Chapter 10: Antibacterial Drugs: Mechanism of Action

The discovery of antimicrobials is one of the great advances in medicine, and their use has substantially reduced morbidity and mortality worldwide. Unfortunately, with widespread antibiotic use, we have witnessed the emergence of multidrug-resistant pathogens and reduced efficacy of many of our most powerful antimicrobials. In addition, we have also recognized many adverse effects of antimicrobials, most notably the rising rates of Clostridium difficile colitis. Further, the cost of medical care is greatly increased due to overuse of antibiotics and treating infections caused by resistant organisms. It is critical that health professionals understand the key concepts behind antimicrobial stewardship at the time they learn about microbial pathogens and antimicrobials.

The worldwide problem of antibiotic resistance makes the need for antimicrobial stewardship evident. The Centers for Disease Control and Prevention estimates that in the United States over 2 million infections with multidrug-resistant pathogens occur each year, leading to more than 20,000 deaths. These pathogens include methicillin-resistant Staphylococcus aureus and extended-spectrum β-lactamase–producing gram-negative rods (e.g., Escherichia coli and Klebsiella pneumoniae). Hospital-associated infections, many of which are caused by antibiotic-resistant bacteria, are estimated to cost billions of dollars each year.

The basic principles of good stewardship are threefold: (1) reduce inappropriate use of antibiotics, (2) encourage targeted treatment with narrow-spectrum drugs, and (3) limit adverse effects (Table 10–1). Inappropriate antibiotic use can occur for many reasons, including the providers’ desire to adhere to patient’s wishes, even when it is not medically appropriate. The most common example of inappropriate antibiotic use is the prescribing of antibiotics for a viral respiratory tract infection. It is estimated that half of all prescriptions given for upper respiratory infections (pharyngitis, sinusitis) are inappropriate.

The concept of targeted treatment refers to making a microbiologic diagnosis promptly and using the most specific antibiotic that has the best safety profile for the patient. If multiple broad-spectrum antibiotics are used as empiric therapy early in infection, then switching to a narrow-spectrum antibiotic as soon as possible should be done. Cultures should be sent prior to starting antibiotics so that the drugs will not reduce the likelihood of isolating the causative organism. Further, switching from intravenous antibiotics to oral dosing reduces the risk of catheter-associated infections. The net result of targeted treatment is to reduce the ability of antibiotics to select for resistant mutants present in the bacterial population.

Limiting the occurrence of adverse effects caused by antibiotics is another major goal of antimicrobial stewardship. Minimizing the duration of antibiotic use to only as long as clinically indicated is a key intervention since duration of exposure correlates closely with the risk of many types of adverse effects. Patients with reduced renal function should have the dose of some antibiotics adjusted based on their estimated glomerular filtration rate.

Antibiotic allergies should be identified and explored in detail. Although reaction to antibiotics may be commonly reported, they may not always be of significance such as some alleged hypersensitivity reactions. If optimal treatment requires a drug such as penicillin to which the patient says he or she is allergic, then skin testing can be employed to determine the accuracy of that claim. In addition, patients should be warned regarding the possibility that certain drugs may cause adverse effects. For example, certain photosensitizing antibiotics may cause a rash when the patient is exposed to sunlight.

The reasons that inappropriate use of antibiotics occurs are varied. Probably, the most important is the lack of knowledge or awareness of the physician. Risk avoidance on the part of the physician is also common. Inadequate microbiologic information plays a role as well. Patient expectation and direct demands for antibiotics contribute to the problem.

In summary, antimicrobial stewardship refers to the effort to improve the treatment of infectious diseases by the appropriate use of antibiotics. This is critical in this era of rising rates of multidrug-resistant pathogens. Targeted therapy with the most appropriate single antibiotic will hopefully improve clinical outcomes and reduce the cost of care.

The most important concept underlying antimicrobial therapy is selective toxicity (i.e., selective inhibition of the growth of the microorganism without damage to the host). Selective toxicity is achieved by exploiting the differences between the metabolism and structure of the microorganism and the corresponding features of human cells. For example, penicillins and cephalosporins are effective antibacterial agents because they prevent the synthesis of peptidoglycan, thereby inhibiting the growth of bacterial but not human cells.

There are four major sites in the bacterial cell that are sufficiently different from the human cell that they serve as the basis for the action of clinically effective drugs: cell wall, ribosomes, nucleic acids, and cell membrane (Table 10–2 and Figure 10–1).

There are far more antibacterial drugs than antiviral drugs. This is a consequence of the difficulty of designing a drug that will selectively inhibit viral replication. Because viruses use many of the normal cellular functions of the host in their growth, it is not easy to develop a drug that specifically inhibits viral functions and does not damage the host cell.

Broad-spectrum antibiotics are active against several types of microorganisms (e.g., tetracyclines are active against many gram-negative rods, chlamydiae, mycoplasmas, and rickettsiae). Narrow-spectrum antibiotics are active against one or very few types (e.g., vancomycin is primarily used against certain gram-positive cocci, namely, staphylococci and enterococci).

In some clinical situations, it is essential to use a bactericidal drug rather than a bacteriostatic one. A bactericidal drug kills bacteria, whereas a bacteriostatic drug inhibits their growth but does not kill them (Figure 10–2). The salient features of the behavior of bacteriostatic drugs are that (1) the bacteria can grow again when the drug is withdrawn, and (2) host defense mechanisms, such as phagocytosis, are required to kill the bacteria. Bactericidal drugs are particularly useful in certain infections (e.g., those that are immediately life-threatening; those in patients whose polymorphonuclear leukocyte count is below 500/μL; and endocarditis, in which phagocytosis is limited by the fibrinous network of the vegetations and bacteriostatic drugs do not effect a cure).

INHIBITION OF CELL WALL SYNTHESIS

1. Inhibition of Bacterial Cell Wall Synthesis

Penicillins

Penicillins (and cephalosporins) act by inhibiting transpeptidases, the enzymes that catalyze the final cross-linking step in the synthesis of peptidoglycan (see Figure 2–5). For example, in S. aureus, transpeptidation occurs between the amino group on the end of the pentaglycine cross-link and the terminal carboxyl group of the D-alanine on the tetrapeptide side chain. Because the stereochemistry of penicillin is similar to that of a dipeptide, D-alanyl-D-alanine, penicillin can bind to the active site of the transpeptidase and inhibit its activity.

Two additional factors are involved in the action of penicillin:

1. The first is that penicillin binds to a variety of proteins in the bacterial cell membrane and cell wall, called penicillin-binding proteins (PBPs). Some PBPs are transpeptidases; the others function in the synthesis of peptidoglycan. Changes in PBPs are in part responsible for an organism’s becoming resistant to penicillin.

2. The second factor is that autolytic enzymes called murein hydrolases (murein is a synonym for peptidoglycan) are activated in penicillin-treated cells and degrade the peptidoglycan. Some bacteria (e.g., strains of S. aureus) are tolerant to the action of penicillin, because these autolytic enzymes are not activated. A tolerant organism is one that is inhibited but not killed by a drug that is usually bactericidal, such as penicillin. Penicillin-treated cells die by rupture as a result of the influx of water into the high-osmotic-pressure interior of the bacterial cell.

Penicillin is bactericidal, but it kills cells only when they are growing. When cells are growing, new peptidoglycan is being synthesized, and transpeptidation occurs. However, in nongrowing cells, no new cross-linkages are required, and penicillin is inactive. Penicillins are therefore more active during the log phase of bacterial cell growth than during the stationary phase (see Chapter 3 for the bacterial cell growth cycle).

Penicillins (and cephalosporins) are called β-lactam drugs because of the importance of the β-lactam ring (Figure 10–3). An intact ring structure is essential for antibacterial activity; cleavage of the ring by penicillinases (β-lactamases) inactivates the drug. The most important naturally occurring compound is benzylpenicillin (penicillin G), which is composed of the 6-aminopenicillanic acid nucleus that all penicillins have, plus a benzyl side chain (see Figure 10–3). Penicillin G is available in three main forms:

1. Aqueous penicillin G, which is metabolized most rapidly.

2. Procaine penicillin G, in which penicillin G is conjugated to procaine. This form is metabolized more slowly and is less painful when injected intramuscularly because the procaine acts as an anesthetic.

3. Benzathine penicillin G, in which penicillin G is conjugated to benzathine. This form is metabolized very slowly and is often called a “depot” preparation.

Benzylpenicillin is one of the most widely used and effective antibiotics. However, it has four disadvantages, three of which have been successfully overcome by chemical modification of the side chain. The three disadvantages are (1) limited effectiveness against many gram-negative rods, (2) hydrolysis by gastric acids, so that it cannot be taken orally, and (3) inactivation by β-lactamases. The limited effectiveness of penicillin G against gram-negative rods is due to the inability of the drug to penetrate the outer membrane of the organism. The fourth disadvantage common to all penicillins that has not been overcome is hypersensitivity, especially anaphylaxis, in some recipients of the drug.

The effectiveness of penicillins against gram-negative rods has been increased by a series of chemical changes in the side chain (Table 10–3). It can be seen that ampicillin and amoxicillin have activity against several gram-negative rods that the earlier penicillins do not have. However, these drugs are not useful against Pseudomonas aeruginosa and K. pneumoniae. Hence, other penicillins were introduced. Generally speaking, as the activity against gram-negative bacteria increases, the activity against gram-positive bacteria decreases.

The second important disadvantageacid hydrolysis in the stomachalso has been addressed by modification of the side chain. The site of acid hydrolysis is the amide bond between the side chain and penicillanic acid nucleus (see Figure 10–3). Minor modifications of the side chain in that region, such as addition of an oxygen (to produce penicillin V) or an amino group (to produce ampicillin), prevent hydrolysis and allow the drug to be taken orally.

The inactivation of penicillin G by β-lactamases is another important disadvantage, especially in the treatment of S. aureus infections. Access of the enzyme to the β-lactam ring is blocked by modification of the side chain with the addition of large aromatic rings containing bulky methyl or ethyl groups (methicillin, oxacillin, nafcillin, etc.; see Figure 10–3). Another defense against β-lactamases is inhibitors such as clavulanic acid, tazobactam, sulbactam, and avibactam. These are structural analogues of penicillin that have little antibacterial activity but bind strongly to β-lactamases and thus protect the penicillin. Combinations, such as amoxicillin and clavulanic acid (Augmentin) and piperacillin plus tazobactam (Zosyn), are in clinical use. Some bacteria resistant to these combinations have been isolated from patient specimens.

Penicillins are usually nontoxic at clinically effective levels. The major disadvantage of these compounds is hypersensitivity, with a reported prevalence of 1% to 10% of patients. The most serious of the hypersensitivity reactions is IgE–mediated, which can give rise to bronchospasm, urticarial rash, and anaphylactic shock (see Chapter 65). Fortunately, this occurs in only 0.5% of patients. Death as a result of anaphylaxis occurs in 0.002% of patients (1 in 50,000 patients). IgG and cell-mediated hypersensitivity reactions are more common and can include nonurticarial skin rashes, hemolytic anemia, nephritis, and drug fever. A maculopapular drug-induced rash is quite common. While these manifestations are not considered to be “true” allergy and are not life threatening, they are adverse reactions, and an alternative antibiotic should be considered for patients with a history of these symptoms.

To determine whether the patient’s allergy is clinically significant, a skin test using penicilloyl-polylysine as the test reagent can be performed. A wheal and flare reaction occurs at the site of injection in allergic individuals. If the patient’s disease requires treatment with penicillin, the patient can be desensitized under the supervision of a trained allergist.

Cephalosporins

Cephalosporins are β-lactam drugs that, like penicillins, also inhibit the cross-linking of peptidoglycan. The structures, however, are different: Cephalosporins have a six-membered ring adjacent to the β-lactam ring and are substituted in two places on the 7-aminocephalosporanic acid nucleus (Figure 10–4), whereas penicillins have a five-membered ring and are substituted in only one place.

The first-generation cephalosporins are active primarily against gram-positive cocci (Table 10–4). Similar to the penicillins, new cephalosporins were synthesized with expansion of activity against gram-negative rods as the goal. These new cephalosporins have been categorized into second, third, and fourth generations, with each generation having expanded coverage against certain gram-negative rods. The fourth- and fifth-generation cephalosporins have activity against many gram-positive cocci as well.

Cephalosporins are effective against a broad range of organisms, are generally well tolerated, and produce fewer hypersensitivity reactions than do the penicillins. Despite the structural similarity, a patient allergic to penicillin has only about a 10% chance of being hypersensitive to cephalosporins also. Most cephalosporins are the products of molds of the genus Cephalosporium; a few, such as cefoxitin, are made by the actinomycete Streptomyces.

The inactivation of cephalosporins by β-lactamases (cephalosporinases) is an important clinical problem. β-Lactamase inhibitors such as tazobactam and avibactam are combined with certain cephalosporins to prevent inactivation of the cephalosporin. For example, the US Food and Drug Administration (FDA) has approved the combination of ceftazidime/avibactam (Avycaz) and ceftolozane/tazobactam (Zerbaxa) for the treatment of intra-abdominal infections and complicated urinary tract infections (UTIs) caused by antibiotic-resistant gram-negative rods.

Carbapenems

Carbapenems are β-lactam drugs that are structurally different from penicillins and cephalosporins. For example, imipenem (N-formimidoylthienamycin), a commonly used carbapenem, has a methylene group in the ring in place of the sulfur (Figure 10–5). Imipenem has one of the widest spectrums of activity of the β-lactam drugs. It has excellent bactericidal activity against many gram-positive, gram-negative, and anaerobic bacteria (Table 10–5). It is effective against most gram-positive cocci (e.g., staphylococci and streptococci), most gram-negative cocci (e.g., Neisseria meningitidis), many gram-negative rods (e.g., Pseudomonas, Haemophilus, and members of the family Enterobacteriaceae such as E. coli), and various anaerobes (e.g., Bacteroides and Clostridium).

Imipenem is especially useful in treating infections caused by gram-negative rods that produce extended-spectrum β-lactamases that make them resistant to all penicillins and cephalosporins. Carbapenems are often the “drugs of last resort” against bacteria resistant to multiple antibiotics and are thus reserved for hospital settings.

Imipenem is prescribed in combination with cilastatin, which is an inhibitor of dehydropeptidase, a kidney enzyme that inactivates imipenem. Other carbapenems, such as ertapenem and meropenem, are not inactivated by dehydropeptidase and are not prescribed in combination with cilastatin.

Imipenem is not inactivated by most β-lactamases; however, carbapenemases produced by K. pneumoniae that degrade imipenem and other carbapenems have emerged. To address the problem of carbapenemase-producing gram-negative rods, the FDA has approved a combination of meropenem-vaborbactam for the treatment of complicated UTIs caused by E. coli, K. pneumoniae, and others. Vaborbactam is a carbapenemase/β-lactamase inhibitor.

Monobactams

Monobactams are also β-lactam drugs that are structurally different from penicillins and cephalosporins. Monobactams are characterized by a β-lactam ring without an adjacent sulfur-containing ring structure (i.e., they are monocyclic) (see Figure 10–5). Aztreonam, currently the most useful monobactam, has excellent activity against many gram-negative rods, such as Enterobacteriaceae and Pseudomonas, but is inactive against gram-positive and anaerobic bacteria. It is resistant to most β-lactamases. It is very useful in patients who are hypersensitive to penicillin because there is no cross-reactivity.

Vancomycin

Vancomycin is a glycopeptide that inhibits cell wall peptidoglycan synthesis by blocking transpeptidation but by a mechanism different from that of the β-lactam drugs. Vancomycin binds directly to the D-alanyl-D-alanine portion of the pentapeptide, which blocks the transpeptidase from binding, whereas the β-lactam drugs bind to and inhibit the activity of the transpeptidase itself.

Vancomycin is a bactericidal agent effective against certain gram-positive bacteria. Its most important use is in the treatment of infections caused by S. aureus strains that are resistant to the penicillinase-resistant penicillins such as nafcillin and methicillin (e.g., methicillin-resistant S. aureus [MRSA]). Note that vancomycin is not a β-lactam drug and, therefore, is not degraded by β-lactamase.

Vancomycin is also used in the treatment of infections caused by Staphylococcus epidermidis, penicillin-resistant Streptococcus pneumoniae, and enterococci. Strains of S. aureus, S. epidermidis, and enterococci with partial or complete resistance to vancomycin have been recovered from patients.

A well-known adverse effect of vancomycin is “red man” syndrome. “Red” refers to the flushing caused by vasodilation induced by histamine release from mast cells and basophils. This is a direct effect of vancomycin on these cells and is not an IgE-mediated response.

Telavancin is a synthetic derivative of vancomycin that both inhibits peptidoglycan synthesis and disrupts bacterial cell membranes. It is used for the treatment of skin and soft tissue infections, especially those caused by MRSA. Oritavancin and dalbavancin are lipoglycopeptide derivatives of vancomycin and teicoplanin, respectively. These drugs inhibit the transpeptidases and transglycosylases required to synthesize the peptidoglycan of gram-positive bacteria. They are effective in the treatment of infections caused by S. aureus, including MRSA, and Enterococcus, including vancomycin-resistant enterococci (VRE).

Cycloserine & Bacitracin

Cycloserine is a structural analogue of D-alanine that inhibits the synthesis of the cell wall dipeptide D-alanyl-D-alanine. It is used as a second-line drug in the treatment of tuberculosis. Bacitracin is a cyclic polypeptide antibiotic that prevents the dephosphorylation of the phospholipid that carries the peptidoglycan subunit across the cell membrane. This blocks the regeneration of the lipid carrier and inhibits cell wall synthesis. Bacitracin is a bactericidal drug useful in the treatment of superficial skin infections but nephrotoxicity limits its systemic use.

Fosfomycin

Fosfomycin inhibits peptidoglycan synthesis by blocking an enzyme required for the production of N-acetyl muramic acid, a major component of the peptidoglycan backbone. Fosfomycin is approved for use in the treatment of uncomplicated UTIs in women caused by gram-negative rods such as E. coli and Proteus species. It is not a β-lactam drug so is useful in treating UTI caused by extended spectrum β-lactamase-producing (ESBL) E. coli.

INHIBITION OF PROTEIN SYNTHESIS

Several drugs inhibit protein synthesis in bacteria without significantly interfering with protein synthesis in human cells. This selectivity is due to the differences between bacterial and human ribosomal proteins, RNAs, and associated enzymes. Bacteria have 70S¹ ribosomes with 50S and 30S subunits, whereas human cells have 80S ribosomes with 60S and 40S subunits.

Chloramphenicol, macrolides such as azithromycin and erythromycin, clindamycin, and linezolid act on the 50S subunit, whereas tetracyclines such as doxycycline and aminoglycosides such as gentamicin act on the 30S subunit. A summary of the modes of action of these drugs is presented in Table 10–6, and a summary of their clinically useful activity is presented in Table 10–7.

1. Drugs That Act on the 30S Subunit

Aminoglycosides

The aminoglycosides in common clinical use are gentamicin, tobramycin, and amikacin. Aminoglycosides are bactericidal drugs especially useful against many gram-negative rods. Certain aminoglycosides are used against other organisms (e.g., streptomycin is used in the multidrug therapy of tuberculosis, and gentamicin is used in combination with penicillin G against enterococci). Aminoglycosides are named for the amino sugar component of the molecule, which is connected by a glycosidic linkage to other sugar derivatives (Figure 10–6).

In 2018, the FDA approved a new aminoglycoside, plazomicin, for the treatment of complicated UTIs. It is active against many aerobic gram-negative rods, such as E. coli, Klebsiella, Proteus, and Pseudomonas, including those resistant to other aminoglycosides.

The two important modes of action of aminoglycosides have been documented best for streptomycin; other aminoglycosides probably act similarly. Both inhibition of the initiation complex and misreading of messenger RNA (mRNA) occur; the former is probably more important for the bactericidal activity of the drug. An initiation complex composed of a streptomycin-treated 30S subunit, a 50S subunit, and mRNA will not functionthat is, no peptide bonds are formed, no polysomes are made, and a frozen “streptomycin monosome” results. Misreading of the triplet codon of mRNA so that the wrong amino acid is inserted into the protein also occurs in streptomycin-treated bacteria. The site of action on the 30S subunit includes both a ribosomal protein and the ribosomal RNA (rRNA). As a result of inhibition of initiation and misreading, membrane damage occurs and the bacterium dies. (In 1993, another possible mode of action was described, namely, that aminoglycosides inhibit ribozyme-mediated self-splicing of rRNA.)

Aminoglycosides have certain limitations in their use: (1) They have a toxic effect both on the kidneys and on the auditory and vestibular portions of the eighth cranial nerve. To avoid toxicity, serum levels of the drug, blood urea nitrogen, and creatinine should be measured. (2) They are poorly absorbed from the gastrointestinal tract and cannot be given orally. (3) They penetrate the spinal fluid poorly and must be given intrathecally in the treatment of meningitis. (4) They are ineffective against anaerobes, because their transport into the bacterial cell requires oxygen.

Tetracyclines

Tetracyclines are a family of antibiotics with bacteriostatic activity against a variety of gram-positive and gram-negative bacteria, mycoplasmas, chlamydiae, and rickettsiae. They inhibit protein synthesis by blocking the aminoacyl transfer RNA (tRNA) from entering the acceptor site on the 30S ribosomal subunit. However, the selective action of tetracycline on bacteria is not at the level of the ribosome, because tetracycline in vitro will inhibit protein synthesis equally well in purified ribosomes from both bacterial and human cells. Its selectivity is based on its greatly increased uptake into susceptible bacterial cells compared with human cells.

Tetracyclines, as the name indicates, have four cyclic rings with different substituents at the three R groups (Figure 10–7). The various tetracyclines (e.g., doxycycline, minocycline, oxytetracycline) have similar antimicrobial activity but different pharmacologic properties. In general, tetracyclines have low toxicity but are associated with some important side effects. One is suppression of the normal flora of the intestinal tract, which can lead to diarrhea and overgrowth by drug-resistant bacteria and fungi. Second is that suppression of Lactobacillus in the vaginal normal flora results in a rise in pH, which allows Candida albicans to grow and cause vaginitis. Third is brown staining of the teeth of fetuses and young children as a result of deposition of the drug in developing teeth; tetracyclines are avid calcium chelators. For this reason, tetracyclines are contraindicated for use in pregnant women and in children younger than 8 years of age. Tetracyclines also chelate iron, and so products containing iron, such as iron-containing vitamins, should not be taken during therapy with tetracyclines. Photosensitivity (rash upon exposure to sunlight) can also occur during tetracycline therapy.

Tigecycline (Tygacil) is the first clinically available member of the glycylcycline class of antibiotics. They have a structure similar to tetracyclines and have the same mechanism of action as tetracyclines; namely, they bind to the 30S ribosomal subunit and inhibit bacterial protein synthesis. They have a similar range of adverse effects. Tigecycline is used to treat skin and skin structure infections caused by methicillin-sensitive and methicillin-resistant S. aureus, group A and group B streptococci, vancomycin-resistant enterococci, E. coli, and Bacteroides fragilis. It is also used to treat complicated intra-abdominal infections caused by a variety of facultative and anaerobic bacteria. In 2018, the FDA approved eravacycline, a drug closely related to tigecycline, for the treatment of complicated intra-abdominal infections.

Also in 2018, the FDA approved omadacycline for the treatment of acute bacterial skin and soft tissue infections, and community-acquired bacterial pneumonia. Omadacycline is a new-generation tetracycline that is effective against gram-positive cocci, for example, methicillin sensitive and resistant S. aureus, vancomycin sensitive, and resistant enterococci, as well as penicillin sensitive and resistant S. pneumoniae. It is also effective against Haemophilus influenzae, E. coli, Legionella, and Mycoplasma. Another advantage of this drug is that is more resistant to various bacterial resistance mechanisms than many other tetracyclines.

2. Drugs That Act on the 50S Subunit

Chloramphenicol

Chloramphenicol is active against a broad range of organisms, including gram-positive and gram-negative bacteria (including anaerobes). It is bacteriostatic against certain organisms, such as Salmonella typhi, but has bactericidal activity against the three important encapsulated organisms that cause meningitis: H. influenzae, S. pneumoniae, and N. meningitidis.

Chloramphenicol inhibits protein synthesis by binding to the 50S ribosomal subunit and blocking the action of peptidyl transferase; this prevents the synthesis of new peptide bonds. It inhibits bacterial protein synthesis selectively, because it binds to the catalytic site of the transferase in the 50S bacterial ribosomal subunit but not to the transferase in the 60S human ribosomal subunit. Chloramphenicol inhibits protein synthesis in the mitochondria of human cells to some extent, since mitochondria have a 50S subunit (mitochondria are thought to have evolved from bacteria). This inhibition may be the cause of the dose-dependent toxicity of chloramphenicol to bone marrow (see next paragraph).

Chloramphenicol is a comparatively simple molecule with a nitrobenzene nucleus (Figure 10–8). Nitrobenzene is a bone marrow depressant and is likely to be involved in the hematologic problems reported with this drug. The most important side effect of chloramphenicol is bone marrow toxicity, of which there are two types. One is a dose-dependent suppression, which is more likely to occur in patients receiving high doses for long periods and which is reversible when administration of the drug is stopped. The other is aplastic anemia, which is caused by an idiosyncratic reaction to the drug. This reaction is not dose-dependent, can occur weeks after administration of the drug has been stopped, and is not reversible. Fortunately, this reaction is rare, occurring in about 1 in 30,000 patients.

One specific toxic manifestation of chloramphenicol is “gray baby” syndrome, in which the infant’s skin appears gray and vomiting and shock occur. This is due to reduced glucuronyl transferase activity in infants, resulting in a toxic concentration of chloramphenicol. Glucuronyl transferase is the enzyme responsible for detoxification of chloramphenicol.

Macrolides

Macrolides are a group of bacteriostatic drugs with a wide spectrum of activity. The name macrolide refers to their large (13–16 carbon) ring structure (Figure 10–9). Azithromycin, erythromycin, and clarithromycin are the main macrolides in clinical use. Azithromycin is used to treat genital tract infections caused by Chlamydia trachomatis and respiratory tract infections caused by Legionella, Mycoplasma, Chlamydia pneumoniae, and S. pneumoniae. Erythromycin has a similar spectrum of activity but has a shorter half-life and so must be taken more frequently and has more adverse effects, especially on the gastrointestinal tract. Clarithromycin is used primarily in the treatment of Helicobacter infections and in the treatment and prevention of Mycobacterium avium-intracellulare infections. An important adverse effect of clarithromycin is prolongation of the QT interval, which may increase the risk of cardiac death.

Macrolides inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit and blocking translocation. They prevent the release of the uncharged tRNA after it has transferred its amino acid to the growing peptide chain. The donor site remains occupied, a new tRNA cannot attach, and protein synthesis stops.

Clindamycin

The most useful clinical activity of this bacteriostatic drug is against anaerobes, both gram-positive bacteria such as Clostridium perfringens and gram-negative bacteria such as B. fragilis.

Clindamycin binds to the 50S subunit and blocks peptide bond formation by an undetermined mechanism. Its specificity for bacteria arises from its inability to bind to the 60S subunit of human ribosomes.

The most important side effect of clindamycin is pseudomembranous colitis, which, in fact, can occur with virtually any antibiotic, whether taken orally or parenterally. The pathogenesis of this potentially severe complication is suppression of the normal flora of the bowel by the drug and overgrowth of a drug-resistant strain of C. difficile. The organism secretes an exotoxin that produces the pseudomembrane in the colon and severe, often bloody diarrhea.

Linezolid

Linezolid is useful for the treatment of vancomycin-resistant enterococci, methicillin-resistant S. aureus and S. epidermidis, and penicillin-resistant pneumococci. It is bacteriostatic against enterococci and staphylococci but bactericidal against pneumococci.

Linezolid binds to the 23S ribosomal RNA in the 50S subunit and inhibits protein synthesis, but the precise mechanism is unknown. It appears to block some early step (initiation) in ribosome formation. Tedizolid is a second-generation drug in the same class as linezolid but is approximately 10 times more effective. It is used for the treatment of skin and soft tissue infections caused by a similar range of bacteria as linezolid and has a similar mechanism of action.

Telithromycin

Telithromycin (Ketek) is the first clinically useful member of the ketolide group of antibiotics. It is similar to the macrolides in general structure and mode of action but is sufficiently different chemically such that organisms resistant to macrolides may be sensitive to telithromycin. It has a wide spectrum of activity against a variety of gram-positive and gram-negative bacteria (including macrolide-resistant pneumococci) and is used in the treatment of community-acquired pneumonia, bronchitis, and sinusitis.

Streptogramins

A combination of two streptogramins, quinupristin and dalfopristin (Synercid), is used for the treatment of bloodstream infections caused by vancomycin-resistant Enterococcus faecium (but not vancomycin-resistant Enterococcus faecalis). It is also approved for use in infections caused by Streptococcus pyogenes, penicillin-resistant S. pneumoniae, methicillin-resistant S. aureus, and methicillin-resistant S. epidermidis.

Streptogramins cause premature release of the growing peptide chain from the 50S ribosomal subunit. The structure and mode of action of streptogramins are different from all other drugs that inhibit protein synthesis, and there is no cross-resistance between streptogramins and these other drugs.

Retapamulin

Retapamulin (Altabax) is the first clinically available member of a new class of antibiotics called pleuromutilins. These drugs inhibit bacterial protein synthesis by binding to the 23S RNA of the 50S subunit and blocking attachment of the donor tRNA. Retapamulin is a topical antibiotic used in the treatment of skin infections, such as impetigo, caused by S. pyogenes and methicillin-sensitive S. aureus. In 2019, lefamulin (Xenleta), a second pleuromulin, was approved for the treatment of community-acquired bacterial pneumonia in adults.

3. Drugs That Inhibit tRNA synthetase

Mupirocin is effective against gram-positive cocci, especially S. aureus, (both MRSA and MSSA), by inhibiting tRNA synthetase. It is used topically in treating minor skin infections such as impetigo. It is also used to reduce nasal carriage of S. aureus in hospital personnel.

INHIBITION OF NUCLEIC ACID SYNTHESIS

The mode of action and clinically useful activity of the important drugs that act by inhibiting nucleic acid synthesis are summarized in Table 10–8.

1. Inhibition of Precursor Synthesis

Sulfonamides

Either alone or in combination with trimethoprim, sulfonamides are useful in a variety of bacterial diseases such as UTIs caused by E. coli, otitis media caused by S. pneumoniae or H. influenzae in children, shigellosis, nocardiosis, and chancroid. In combination, they are also the drugs of choice for two additional diseases, toxoplasmosis and Pneumocystis pneumonia. The sulfonamides are a large family of bacteriostatic drugs that are produced by chemical synthesis. In 1935, the parent compound, sulfanilamide, became the first clinically effective antimicrobial agent.

The mode of action of sulfonamides is to block the synthesis of tetrahydrofolic acid, which is required as a methyl donor in the synthesis of the nucleic acid precursors adenine, guanine, and thymine. Sulfonamides are structural analogues of p-aminobenzoic acid (PABA). PABA condenses with a pteridine compound to form dihydropteroic acid, a precursor of tetrahydrofolic acid (Figure 10–10). Sulfonamides compete with PABA for the active site of the enzyme dihydropteroate synthetase. This competitive inhibition can be overcome by an excess of PABA.

The basis of the selective action of sulfonamides on bacteria is that many bacteria synthesize their folic acid from PABA-containing precursors, whereas human cells require preformed folic acid as an exogenous nutrient because they lack the enzymes to synthesize it. Human cells therefore bypass the step at which sulfonamides act. Bacteria that can use preformed folic acid are similarly resistant to sulfonamides.

The p-amino group on the sulfonamide is essential for its activity. Modifications are therefore made on the sulfonic acid side chain. Sulfonamides are inexpensive and infrequently cause side effects. However, drug-related fever, rashes, photosensitivity (rash upon exposure to sunlight), and bone marrow suppression can occur. They are the most common group of drugs that cause erythema multiforme and its more severe forms, Stevens-Johnson syndrome and toxic epidermal necrolysis.

Trimethoprim

Trimethoprim also inhibits the production of tetrahydrofolic acid but by a mechanism different from that of the sulfonamides (i.e., it inhibits the enzyme dihydrofolate reductase) (see Figure 10–10). Its specificity for bacteria is based on its much greater affinity for bacterial reductase than for the human enzyme.

Trimethoprim is used most frequently together with sulfamethoxazole. Note that both drugs act on the same pathway but at different sitesto inhibit the synthesis of tetrahydrofolate. The advantages of the combination are that (1) bacterial mutants resistant to one drug will be inhibited by the other and (2) the two drugs can act synergistically (i.e., when used together, they cause significantly greater inhibition than the sum of the inhibition caused by each drug separately).

Trimethoprim-sulfamethoxazole is clinically useful in the treatment of UTIs, Pneumocystis pneumonia, and shigellosis. It also is used for prophylaxis in granulopenic patients to prevent opportunistic infections.

2. Inhibition of DNA Synthesis

Fluoroquinolones

Fluoroquinolones are bactericidal drugs that block bacterial DNA synthesis by inhibiting DNA gyrase (topoisomerase). Fluoroquinolones, such as ciprofloxacin (Figure 10–11), levofloxacin, norfloxacin, ofloxacin, and others, are active against a broad range of organisms that cause infections of the lower respiratory tract, intestinal tract, urinary tract, and skeletal and soft tissues. Nalidixic acid, which is a quinolone but not a fluoroquinolone, is much less active and is used only for the treatment of UTIs. Fluoroquinolones should not be given to pregnant women and children under the age of 18 years because they damage growing bone and cartilage.

In 2018, the FDA approved delafloxacin for the treatment of acute bacterial skin and soft tissue infections. Delafloxacin is a fluoroquinolone that is effective against gram-positive cocci, for example, methicillin sensitive and resistant S. aureus, as well as S. pyogenes and E. faecalis. It is effective against gram-negative bacteria, for example, E. coli, Klebsiella, and Pseudomonas. It is also effective against Legionella and Mycoplasma, and against anaerobes such as Bacteroides. It is the only fluoroquinolone approved for the treatment of MRSA.

The FDA has issued a warning regarding the possibility of Achilles tendonitis and tendon rupture associated with fluoroquinolone use, especially in those over 60 years of age and in patients receiving corticosteroids, such as prednisone. In view of this, the FDA recommends that fluoroquinolones not be used in the treatment of acute sinusitis and uncomplicated UTIs. Another important adverse effect of fluoroquinolones is peripheral neuropathy, the symptoms of which include pain, burning, numbness, or tingling in the arms or legs. Another rare but serious adverse effect is rupture of the aorta.

3. Inhibition of mRNA Synthesis

Rifampin is used primarily for the treatment of tuberculosis in combination with other drugs and for prophylaxis in close contacts of patients with meningitis caused by either N. meningitidis or H. influenzae. It is also used in combination with other drugs in the treatment of prosthetic-valve endocarditis caused by S. epidermidis. In 2018, the FDA approved the use of oral rifampin for the treatment of travelers diarrhea caused by E. coli. With the exception of the short-term prophylaxis of meningitis and treatment of travelers diarrhea, rifampin is given in combination with other drugs because resistant mutants appear at a high rate when it is used alone.

The selective mode of action of rifampin is based on blocking mRNA synthesis by bacterial RNA polymerase without affecting the RNA polymerase of human cells. Rifampin is red, and the urine, saliva, and sweat of patients taking rifampin often turn orange; this is disturbing but harmless. Rifampin is excreted in high concentration in saliva, which accounts for its success in the prophylaxis of bacterial meningitis since the organisms are carried in the throat.

Rifabutin, a rifampin derivative with the same mode of action as rifampin, is useful in the prevention of disease caused by Mycobacterium avium-intracellulare in patients with severely reduced numbers of helper T cells (e.g., acquired immunodeficiency syndrome [AIDS] patients). Note that rifabutin does not increase cytochrome P450 as much as rifampin, so rifabutin is used in human immunodeficiency virus (HIV)/AIDS patients taking protease inhibitors or non-reverse transcriptase inhibitors (NRTI).

Fidaxomicin (Dificid) inhibits the RNA polymerase of C. difficile. It is used in the treatment of pseudomembranous colitis and in preventing relapses of this disease. It specifically inhibits C. difficile and does not affect the gram-negative normal flora of the colon.

ALTERATION OF CELL MEMBRANE FUNCTION

There are few antimicrobial compounds that act on the cell membrane because the structural and chemical similarities of bacterial and human cell membranes make it difficult to provide sufficient selective toxicity.

Polymyxins are a family of polypeptide antibiotics of which the clinically most useful compound is polymyxin E (colistin). Colistin is active against gram-negative rods, especially P. aeruginosa, Acinetobacter baumannii, and carbapenemase-producing Enterobacteriaceae. Most strains of these highly antibiotic-resistant bacteria are sensitive to colistin, although rare isolates from patients are resistant.

Polymyxins are cyclic peptides composed of 10 amino acids, six of which are diaminobutyric acid. The positively charged free amino groups act like a cationic detergent to disrupt the phospholipid structure of the cell membrane.

Daptomycin is a cyclic lipopeptide that disrupts the cell membranes of gram-positive cocci. It is bactericidal for organisms such as S. aureus, S. epidermidis, S. pyogenes, E. faecalis, and E. faecium, including methicillin-resistant strains of S. aureus and S. epidermidis, vancomycin-resistant strains of S. aureus, and vancomycin-resistant strains of E. faecalis and E. faecium. It is approved for use in complicated skin and soft tissue infections caused by these bacteria.

ADDITIONAL DRUG MECHANISMS

Isoniazid, or isonicotinic acid hydrazide (INH), is a bactericidal drug highly specific for Mycobacterium tuberculosis and other mycobacteria. It is used in combination with other drugs to treat tuberculosis and by itself to prevent tuberculosis in exposed persons. Because it penetrates human cells well, it is effective against the organisms residing within macrophages. The structure of isoniazid is shown in Figure 10–12.

INH inhibits mycolic acid synthesis, which explains why it is specific for mycobacteria and relatively nontoxic for humans. The drug inhibits a reductase required for the synthesis of the long-chain fatty acids called mycolic acids that are an essential constituent of mycobacterial cell walls. The active drug is probably a metabolite of INH formed by the action of catalase peroxidase because deletion of the gene for these enzymes results in resistance to the drug. Its main side effect is liver toxicity. It is given with pyridoxine to prevent neurologic complications.

Metronidazole (Flagyl) is bactericidal against anaerobic bacteria such as B. fragilis. It is used to treat bacterial vaginosis and can be used for non-serious colitis caused by C. difficile. It is also effective against infections caused by certain protozoa such as Giardia and Trichomonas. Metronidazole is a prodrug that is activated to the active compound within anaerobic bacteria by ferredoxin-mediated reduction of its nitro group.

This drug has two possible mechanisms of action, and it is unclear which is more important. The first, which explains its specificity for anaerobes, is its ability to act as an electron sink. By accepting electrons, the drug deprives the organism of required reducing power. In addition, when electrons are acquired, the drug ring is cleaved and a toxic intermediate is formed that damages DNA. The precise nature of the intermediate and its action is unknown. The structure of metronidazole is shown in Figure 10–12.

The second mode of action of metronidazole relates to its ability to inhibit DNA synthesis. The drug binds to DNA and causes strand breakage, which prevents its proper functioning as a template for DNA polymerase.

Nitrofurantoin is a urinary tract antiseptic that is useful in the treatment of uncomplicated lower UTIs. It is concentrated in the urine to reach bactericidal levels but does not reach cidal levels systemically so is not useful for infections outside the urinary tract.

Nitrofurantoin acts by binding to DNA. Its selective toxicity for bacteria is dependent upon the ability of bacteria to form larger amounts of the highly reactive reduced form of the drug compared to the amount formed in human cells.

Ethambutol is a bacteriostatic drug used in the treatment of infections caused by M. tuberculosis and many of the atypical mycobacteria. It acts by inhibiting the synthesis of arabinogalactan, which functions as a link between the mycolic acids and the peptidoglycan of the organism.

Pyrazinamide (PZA) is a bactericidal drug used in the treatment of tuberculosis but not in the treatment of most atypical mycobacterial infections. PZA is particularly effective against semidormant organisms in the lesion, which are not affected by INH or rifampin. The mechanism of action of PZA is uncertain, but there is evidence that it acts by inhibiting a fatty acid synthetase that prevents the synthesis of mycolic acid. It is converted to the active intermediate, pyrazinoic acid, by an amidase in the mycobacteria.

In most instances, the antimicrobial agents described in this chapter are used for the treatment of infectious diseases. However, there are times when they are used to prevent diseases from occurringa process called chemoprophylaxis.

Chemoprophylaxis is used in three circumstances: prior to surgery, in immunocompromised patients, and in people with normal immunity who have been exposed to certain pathogens. Table 10–9 describes the drugs and the situations in which they are used. For more information, see the chapters on the individual organisms.

Of particular importance is the prevention of endocarditis in high-risk patients undergoing dental surgery by using amoxicillin perioperatively. High-risk patients include those who have unrepaired damage to their heart valve, have a prosthetic heart valve, or have previously had infective endocarditis. Prophylaxis to prevent endocarditis in patients undergoing gastrointestinal or genitourinary tract surgery is not recommended.

Cefazolin is often used to prevent staphylococcal infections in patients undergoing orthopedic surgery, including prosthetic joint implants, and in vascular graft surgery. Chemoprophylaxis is unnecessary in those with an implanted dialysis catheter, a cardiac pacemaker, or a ventriculoperitoneal shunt.

In contrast to the chemical antibiotics previously described in this chapter, probiotics are live, nonpathogenic bacteria (or yeasts) that may be effective in the treatment or prevention of certain human diseases. The suggested basis for the possible beneficial effect lies either in providing colonization resistance by which the nonpathogen excludes the pathogen from binding sites on the mucosa, in enhancing the immune response against the pathogen, or in reducing the inflammatory response against the pathogen. For example, the oral administration of live Lactobacillus rhamnosus strain GG significantly reduces the number of cases of nosocomial diarrhea in young children. Also, the yeast Saccharomyces boulardii reduces the risk of antibiotic-associated diarrhea caused by C. difficile. Adverse effects are few; however, serious complications have arisen in highly immunosuppressed patients and in patients with indwelling vascular catheters.