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The antimicrobial agents discussed in this chapter may be assigned to three groups:
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Bacteriostatic, protein-synthesis inhibitors that target the ribosome, such as tetracyclines and glycylcyclines, chloramphenicol, macrolides and ketolides, lincosamides (clindamycin), streptogramins (quinupristin/dalfopristin), oxazolidinones (linezolid), and aminocyclitols (spectinomycin).
Agents acting on the cell wall or cell membrane such as polymyxins, glycopeptides (vancomycin and teicoplanin), and lipopeptides (daptomycin).
Miscellaneous compounds acting by diverse mechanisms with limited indications: bacitracin and mupirocin.
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TETRACYCLINES AND GLYCYLCYCLINES
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Sources and Chemistry. The tetracyclines are close congeners of polycyclic naphthacenecarboxamide. Structural formulas of the tetracyclines are shown in Table 55–1.
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Chlortetracycline, the prototype of this class, was introduced in 1948 but is no longer marketed in the U.S. Oxytetracycline is a natural product elaborated by Streptomyces rimosus. Tetracycline is a semisynthetic derivative of chlortetracycline. Demeclocycline is the product of a mutant strain of S. aureofaciens, and methacycline (not available in the U.S.), doxycycline, and minocycline all are semisynthetic derivatives.
Because of their activity against Rickettsia, aerobic and anaerobic gram-positive and gram-negative bacteria, and Chlamydia, tetracyclines became known as "broad-spectrum" antibiotics. The spread of antimicrobial resistance has eroded the activity of tetracyclines against many gram-positive and gram-negative organisms. A new group of tetracycline derivatives, the glycyclines, have regained much of this activity. The glycylcyclines are synthetic analogs of the tetracyclines; the glycycline currently approved is tigecycline, the 9-tert-butyl-glycylamido derivative of minocycline.
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Antimicrobial Activity. Tetracyclines are bacteriostatic antibiotics with activity against a wide range of aerobic and anaerobic gram-positive and gram-negative bacteria.
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They also are effective against some microorganisms, such as Rickettsia, Coxiella burnetii, Mycoplasma pneumoniae, Chlamydia spp., Legionella spp., Ureaplasma, some atypical mycobacteria, and Plasmodium spp., that are resistant to cell-wall-active antimicrobial agents. The tetracyclines are active against many spirochetes, including Borrelia recurrentis, Borrelia burgdorferi (Lyme disease), Treponema pallidum (syphilis), and Treponema pertenue. Tetracyclines are active against Chlamydia and Mycoplasma. Some nontuberculosis strains of mycobacteria (e.g., M. marinum) also are susceptible. They are not active against fungi. Demeclocycline, tetracycline, minocycline, and doxycycline are available in the U.S. for systemic use. Other derivatives are available in other countries. The more lipophilic drugs, minocycline and doxycycline, usually are the most active by weight, followed by tetracycline. Resistance of a bacterial strain to any one member of the class may or may not result in cross-resistance to other tetracyclines. Tigecycline is generally, although not universally, active against organisms that are susceptible to tetracyclines as well as those with acquired resistance to tetracyclines (Gales et al., 2008).
Bacterial strains with tetracycline minimum inhibitory concentrations (MICs) ≤4 μg/mL are considered susceptible except for Haemophilus influenzae and Streptococcus pneumoniae, whose susceptibility breakpoints (defined as the upper limit of ...