile, Burkholderia cepacia, and Stenotrophomonas maltophilia are resistant. Imipenem is inactivated by dehydropeptidases in renal tubules, resulting in low urinary concentrations. Consequently, it is administered together with an inhibitor of renal dehydropeptidase, cilastatin, for clinical use.

Meropenem is similar to imipenem but has slightly greater activity against gram-negative aerobes and slightly less activity against gram-positives. It is not significantly degraded by renal dehydropeptidase and does not require an inhibitor. Ertapenem is less active than meropenem or imipenem against Pseudomonas aeruginosa and acinetobacter species. It is not degraded by renal dehydropeptidase.

Carbapenems penetrate body tissues and fluids well, including the cerebrospinal fluid. All are cleared renally, and the dose must be reduced in patients with renal insufficiency. The usual dose of imipenem is 0.25–0.5 g given intravenously every 6–8 hours (half-life 1 hour). The usual adult dose of meropenem is 1 g intravenously every 8 hours. Ertapenem has the longest half-life (4 hours) and is administered as a once-daily dose of 1 g intravenously or intramuscularly. Intramuscular ertapenem is irritating, and for that reason the drug is formulated with 1% lidocaine for administration by this route. A carbapenem is indicated for infections caused by susceptible organisms that are resistant to otheravailable drugs and for treatment of mixed aerobic and anaerobic infections.

Carbapenems are active against many highly penicillin-resistant strains of pneumococci.

A carbapenem is the - lactam antibiotic of choice for treatment of enterobacter infections, since it is resistant to destruction by the lactamase produced by these organisms.

Strains of Pseudomonas aeruginosa may rapidly

develop resistance to imipenem or meropenem, so simultaneous use of an aminoglycoside is recommended for infections caused by those organisms. Ertapenem is insufficiently active against P

aeruginosa and should not be used to treat infections caused by that organism. Imipenem or meropenem with or without an aminoglycoside may be effective treatment for febrile neutropenic patients.

The most common adverse effects of carbapenems—which tend to be more common with imipenem—are nausea, vomiting, diarrhea, skin rashes, and reactions at the infusion sites.

Excessive levels of imipenem in patients with renal failure may lead to seizures. Meropenem and ertapenem are less likely to cause seizures than imipenem. Patients allergic to penicillins may be allergic to carbapenems as well.

Other Inhibitors of Cell Wall Synthesis

Vancomycin

Vancomycin is an antibiotic produced by Streptococcus orientalis. With the single exception of flavobacterium, it is active only against gram-positive bacteria, particularly staphylococci.

Vancomycin is a glycopeptide of molecular weight 1500. It is water-soluble and quite stable.

Mechanisms of Action & Basis of Resistance

Vancomycin inhibits cell wall synthesis by binding firmly to the D-Ala-D-Ala terminus of nascent peptidoglycan pentapeptide (Figure 43–5). This inhibits the transglycosylase, preventing further elongation of peptidoglycan and cross-linking. The peptidoglycan is thus weakened and the cell becomes susceptible to lysis. The cell membrane is also damaged, which contributes to the antibacterial effect. Resistance to vancomycin in enterococci is due to modification of the D-Ala-D-Ala binding site of the peptidoglycan building block in which the terminal D-Ala is replaced by D-lactate. This results in the loss of a critical hydrogen bond that facilitates high-affinity binding of vancomycin to its target and loss of activity. This mechanism is also present in vancomycin-resistant S aureus strains (MIC 32 g/mL), which have acquired the enterococcal resistance determinants. The mechanism for reduced vancomycin susceptibility of vancomycin-intermediate strains (MICs = 8–16 g/mL) is not known.

Antibacterial Activity

Vancomycin is bactericidal for gram-positive bacteria in concentrations of 0.5–10 g/mL. Most pathogenic staphylococci, including those producing lactamase and those resistant to nafcillin and methicillin, are killed by 4 g/mL or less. Vancomycin kills staphylococci relatively slowly and only if cells are actively dividing; the rate is less than that of the penicillins both in vitro and in vivo. Vancomycin is synergistic with gentamicin and streptomycin against E faecium and E faecalis strains that do not exhibit high levels of aminoglycoside resistance.

Pharmacokinetics

Vancomycin is poorly absorbed from the intestinal tract and is administered orally only for the treatment of antibiotic-associated enterocolitis caused by Clostridium difficile. Parenteral doses must be administered intravenously.

A 1 hour intravenous infusion of 1 g produces blood levels of 15–30 g/mL for 1–2 hours. The drug is widely distributed in the body. Cerebrospinal fluid levels 7–30% of simultaneous serum concentrations are achieved if there is meningeal inflammation. Ninety percent of the drug is excreted by glomerular filtration. In the presence of renal insufficiency, striking accumulation may occur (Table 43–2). In functionally anephric patients, the The main indication for parenteral vancomycin is sepsis or endocarditis caused by methicillinresistant staphylococci. However, vancomycin is not as effective as an antistaphylococcal penicillin for treatment of serious infections such as endocarditis caused by methicillin-susceptible strains.

Vancomycin in combination with gentamicin• Pharyngitis, tonsillitis

 is an alternative regimen for treatment of enterococcal endocarditis in a patient with serious penicillin allergy.

Vancomycin (in combination with cefotaxime, ceftriaxone, or rifampin) is also recommended for treatment of meningitis suspected or known to be caused by a highly penicillin-resistant strain of pneumococcus (ie, MIC > 1 g/mL). The recommended dosage is 30 mg/kg/d in two or three divided doses. A typical dosing regimen for most infections in adults with normal renal function is 1 g every 12 hours. The dosage in children is

40 mg/kg/d in three or four divided doses. Clearance of vancomycin is directly proportionate to creatinine clearance, and the dose is reduced accordingly in patients with renal insufficiency should have serum concentrations checked. Recommended peak serum concentrations are 20–50 g/mL, and trough concentrations are 5–15 g/mL.

Oral vancomycin, 0.125–0.25 g every 6 hours, is used to treat antibiotic-associated enterocolitis caused by Clostridium difficile. However, because of the emergence of vancomycin-resistant enterococci and the strong selective pressure of oral vancomycin for these resistant organisms, metronidazole is strongly preferred as initial therapy and vancomycin should be reserved for treatment of refractory cases.

Adverse Reactions

Adverse reactions are encountered in about 10% of cases. Most reactions are minor. Vancomycin is irritating to tissue, resulting in phlebitis at the site of injection. Chills and fever may occur. Ototoxicity is rare and nephrotoxicity uncommon with current preparations. However, administration with another ototoxic or nephrotoxic drug, such as an aminoglycoside, increases the risk of these toxicities. Ototoxicity can be minimized by maintaining peak serum concentrations below 60 g/mL. Among the more common reactions is the so-called "red man" or "red neck" syndrome. This infusion-related flushing is caused by release of histamine. It can be largely prevented by prolonging the infusion period to 1–2 hours or increasing the dosing interval.

Teicoplanin

Teicoplanin is a glycopeptide antibiotic that is very similar to vancomycin in mechanism of action and antibacterial spectrum. Unlike vancomycin, it can be given intramuscularly as well as intravenously. Teicoplanin has a long half-life (45–70 hours), permitting once-daily dosing. This drug is available in Europe but has not been approved for use in the United States.

Fosfomycin

Fosfomycin trometamol, a stable salt of fosfomycin (phosphonomycin), inhibits a very early stage of bacterial cell wall synthesis (Figure 43–5). An analog of phosphoenolpyruvate, it is structurally unrelated to any other antimicrobial agent. It inhibits the cytoplasmic enzyme enolpyruvate transferase by covalently binding to the cysteine residue of the active site and blocking the addition of phosphoenolpyruvate to UDP-N-acetylglucosamine. This reaction is the first step in the formation of UDP-N-acetylmuramic acid, the precursor of N-acetylmuramic acid, which is found only in bacterial cell walls. The drug is transported into the bacterial cell by glycerophosphate or glucose 6-phosphate transport systems. Resistance is due to inadequate transport of drug into the cell.

Fosfomycin is active against both gram-positive and gram-negative organisms at concentrations 125 g/mL. Susceptibility tests should be performed in growth medium supplemented with glucose 6-phosphate to minimize false-positive indications of resistance. In vitro synergism occurs when fosfomycin is combined with -lactam antibiotics, aminoglycosides, or fluoroquinolones.

Fosfomycin trometamol is available in both oral and parenteral formulations, though only the oral preparation is approved for use in the United States. Oral bioavailability is approximately 40%. Peak serum concentrations are 10 g/mL and 30 g/mL following a 2 g or 4 g oral dose, respectively. The half-life is approximately 4 hours. The active drug is excreted by the kidney, with urinary concentrations exceeding MICs for most urinary tract pathogens.

Fosfomycin is approved for use as a single 3 g dose for treatment of uncomplicated lower urinary tract infections in women.

The drug appears to be safe for use in pregnancy.

Bacitracin

Bacitracin is a cyclic peptide mixture first obtained from the Tracy strain of Bacillus subtilis in 1943. It is active against gram-positive microorganisms. Bacitracin inhibits cell wall formation by interfering with dephosphorylation in cycling of the lipid carrier that transfers peptidoglycan subunits to the growing cell wall (Figure 43–5). There is no cross-resistance between bacitracin and other antimicrobial drugs.

Bacitracin is markedly nephrotoxic if administered systemically, producing proteinuria, hematuria, and nitrogen retention. Hypersensitivity reactions (eg, skin rashes) are rare. Because of its marked toxicity when used systemically, it is limited to topical use. Bacitracin is poorly absorbed. Topical application results in local antibacterial activity without significant systemic toxicity. The small amounts of bacitracin that are absorbed are excreted by glomerular filtration.

Bacitracin, 500 units/g in an ointment base (often combined with polymyxin or neomycin), is useful for the suppression of mixed bacterial flora in surface lesions of the skin, in wounds, or on mucous membranes. Solutions of bacitracin containing 100–200 units/mL in saline can be employed for irrigation of joints, wounds, or the pleural cavity.

Cycloserine

Cycloserine is an antibiotic produced by Streptomyces orchidaceus. It is water-soluble and very unstable at acid pH. Cycloserine inhibits many gram-positive and gram-negative organisms, but it is used almost exclusively to treat tuberculosis caused by strains of M tuberculosis resistant to firstline agents. Cycloserine is a structural analog of D-alanine and inhibits the incorporation of Dalanine into peptidoglycan pentapeptide by inhibiting alanine racemase, which converts L-alanine to D-alanine, and D-alanyl-D-alanine ligase (Figure 43–5). After ingestion of 0.25 g of cycloserine blood levels reach 20–30 g/mL—sufficient to inhibit many strains of mycobacteria and gramnegative bacteria. The drug is widely distributed in tissues. Most of the drug is excreted in active form into the urine. The dosage for treating tuberculosis is 0.5 to 1 g/d in two or three divided doses.

Testing penicillin

Cycloserine causes serious dose-related central nervous system toxicity with headaches, tremors, acute psychosis, and convulsions. If oral dosages are maintained below 0.75 g/d, such effects can usually be avoided.

Chloramphenicol, Tetracyclines, Macrolides, Clindamycin, Streptogramins

Chloramphenicol

Crystalline chloramphenicol is a neutral, stable compound with the following structure:

It is soluble in alcohol but poorly soluble in water. Chloramphenicol succinate, which is used for parenteral administration, is highly water-soluble. It is hydrolyzed in vivo with liberation of free chloramphenicol.

Antimicrobial Activity

Chloramphenicol is a potent inhibitor of microbial protein synthesis. It binds reversibly to the 50S subunit of the bacterial ribosome (Figure 44–1). It inhibits the peptidyl transferase step of protein synthesis. Chloramphenicol is a bacteriostatic broad-spectrum antibiotic that is active against both aerobic and anaerobic gram-positive and gram-negative organisms. It is active also against rickettsiae but not chlamydiae. Most gram-positive bacteria are inhibited at concentrations of 1–10 g/mL, and many gram-negative bacteria are inhibited by concentrations of 0.2–5 g/mL.

Figure 44–1.

 

Haemophilus influenzae, Neisseria meningitidis, and some strains of bacteroides are highly susceptible, and for them chloramphenicol may be bactericidal.

Low-level resistance may emerge from large populations of chloramphenicol-susceptible cells by selection of mutants that are less permeable to the drug. Clinically significant resistance is due to production of chloramphenicol acetyltransferase, a plasmid-encoded enzyme that inactivates the drug.

Pharmacokinetics

The usual dosage of chloramphenicol is 50–100 mg/kg/d. After oral administration, crystalline chloramphenicol is rapidly and completely absorbed. A 1 g oral dose produces blood levels between 10 and 15 g/mL. Chloramphenicol palmitate is a prodrug that is hydrolyzed in the intestine to yield free chloramphenicol. The parenteral formulation, chloramphenicol succinate, yields free chloramphenicol by hydrolysis, giving blood levels somewhat lower than those achieved with orally administered drug. After absorption, chloramphenicol is widely distributed to virtually all tissues and body fluids, including the central nervous system and cerebrospinal fluid such that the concentration of chloramphenicol in brain tissue may be equal to that in serum. The drug penetrates

cell membranes readily. Most of the drug is inactivated either by conjugation with glucuronic acid (principally in the liver) or by reduction to inactive aryl amines.

Excretion of active

chloramphenicol (about 10% of the total dose administered) and of inactive degradation products (about 90% of the total) occurs by way of the urine. A small amount of active drug is excreted into bile or feces. The systemic dosage of chloramphenicol need not be altered in renal insufficiency, but it must be reduced markedly in hepatic failure. Newborns less than a week old and premature infants also clear chloramphenicol less well, and the dosage should be reduced to 25 mg/kg/d.

Clinical Uses

Because of potential toxicity, bacterial resistance, and the availability of other effective drugs (eg, cephalosporins), chloramphenicol is all but obsolete as a systemic drug. It may be considered for treatment of serious rickettsial infections, such as typhus or Rocky Mountain spotted fever, in children for whom tetracyclines are contraindicated, ie, those under 8 years of age.

It is an alternative to a -lactam antibiotic for treatment of meningococcal meningitis occurring in patients who have major hypersensitivity reactions to penicillin or bacterial meningitis caused by penicillinresistant strains of pneumococci. The dosage is 50–100 mg/kg/d in four divided doses. Chloramphenicol is occasionally used topically in the treatment of eye infections because of its wide antibacterial spectrum and its penetration of ocular tissues and the aqueous humor. It is ineffective for chlamydial infections.

Adverse Reactions

Gastrointestinal Disturbances

Adults occasionally develop nausea, vomiting, and diarrhea. This is rare in children. Oral or vaginal candidiasis may occur as a result of alteration of normal microbial flora. Bone Marrow Disturbances

Chloramphenicol commonly causes a dose-related reversible suppression of red cell production at dosages exceeding 50 mg/kg/d after 1–2 weeks. Aplastic anemia is a rare consequence of chloramphenicol administration by any route. It is an idiosyncratic reaction unrelated to dose, though it occurs more frequently with prolonged use. It tends to be irreversible and can be fatal. Aplastic anemia probably develops in one of every 24,000–40,000 patients who have taken chloramphenicol.

Toxicity for Newborn Infants

Newborn infants lack an effective glucuronic acid conjugation mechanism for the degradation and detoxification of chloramphenicol. Consequently, when infants are given dosages above 50 mg/kg/d, the drug may accumulate, resulting in the gray baby syndrome, with vomiting, flaccidity, hypothermia, gray color, shock, and collapse. To avoid this toxic effect, chloramphenicol should be used with caution in infants and the dosage limited to 50 mg/kg/d or less (during the first week of life) in full-term infants and 25 mg/kg/d in premature infants.

Interaction with Other Drugs

Chloramphenicol inhibits hepatic microsomal enzymes that metabolize several drugs. Half-lives are prolonged, and the serum concentrations of phenytoin, tolbutamide, chlorpropamide, and warfarin are increased. Like other bacteriostatic inhibitors of microbial protein synthesis, chloramphenicol can antagonize bactericidal drugs such as penicillins or aminoglycosides.

Tetracyclines

All of the tetracyclines have the basic structure shown below:

Free tetracyclines are crystalline amphoteric substances of low solubility. They are available as hydrochlorides, which are more soluble. Such solutions are acid and, with the exception of chlortetracycline, fairly stable. Tetracyclines chelate divalent metal ions, which can interfere with their absorption and activity.

Antimicrobial Activity

Tetracyclines are broad-spectrum bacteriostatic antibiotics that inhibit protein synthesis. They are active against many gram-positive and gram-negative bacteria, including anaerobes, rickettsiae, chlamydiae, mycoplasmas, and L forms; and against some protozoa, eg, amebas. The antibacterial activities of most tetracyclines are similar except that tetracycline-resistant strains may remain susceptible to doxycycline or minocycline, drugs that are less rapidly transported by the pump that is responsible for resistance (see Resistance). Differences in clinical efficacy are minor and attributable largely to features of absorption, distribution, and excretion of individual drugs.

Tetracyclines enter microorganisms in part by passive diffusion and in part by an energy-dependent process of active transport. Susceptible cells concentrate the drug intracellularly. Once inside the cell, tetracyclines bind reversibly to the 30S subunit of the bacterial ribosome, blocking the binding of aminoacyl-tRNA to the acceptor site on the mRNA-ribosome complex (Figure 44–1). This prevents addition of amino acids to the growing peptide.

Resistance

Three mechanisms of resistance to tetracycline have been described: (1) decreased intracellular accumulation due to either impaired influx or increased efflux by an active transport protein pump; (2) ribosome protection due to production of proteins that interfere with tetracycline binding to the ribosome; and (3) enzymatic inactivation of tetracyclines. The most important of these is production of an efflux pump. The pump protein is encoded on a plasmid and may be transmitted by transduction or by conjugation. Because these plasmids commonly encode resistance genes for other drugs, eg, aminoglycosides, sulfonamides, and chloramphenicol, tetracycline resistance is a marker for resistance to multiple drugs.

Pharmacokinetics

Tetracyclines mainly differ in their absorption after oral administration and their elimination. Absorption after oral administration is approximately 30% for chlortetracycline; 60–70% for tetracycline, oxytetracycline, demeclocycline, and methacycline; and 95–100% for doxycycline and minocycline. A portion of an orally administered dose of tetracycline remains in the gut lumen, modifies intestinal flora, and is excreted in the feces. Absorption occurs mainly in the upper small intestine and is impaired by food (except doxycycline and minocycline); by divalent cations (Ca2+, Mg2+, Fe2+) or Al3+; by dairy products and antacids, which contain multivalent cations; and by alkaline pH. Specially buffered tetracycline solutions are formulated for intravenous administration.

Tetracyclines are 40–80% bound by serum proteins. Oral dosages of 500 mg every 6 hours of tetracycline hydrochloride or oxytetracycline produce peak blood levels of 4–6 g/mL. Peak levels of 2–4 g/mL are achieved with a 200 mg dose of doxycycline or minocycline. Intravenously injected tetracyclines give somewhat higher levels only temporarily. Tetracyclines are distributed widely to tissues and body fluids except for cerebrospinal fluid, where concentrations are 10–25%

of those in serum. Minocycline reaches very high concentrations in tears and saliva, which makes it useful for eradication of the meningococcal carrier state.

Tetracyclines cross the placenta to reach the fetus and are also excreted in milk. As a result of chelation with calcium, tetracyclines are bound to—and damage—growing bones and teeth.

Carbamazepine, phenytoin, barbiturates, and chronic alcohol ingestion may shorten the half-life of doxycycline 50% by induction of hepatic enzymes that metabolize the drug.

Tetracyclines are excreted mainly in bile and urine. Concentrations in bile exceed those in serum tenfold. Some of the drug excreted in bile is reabsorbed from the intestine (enterohepatic circulation) and contributes to maintenance of serum levels. Ten to 50 percent of various tetracyclines is excreted into the urine, mainly by glomerular filtration. Ten to 40 percent of the drug in the body is excreted in feces. Doxycycline, in contrast to other tetracyclines, is eliminated by nonrenal mechanisms, does not accumulate significantly in renal failure, and requires no dosage adjustment, making it the tetracycline of choice for use in the setting of renal insufficiency.

Tetracyclines are classified as short-acting (chlortetracycline, tetracycline, oxytetracycline), intermediate-acting (demeclocycline and methacycline), or long-acting (doxycycline and minocycline) based on serum half-lives of 6–8 hours, 12 hours, and 16–18 hours, respectively. The almost complete absorption and slow excretion of doxycycline and minocycline allow for oncedaily dosing.

Clinical Uses

A tetracycline is the drug of choice in infections with Mycoplasma pneumoniae, chlamydiae, rickettsiae, and some spirochetes. They are used in combination regimens to treat gastric and duodenal ulcer disease caused by Helicobacter pylori. They may be employed in various grampositive and gram-negative bacterial infections, including vibrio infections, provided the organism is not resistant. In cholera, tetracyclines rapidly stop the shedding of vibrios, but tetracycline resistance has appeared during epidemics. Tetracyclines remain effective in most chlamydial infections, including sexually transmitted diseases. Tetracyclines are no longer recommended for treatment of gonococcal disease because of resistance. A tetracycline—usually in combination with an aminoglycoside—is indicated for plague, tularemia, and brucellosis. Tetracyclines are sometimes employed in the treatment of protozoal infections, eg, those due to Entamoeba histolytica or

Plasmodium falciparum. Other uses include treatment of acne, exacerbations of bronchitis, community-acquired pneumonia, Lyme disease, relapsing fever, leptospirosis, and some nontuberculous mycobacterial infections (eg, Mycobacterium marinum). Tetracyclines formerly were used for a variety of common infections, including bacterial gastroenteritis, pneumonia (other than mycoplasmal or chlamydial pneumonia), and urinary tract infections. However, many strains of bacteria causing these infections now are resistant, and other agents have largely supplanted tetracyclines.

Minocycline, 200 mg orally daily for 5 days, can eradicate the meningococcal carrier state, but because of side-effects and resistance of many meningococcal strains, rifampin is preferred.

Demeclocycline inhibits the action of ADH in the renal tubule and has been used in the treatment of inappropriate secretion of ADH or similar peptides by certain tumors.

Oral Dosage

The oral dosage for rapidly excreted tetracyclines, equivalent to tetracycline hydrochloride, is 0.25– 0.5 g four times daily for adults and 20–40 mg/kg/d for children (8 years of age and older). For severe systemic infections, the higher dosage is indicated, at least for the first few days. The daily dose is 600 mg for demeclocycline or methacycline, 100 mg once or twice a day for doxycycline, and 100 mg twice a day for minocycline. Doxycycline is the tetracycline of choice because it can be given as a once-daily dose and its absorption is not significantly affected by food. All tetracyclines chelate with metals, and none should be administered with milk, antacids, or ferrous sulfate. To avoid deposition in growing bones or teeth, tetracyclines should be avoided for pregnant women and for children under 8 years of age.

Parenteral Dosage

Several tetracyclines are available for intravenous injection in doses of 0.1–0.5 g every 6–12 hours (similar to oral doses), depending on the agent. Intramuscular injection is not recommended because of pain and inflammation at the injection site. Doxycycline is the preferred agent, at a dosage of 100 mg every 12–24 hours.

Adverse Reactions

Hypersensitivity reactions (drug fever, skin rashes) to tetracyclines are uncommon. Most adverse

effects are due to direct toxicity of the drug or to alteration of microbial flora. Gastrointestinal Adverse Effects Nausea, vomiting, and diarrhea are the most common reasons for discontinuing tetracycline medication. These effects are attributable to direct local irritation of the intestinal tract. Nausea, anorexia, and diarrhea can usually be controlled by administering the drug with food or carboxymethylcellulose, reducing drug dosage, or discontinuing the drug.

Tetracyclines modify the normal flora, with suppression of susceptible coliform organisms and overgrowth of pseudomonas, proteus, staphylococci, resistant coliforms, clostridia, and candida. This can result in intestinal functional disturbances, anal pruritus, vaginal or oral candidiasis, or enterocolitis with shock and death. Pseudomembranous enterocolitis associated with Clostridium difficile should be treated with metronidazole.

Bony Structures and Teeth

Tetracyclines are readily bound to calcium deposited in newly formed bone or teeth in young children. When the drug is given during pregnancy, it can be deposited in the fetal teeth, leading to fluorescence, discoloration, and enamel dysplasia; it can also be deposited in bone, where it may cause deformity or growth inhibition. If the drug is given for long periods to children under 8 years of age, similar changes can result.

Liver Toxicity

Tetracyclines can probably impair hepatic function, especially during pregnancy, in patients with preexisting hepatic insufficiency and when high doses are given intravenously. Hepatic necrosis has been reported with daily doses of 4 g or more intravenously.

Kidney Toxicity

Renal tubular acidosis and other renal injury resulting in nitrogen retention have been attributed to the administration of outdated tetracycline preparations. Tetracyclines given along with diuretics may produce nitrogen retention. Tetracyclines other than doxycycline may accumulate to toxic levels in patients with impaired kidney function.

Local Tissue Toxicity

Intravenous injection can lead to venous thrombosis. Intramuscular injection produces painful local irritation and should be avoided.

Photosensitization

Systemic tetracycline administration, especially of demeclocycline, can induce sensitivity to sunlight or ultraviolet light, particularly in fair-skinned persons.

Vestibular Reactions

Dizziness, vertigo, nausea, and vomiting have been particularly noted with doxycycline at doses above 100 mg. With dosages of 200–400 mg/d of minocycline, 35–70% of patients will have these reactions.

Medical & Social Implications of Overuse

Tetracyclines have been extensively used in animal feeds to enhance growth. This practice has contributed to the spread of tetracycline resistance among enteric bacteria and of plasmids that encode tetracycline resistance genes.

 

Chloramphenicol, Tetracyclines, Macrolides, Clindamycin, Streptogramins

Macrolides

The macrolides are a group of closely related compounds characterized by a macrocyclic lactone ring (usually containing 14 or 16 atoms) to which deoxy sugars are attached. The prototype drug, erythromycin, which consists of two sugar moieties attached to a 14-atom lactone ring, was obtained in 1952 from Streptomyces erythreus. Clarithromycin and azithromycin are semisynthetic derivatives of erythromycin.

Erythromycin

Chemistry

The general structure of erythromycin is shown above with the macrolide ring and the sugars desosamine and cladinose.

It is poorly soluble in water (0.1%) but dissolves readily in organic solvents. Solutions are fairly stable at 4 °C but lose activity rapidly at 20 °C and at acid pH. Erythromycins are usually dispensed as various esters and salts.

Antimicrobial Activity

Erythromycin is effective against gram-positive organisms, especially pneumococci, streptococci, staphylococci, and corynebacteria, in plasma concentrations of 0.02–2 g/mL. Mycoplasma, legionella, Chlamydia trachomatis, C psittaci, C pneumoniae, helicobacter, listeria, and certain mycobacteria (Mycobacterium kansasii, Mycobacterium scrofulaceum) are also susceptible. Gramnegative organisms such as neisseria species, Bordetella pertussis, Bartonella henselae, and B quintana (etiologic agents of cat-scratch disease and bacillary angiomatosis), some rickettsia species, Treponema pallidum, and campylobacter species are susceptible. Haemophilus influenzae is somewhat less susceptible.

The antibacterial action of erythromycin may be inhibitory or bactericidal, particularly at higher concentrations, for susceptible organisms. Activity is enhanced at alkaline pH. Inhibition of protein synthesis occurs via binding to the 50S ribosomal RNA. Protein synthesis is inhibited because aminoacyl translocation reactions and the formation of initiation complexes are blocked (Figure 44–1).

Resistance

Resistance to erythromycin is usually plasmid-encoded. Three mechanisms have been identified: (1) reduced permeability of the cell membrane or active efflux; (2) production (by Enterobacteriaceae) of esterases that hydrolyze macrolides; and (3) modification of the ribosomal binding site (so-called ribosomal protection) by chromosomal mutation or by a macrolide-inducible or constitutive

methylase. Efflux and methylase production account for the vast majority of cases of resistance in gram-positive organisms. Cross-resistance is complete between erythromycin and the other macrolides. Constitutive methylase production also confers resistance to structurally unrelated but mechanistically similar compounds such as clindamycin and streptogramin B (so-called macrolidelincosamide

streptogramin, or MLS-type B, resistance), which share the same ribosomal binding site. Because nonmacrolides are poor inducers of the methylase, strains expressing an inducible methylase will appear susceptible in vitro. However, constitutive mutants that are resistant can be selected out and emerge during therapy with clindamycin.

Pharmacokinetics

Erythromycin base is destroyed by stomach acid and must be administered with enteric coating. Food interferes with absorption. Stearates and esters are fairly acid-resistant and somewhat better absorbed. The lauryl salt of the propionyl ester of erythromycin (erythromycin estolate) is the bestabsorbed oral preparation. Oral dosage of 2 g/d results in serum erythromycin base and ester concentrations of approximately 2 g/mL. However, only the base is microbiologically active, and

its concentration tends to be similar regardless of the formulation. A 500 mg intravenous dose of erythromycin lactobionate produces serum concentrations of 10 g/mL 1 hour after dosing. The serum half-life is approximately 1.5 h normally and 5 hours in patients with anuria. Adjustment for renal failure is not necessary. Erythromycin is not removed by dialysis. Large amounts of an administered dose are excreted in the bile and lost in feces, and only 5% is excreted in the urine. Absorbed drug is distributed widely except to the brain and cerebrospinal fluid. Erythromycin is taken up by polymorphonuclear leukocytes and macrophages. It traverses the placenta and reaches the fetus.

Clinical Uses

An erythromycin is the drug of choice in corynebacterial infections (diphtheria, corynebacterial sepsis, erythrasma); in respiratory, neonatal, ocular, or genital chlamydial infections; and in treatment of community-acquired pneumonia because its spectrum of activity includes the pneumococcus, mycoplasma, and legionella. Erythromycin is also useful as a penicillin substitute in penicillin-allergic individuals with infections caused by staphylococci (assuming that the isolate is susceptible), streptococci, or pneumococci. Emergence of erythromycin resistance in strains of group A streptococci and pneumococci (penicillin-resistant pneumococci in particular) has made macrolides less attractive as first-line agents for treatment of pharyngitis, skin and soft tissue infections, and pneumonia. Erythromycin has been recommended as prophylaxis against endocarditis during dental procedures in individuals with valvular heart disease, though clindamycin, which is better tolerated, has largely replaced it. Although erythromycin estolate is the best-absorbed salt, it imposes the greatest risk of adverse reactions. Therefore, the stearate or succinate salt may be preferred.

The oral dosage of erythromycin base, stearate, or estolate is 0.25–0.5 g every 6 hours (for children, 40 mg/kg/d). The dosage of erythromycin ethylsuccinate is 0.4–0.6 g every 6 hours. Oral erythromycin base (1 g) is sometimes combined with oral neomycin or kanamycin for preoperative preparation of the colon. The intravenous dosage of erythromycin gluceptate or lactobionate is 0.5– 1.0 g every 6 hours for adults and 20–40 mg/kg/d for children. The higher dosage is recommended when treating pneumonia caused by legionella species.

Adverse Reactions

Gastrointestinal Effects

Anorexia, nausea, vomiting, and diarrhea occasionally accompany oral administration. Gastrointestinal intolerance, which is due to a direct stimulation of gut motility, is the most frequent reason for discontinuing erythromycin and substituting another antibiotic.

Liver Toxicity

Erythromycins, particularly the estolate, can produce acute cholestatic hepatitis (fever, jaundice, impaired liver function), probably as a hypersensitivity reaction. Most patients recover from this, but hepatitis recurs if the drug is readministered. Other allergic reactions include fever, eosinophilia, and rashes.

Drug Interactions

Erythromycin metabolites can inhibit cytochrome P450 enzymes and thus increase the serum concentrations of numerous drugs, including theophylline, oral anticoagulants, cyclosporine, and methylprednisolone. Erythromycin increases serum concentrations of oral digoxin by increasing its bioavailability.

Clarithromycin

Clarithromycin is derived from erythromycin by addition of a methyl group and has improved acid stability and oral absorption compared with erythromycin. Its mechanism of action is the same as that of erythromycin. Clarithromycin and erythromycin are virtually identical with respect to antibacterial activity except that clarithromycin is more active against Mycobacterium avium Complex leprae and Toxoplasma gondii. Erythromycin-resistant streptococci and staphylococci are also

resistant to clarithromycin. A 500 mg dose produces serum concentrations of 2–3 g/mL. The longer half-life of clarithromycin (6 hours) compared with erythromycin permits twice-daily dosing. The recommended dosage is

250–500 mg twice daily. Clarithromycin penetrates most tissues well, with concentrations equal to or exceeding serum concentrations.

Clarithromycin is metabolized in the liver. The major metabolite is 14-hydroxyclarithromycin, which also has antibacterial activity. A portion of active drug and this major metabolite is eliminated in the urine, and dosage reduction (eg, a 500 mg loading dose, then 250 mg once or twice daily) is recommended for patients with creatinine clearances less than 30 mL/min.

Clarithromycin has drug interactions similar to those described for erythromycin. The advantages of clarithromycin compared with erythromycin are lower frequency of gastrointestinal intolerance and less frequent dosing. Except for the specific organisms noted above, the two drugs are otherwise therapeutically very similar, and the choice of one over the other usually turns on issues of cost (clarithromycin being much more expensive) and tolerability.

Azithromycin

Azithromycin, a 15-atom lactone macrolide ring compound, is derived from erythromycin by addition of a methylated nitrogen into the lactone ring of erythromycin. Its spectrum of activity and clinical uses are virtually identical to those of clarithromycin. Azithromycin is active against M avium complex and T gondii. Azithromycin is slightly less active than erythromycin and clarithromycin against staphylococci and streptococci and slightly more active against H influenzae. Azithromycin is highly active against chlamydia. Azithromycin differs from erythromycin and clarithromycin mainly in pharmacokinetic properties.

A 500 mg dose of azithromycin produces relatively low serum concentrations of approximately 0.4 g/mL. However, azithromycin penetrates into most tissues (except cerebrospinal fluid) and phagocytic cells extremely well, with tissue concentrations exceeding serum concentrations by 10- to 100-fold. The drug is slowly released from tissues (tissue half-life of 2–4 days) to produce an elimination half-life approaching 3 days. These unique properties permit once-daily dosing and

shortening of the duration of treatment in many cases. For example, a single 1 g dose of

azithromycin is as effective as a 7-day course of doxycycline for chlamydial cervicitis and urethritis. Community-acquired pneumonia can be treated with azithromycin given as a 500 mg loading dose, followed by a 250 mg single daily dose for the next 4 days.

Azithromycin is rapidly absorbed and well tolerated orally. It should be administered 1 hour before or 2 hours after meals. Aluminum and magnesium antacids do not alter bioavailability but delay absorption and reduce peak serum concentrations. Because it has a 15-member (not 14-member) lactone ring, azithromycin does not inactivate cytochrome P450 enzymes and therefore is free of the drug interactions that occur with erythromycin and clarithromycin.

Clindamycin penetrates well into most tissues, with brain and cerebrospinal fluid being important exceptions. It penetrates well into abscesses and is actively taken up and concentrated by phagocytic cells. Clindamycin is metabolized by the liver, and both active drug and active metabolites are excreted in bile. The half-life is about 2.5 hours in normal individuals, increasing to 6 hours in patients with anuria. No dosage adjustment is required for renal failure.

Clinical Uses

Clindamycin is indicated for treatment of severe anaerobic infection caused by bacteroides and other anaerobes that often participate in mixed infections. Clindamycin, sometimes in combination with an aminoglycoside or cephalosporin, is used to treat penetrating wounds of the abdomen and the gut; infections originating in the female genital tract, eg, septic abortion and pelvic abscesses; or

aspiration pneumonia. Clindamycin is now recommended instead of erythromycin for prophylaxis of endocarditis in patients with valvular heart disease who are undergoing certain dental procedures.

Clindamycin plus primaquine is an effective alternative to trimethoprim-sulfamethoxazole for moderate to moderately severe Pneumocystis carinii pneumonia in AIDS patients. It is also used in combination with pyrimethamine for AIDS-related toxoplasmosis of the brain.

Adverse Effects

Common adverse effects are diarrhea, nausea, and skin rashes. Impaired liver function (with or without jaundice) and neutropenia sometimes occur. Severe diarrhea and enterocolitis have followed clindamycin administration. Antibiotic-associated colitis that has followed administration of clindamycin and other drugs is caused by toxigenic C difficile. This potentially fatal complication must be recognized promptly and treated with metronidazole, 500 mg orally or intravenously three times a day (the preferred therapy), or vancomycin, 125 mg orally four times a day (less desirable given the increasing prevalence of vancomycin-resistant enterococci). Relapse may occur. Variations in the local prevalence of C difficile may account for the great differences in incidence of antibiotic-associated colitis. For unknown reasons, neonates given clindamycin may become colonized with toxigenic C difficile but do not develop colitis.

 

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