1. Semisynthetic Penicillin

4. The penicillins are a large group of bactericidal compounds. They can be subdivided and classified by their chemical structure and spectrum of activity. The structure common to all penicillins is a -lactam ring fused with a thiazolidine nucleus. The antimicrobial activity of penicillin resides in the -lactam ring. Splitting of the -lactam ring by either acid hydrolysis or -lactamases results in the formation of penicilloic acid, a product without antibiotic activity. Addition of various side chains (R) to the basic penicillin molecule creates classes of compounds with the same mechanism of action as penicillin but with different chemical and biological properties. For example, some analogues are resistant to hydrolysis by acid or - lactamase; some have an extended the spectrum of antibacterial activity; and others show improved absorption from the intestinal tract.

5. Penicillins may be classified into four groups: Natural penicillins (G and V), Antistaphylococcal (penicillinaseresistant) penicillins, Aminopenicillins, and Antipseudomonal penicillins. Natural penicillins have therapeutic effects limited to streptococci and a few gram-negative organisms. The antistaphylococcal (penicillinase-resistant) penicillins treat infections caused by streptococci and staphylococci but do not affect MRSA. The aminopenicillins are effective against streptococci, enterococci, and some gram-negative organisms but have variable activity against staphylococci and are ineffective against P. aeruginosa. The antipseudomonal penicillins retain activity against streptococci and possess additional effects against gram-negative organisms, including various Enterobacteriaceae and Pseudomonas

13. MECHANISM OF ACTION The final reaction in bacterial cell wall synthesis is a cross-linking of adjacent peptidoglycan (murein) strands by a transpeptidation reaction. In this reaction, bacterial transpeptidases cleave the terminal D-alanine from a pentapeptide on one peptidoglycan strand and thencross-link it with the pentapeptide of another peptidoglycan strand. The cross-linked peptidoglycan (murein) strands give structural integrity to cell walls and permit bacteria to survive environments that do not match the organism’s internal osmotic pressure. The -lactam antibiotics structurally resemble the terminal D-alanyl-D-alanine (D-Ala-D- Ala) in the pentapeptides on peptidoglycan (murein) (Fig. 45.1). Bacterial transpeptidases covalently bind the –lactam antibiotics at the enzyme active site, and the resultant acyl enzyme molecule is stable and inactive. The intact -lactam ring is required for antibiotic action. The -lactam ring modifies the active serine site on transpeptidases and blocks further enzyme function. In addition to transpeptidases, other penicillin-binding proteins (PBPs) function as transglycosylases and carboxypeptidases. All of the PBPs are involved with assembly, maintenance, or regulation of peptidoglycan cell wall synthesis. When - lactam antibiotics inactivate PBPs, the consequence to the bacterium is a structurally weakened cell wall, aberrant morphological form, cell lysis, and death.

14. A number of antibiotics produced by fungi of the genus Cephalosporium have been identified. These antibiotics called cephalosporins contain, in common with the penicillins, a -lactam ring. In addition to the numerous penicillins and cephalosporins in use, three other classes of -lactam antibiotics are available for clinical use, these are: carbapenems, carbacephems, and monobactams. All -lactam antibiotics have the same bactericidal mechanism of action. They block a critical step in bacterial cell wall synthesis.

15. The cephalosporins are semisynthetic antibiotics derived from products of various microorganisms, including Cephalosporium and Streptomyces. All cephalosporins have a 7-aminocephalosporanic acid composed of a dihydrothiazine ring fused to a –lactam. The different pharmacological, pharmacokinetic, and antibacterial properties of individual cephalosporins result from substitution of various groups on the basic molecule. Cephalosporins also vary in acid stability and - lactamase susceptibility. The -lactamases (penicillinases) inactivate some cephalosporins but are much less efficient than are the cephalosporinases (-lactamases specific for the cephalosporins). Resistance to cephalosporins also results from modification of microbial PBPs.

30. Aminoglycosides are hydrophilic, polycationic, aminecontaining carbohydrates that are usually composed of three to five rings.Most aminoglycosides are either natural products or derivatives of soil actinomycetes. They are often secreted by these actinomycetes as mixtures of closely related compounds. The polycationic aminoglycoside chemical structure results in a binding both to the anionic outer bacterial membrane and to anionic phospholipids in the cell membranes of mammalian renal proximal tubular cells.The former contributes to the bactericidal effects of these compounds, while the latter binding accounts for their toxicity. Because of their hydrophilicity, the transport of aminoglycosides across the hydrophobic lipid bilayer of eukaryotic cell membranes is impeded.

35. MECHANISM OF ANTIBACTERIAL ACTION The antibacterial actions of the aminoglycosides involve two possibly synergistic effects. First, the positively charged aminoglycoside binds to negatively charged sites on the outer bacterial membrane, thereby disrupting membrane integrity. It is likely that the aminoglycoside-induced bacterial outer membrane degradation accounts for the rapid concentrationdependent bactericidal effect of these compounds. Second, aminoglycosides bind to various sites on bacterial 30S ribosomal subunits, disrupting the initiation of protein synthesis and inducing errors in the translation of messenger RNA to peptides. They also bind to sites on bacterial 50S ribosomal subunits, although the significance of this binding is uncertain. In addition, they have a postantibiotic effect; that is, they continue to suppress bacterial regrowth even after removal of the antibiotic from the bacterial microenvironment. It is likely that ribosome disruption accounts for this postantibiotic activity.

36. SULFONAMIDES Chemistry, Structure, and Function The sulfonamides are a large group of compounds that are structural analogues of p-aminobenzoic acid (PABA). They differ primarily in the substituents on either the amido group (SO2-NH-R) or the amino group(-NH2) of the sulfanilamide nucleus. Substitutions on the sulfonamide group modify the drug’s solubility characteristics, resulting in congeners with different rates of absorption and excretion. One group of sulfonamides remains largely unabsorbed in the gastrointestinal (GI) tract following oral administration. Sulfadiazine, for example, produces changes only on local gut bacterial flora and finds wide use in presurgical bowel sterilization. Other sulfonamides, such as sulfisoxazole, are rapidly absorbed and highly soluble, and they undergo rapid urinary excretion, mainly in the unaltered form. A third group are rapidly absorbed and slowly excreted and maintain adequate blood levels for up to 24 hours (e.g., sulfamethoxazole). These drugs are useful in treating chronic urinary infections. Finally, some sulfonamides (e.g., sulfacetamide and sulfadiazine [silver salt]) are designed for topical use such as in infection of the eye and in burn patients.

37. Mechanism of Action and Resistance Both sulfonamides and trimethoprim (not a sulfonamide) sequentially interfere with folic acid synthesis by bacteria. Folic acid functions as a coenzyme in the transfer of one-carbon units required for the synthesis of thymidine, purines, and some amino acids and consists of three components: a pteridine moiety, PABA,and glutamate (Fig. 44.1). The sulfonamides, as structural analogues, competitively block PABA incorporation; sulfonamides inhibit the enzyme dihydropteroate synthase, which is necessary for PABA to be incorporated into dihydropteroic acid, an intermediate compound in the formation of folinic acid. Since the sulfonamides reversibly block the synthesis of folic acid, they are bacteriostatic drugs. Humans cannot synthesize folic acid and must acquire it in the diet; thus, the sulfonamides selectively inhibit microbial growth. Resistance to the sulfonamides can be the result of decreased bacterial permeability to the drug, increased production of PABA, or production of an altered dihydropteroate synthetase that exhibits low affinity for sulfonamides. The latter mechanism of resistance is plasmid mediated. Active efflux of the sulfonamides has also been reported to play a role in resistance. The inhibitory effect of the sulfonamides also can be reversed by the presence of pus, tissue fluids, and drugs that contain releasable PABA.

40. Antibacterial Spectrum The macrolides are effective against a number of organisms, including Mycoplasma spp., H. influenzae, Streptococcus spp. (including S. pyogenes and S. pneumoniae), staphylococci, gonococci, Legionella pneumophila, and other Legionella spp. There has been increasing resistance of S. pneumoniae to macrolides worldwide. This is true especially if the strain is resistant to penicillin. This resistance includes not only erythromycin but also clarithromycin and azithromycin. Approximately 10 to 15% of S. pneumoniae in the United States show complete resistance to macrolides. Staphylococci resistant to erythromycin are resistant to all macrolides.The hemolytic streptococci also exhibit varying degrees of cross-resistance to the macrolides and to lincomycin and clindamycin, although the macrolides are chemically unrelated to the last two agents. There are only minor variations in the antibacterial spectrum of the newer macrolides. Clarithromycin is very active against H. influenzae, Legionella, and Mycobacterium avium- intracellulare, whereas azithromycin is superior against Branhamella, Neisseria, and H. influenzae but less active against mycobacterial species. Clarithromycin and azithromycin have significant activity against Mycobacterium avium complex (MAC), and it is one of the drugs of choice in treating disseminated MAC. Both azithromycin and clarithromycin can be used prophylactically in HIV and AIDS patients to help prevent disseminated MAC.

41. Clinical Uses Although erythromycin is a well-established antibiotic, there are relatively few primary indications for its use. These indications include the treatment of Mycoplasma pneumoniae infections, eradication of Corynebacterium diphtheriae from pharyngeal carriers, the early preparoxysmal stage of pertussis, chlamydial infections, and more recently, the treatment of Legionnaires’ disease, Campylobacter enteritis, and chlamydial conjunctivitis, and the prevention of secondary pneumonia in neonates. Erythromycin is effective in the treatment and prevention of S. pyogenes and other streptococcal infections, but not those caused by the more resistant fecal streptococci. Staphylococci are generally susceptible to erythromycin, so this antibiotic is a suitable alternative drug for the penicillin-hypersensitive individual. It is a second-line drug for the treatment of gonorrhea and syphilis.Although erythromycin is popular for the treatment of middle ear and sinus infections, including H. influenzae, possible erythromycin-resistant S. pneumoniaeis a concern.

42. TETRACYCLINES Structure and Mechanism of Action Although all tetracyclines have a similar mechanism of action, they have different chemical structures and are produced by different species of Streptomyces. In addition,structural analogues of these compounds have been synthesized to improve pharmacokinetic properties and antimicrobial activity.While several biological processes in the bacterial cells are modified by the tetracyclines, their primary mode of action is inhibition of protein synthesis.Tetracyclines bind to the 30S ribosome and thereby prevent the binding of aminoacyl transfer RNA (tRNA) to the A site (acceptor site) on the 50S ribosomal unit. The tetracyclines affect both eukaryotic and prokaryotic cells but are selectively toxic for bacteria, because they readily penetrate microbial membranes and accumulate in the cytoplasm through an energydependent tetracycline transport system that is absent from mammalian cells. Resistance is related largely to changes in cell permeability and a resultant decreased accumulation of drug due to increased efflux from the cell by an energydependent mechanism. Other mechanisms, such as production of a protein that alters the interaction of tetracycline with the ribosome and enzymatic inactivation of the drug, have been reported.

43. Antibacterial Spectrum The tetracyclines display broad-spectrum activity and are effective against both gram-positive and gram-negative bacteria, including Rickettsia, Coxiella, Mycoplasma, and Chlamydia spp.. Tetracycline resistance has increased among pneumococci and gonococci, which limits their use in the treatment of infections caused by these organisms. Although several congeners of the tetracyclines are available, they all have a similar spectrum of in vitro activity. Minocycline is somewhat more active and oxytetracycline and tetracycline are somewhat less active than other members of this group.

44. Clinical Uses There is little difference in clinical response among the various tetracyclines. The selection of an agent, therefore, is based on tolerance, ease of administration, and cost. The restriction of their use in pregnancy and in patients under the age of 8 years applies to all preparations. Two tetracyclines have sufficiently distinctive features to warrant separate mention. Doxycycline, with its longer half-life and lack of nephrotoxicity, is a popular choice for patients with preexisting renal disease or those who are at risk for developing renal insufficiency. The lack of nephrotoxicity is related mainly to biliary excretion, which is the primary route of doxycycline elimination. Doxycycline is the preferred parenteral tetracycline. Doxycycline is a potential first-line agent in the prophylaxis of anthrax after exposure. Doxycycline is the treatment of choice for the primary stage of Lyme disease in adults and children older than 8 years. Minocycline is an effective alternative to rifampin for eradication of meningococci, including sulfonamideresistant strains, from the nasopharynx. However, the high incidence of dose-related vestibular side effects renders it less acceptable. Although minocycline has good in vitro activity against Nocardia spp., further studies are necessary to confirm its clinical efficacy. The tetracyclines are still the drugs of choice for treatment of cholera, diseases caused by Rickettsia and Coxiella, granuloma inguinale, relapsing fever,

45. CHLORAMPHENICOL Mechanism of Action Chloramphenicol (Chloromycetin) is a nitrobenzene derivative that affects protein synthesis by binding to the 50S ribosomal subunit and preventing peptide bond formation. It prevents the attachment of the amino acid end of aminoacyl-tRNA to the A site, hence the association of peptidyltransferase with the amino acid substrate. Resistance due to changes in the ribosomebinding site results in a decreased affinity for the drug, decreased permeability, and plasmids that code for enzymes that degrade the antibiotic. The drug-induced inhibition of mitochondrial protein synthesis is probably responsible for the associated toxicity.

48. Clinical Use The potentially fatal nature of chloramphenicolinduced bone marrow suppression restricts its use to a few life-threatening infections in which the benefits outweigh the risks. There is no justification for its use in treating minor infections. Since effective CSF levels are obtained, it used to be a choice for treatment of specific bacterial causes of meningitis: Haemophilus influenzae, Neisseria meningitidis, and S. pneumoniae. Additionally, it was effective against H. influenzae–related arthritis, osteomyelitis, and epiglottitis. The development of -lactamase-producing strains of H. influenzae increased the use of chloramphenicol. However, with the advent of third-generation cephalosporins such as ceftriaxone and cefotaxime, chloramphenicol use has significantly decreased. If the patient is hypersensitive to -lactams, chloramphenicol administration is appropriate therapy for meningitis caused by N. meningitidis and S. pneumoniae.

49. Chloramphenicol remains a major treatment of typhoid and paratyphoid fever in developing countries. However, with increasing resistance to ampicillin, trimethoprim-sulfamethoxazole and, to some extent, chloramphenicol, fluoroquinolones and some third-generation cephalosporins (e.g., ceftriaxone) have become the drugs of choice. Salmonella infections, such as osteomyelitis, meningitis and septicemia, have also been indications for chloramphenicol use. Nevertheless, antibiotic resistance patterns can be a problem Chloramphenicol also is widely used for the topical treatment of eye infections. It is a very effective agent because of its extremely broad spectrum of activity and its ability to penetrate ocular tissue. The availability of safer, less irritating instilled ophthalmic antibiotics and the increase in fatal aplastic anemia associated with the use of this dosage form suggest that this agent might best be withdrawn. Chloramphenicol is an alternative to tetracycline for rickettsial diseases, especially in children younger than 8 years, and alone or in combination with other antibiotics, it has been used to treat vancomycin-resistant enterococci. Another indication for chloramphenicol is in the treatment of serious anaerobic infections caused by penicillin-resistant bacteria, such as B. fragilis. Clindamycin and metronidazole are now preferred for treatment of anaerobic infections. Chloramphenicol, in combination with surgical drainage, is useful in treating cerebral abscesses caused by anaerobic bacteria, particularly those that are resistant to penicillin.

50. LINCOSAMIDES Mechanism of Action The lincosamide family of antibiotics includes lincomycin (Lincocin) and clindamycin (Cleocin), both of which inhibit protein synthesis. They bind to the 50S ribosomal subunit at a binding site close to or overlapping the binding sites for chloramphenicol and erythromycin. They block peptide bond formation by interference at either the A or P site on the ribosome. Lincomycin is no longer available for human use in the United States.

51. Clinical Uses Clindamycin is highly active against staphylococci and streptococci other than enterococci. Also, clindamycin has significant antibacterial activity against S. pyogenes (group A strep). However, the adverse reaction of pseudomembranous colitis has limited its use to individuals who are unable to tolerate other antibiotics and to the treatment of penicillin-resistant anaerobic bacterial infections. Clindamycin has shown excellent activity topically against Corynebacterium acnes in patients with recalcitrant cystic facial acne who cannot tolerate tetracyclines. Precautions should be given to all patients using the topical preparations, since the development of colitis is possible. Both clindamycin and choramphenicol have excellent activity against anaerobic bacteria but have potentially life-threatening adverse reactions and should not be used without good justification.

52. Bacitracin and the polymyxins are polypeptide antibiotics. They are relatively toxic drugs and have had only limited use in chemotherapy until recently. Vancomycin, a glycopeptide, although not without side effects, is widely used.Teicoplanin is a new glycopeptide antibiotic that may be beneficial against certain infections caused by gram-positive organisms. The mechanisms of action of this group vary. Bacitracin and the glycopeptides affect cell wall synthesis, whereas the polymyxins affect the cell membrane. Bacitracin and the glycopeptides are used for the treatment of infections caused by gram-positive bacteria; the polymyxins are used for treating gram-negative infections and are active against Pseudomonas aeruginosa.

53. BACITRACIN Structure and Mechanism of Action Bacitracin is a mixture of polypeptide antibiotics produced by Bacillus subtilis. As with penicillin, it contains a thiazolidine nucleus attached through L-leucine to a peptide composed of both D- and L-amino acids.However, it does not contain a -lactam ring. Bacitracin prevents cell wall synthesis by binding to a lipid pyrophosphate carrier that transports cell wall precursors to the growing cell wall. Bacitracin inhibits the dephosphorylation of this lipid carrier, a step essential to the carrier molecule’s ability to accept cell wall constituents for transport. Antimicrobial Spectrum Bacitracin inhibits gram-positive cocci, including Staphylococcus aureus, streptococci, a few gram-negative organisms, and one anaerobe, Clostridium difficile. Absorption, Distribution, and Excretion Bacitracin is primarily a topical antibiotic. Previously, it was administered intramuscularly, but the toxicity associated with its parenteral administration has precluded systemic use.The bacitracins are not absorbed from the gastrointestinal tract following oral administration.

54. Clinical Uses Bacitracin is highly active against staphylococci, Streptococcus pyogenes, and C. difficile. Its high degree of activity against the group A streptococci is used in the laboratory as a means of differentiating between the Lancefield group A streptococci and other streptococci. Bacitracin is well tolerated topically and orally and is frequently used in combination with other agents Tetracyclines, Chloramphenicol, Macrolides, and Lincosamides 553 tably polymyxin B and neomycin) in the form of creams, ointments, and aerosol preparations. Hydrocortisone has been added to the combination for its antiinflammatory effects. Bacitracin preparations are effective in the treatment of impetigo and other superficial skin infections. However, poststreptococcal nephritis has followed the topical treatment of impetigo, and therefore oral penicillin therapy is preferred. Bacitracin has been used with limited success for eradication of S. aureus in the nares. Because of the risk of serious nephrotoxicity, the parenteral use of bacitracin is not justified.

55. GLYCOPEPTIDES: VANCOMYCIN AND TEICOPLANIN Structure and Mechanism of Action Vancomycin (Vancocin) is a complex tricyclic glycopeptide antibiotic produced by Streptomyces orientalis, while teicoplanin (Targocid) is derived from Actinoplanes (Actinomyces) teichomyceticus. Teicoplanin has two major components: a phosphoglycolipid (A1) and five chlorine-containing glycopeptides (A2). It is available as an investigational drug. The glycopeptides are inhibitors of cell wall synthesis. They bind to the terminal carboxyl group on the Dalanyl- D-alanine terminus of the N-acetylglucosamine- N-acetylmuramic acid peptide and prevent polymerization of the linear peptidoglycan by peptidoglycan synthase.They are bactericidal in vitro.