Antibacterial Activity

Erythromycin usually is bacteriostatic. It is most active in vitro against aerobic gram-positive cocci and bacilli. Cross-resistance is complete. The prevalence of macrolide resistance among group A streptococcal isolates is related to consumption of macrolide antibiotics within the population. Only 5% of penicillin-susceptible strains are macrolide-resistant, whereas 50% or more of penicillin-resistant strains may be macrolide-resistant. Staphylococci are not reliably sensitive to erythro-mycin. Macrolide-resistant strains of S. aureus are potentially also resistant to clindamycin and streptogramin B (quinupristin). Gram-positive bacilli are sensitive to erythromycin, including Clostridium perfringens, Corynebacterium diphtheriae, and Listeria monocytogenes.

Erythromycin is inactive against most aerobic enteric gram-negative bacilli but has modest activity in vitro against other gram-negative organisms, including H. influenzae and N. meningitidis, and good activity against most strains of N. gonorrhoeae. Useful antibacterial activity also is observed against Pasteurella multocida, Borrelia spp., and Bordetella pertussis. Resistance is

Nascent polypeptide chain

P site

A site

Macrolides

Macrolides tRNA'

FIGURE 46-3 Inhibition of bacterial protein synthesis by the macrold antibiotics erythromycin, clarithromycin, and azithromycin. Macrolide antibiotics are bacteriostatic agents that inhibit protein synthesis by binding reversibly to the 50S ribosomal subunits of sensitive organisms. Erythromycin apparently inhibits the translocation step wherein the nascent peptide chain temporarily residing at the A site of the transferase reaction fails to move to the P, or donor, site. Alternatively, macrolides may bind and cause a conformational change that terminates protein synthesis by indirectly interfering with transpeptidation and translocation. See Figure 46-1 and its legend for additional information.

Transferase site mRNA template tRNA'

FIGURE 46-3 Inhibition of bacterial protein synthesis by the macrold antibiotics erythromycin, clarithromycin, and azithromycin. Macrolide antibiotics are bacteriostatic agents that inhibit protein synthesis by binding reversibly to the 50S ribosomal subunits of sensitive organisms. Erythromycin apparently inhibits the translocation step wherein the nascent peptide chain temporarily residing at the A site of the transferase reaction fails to move to the P, or donor, site. Alternatively, macrolides may bind and cause a conformational change that terminates protein synthesis by indirectly interfering with transpeptidation and translocation. See Figure 46-1 and its legend for additional information.

common for B. fragilis. Macrolides are usually active against Campylobacter jejuni. Erythromycin is active against M. pneumoniae and Legionella pneumophila. Most strains of C. trachomatis are inhibited by erythromycin.

Clarithromycin is slightly more potent than erythromycin against sensitive strains of streptococci and staphylococci and has modest activity against H. influenzae and N. gonorrhoeae. Clarithromycin has good activity against M. catarrhalis, Chlamydia spp., L. pneumophila, B. burgdorferi, Mycoplasma pneumoniae, and H. pylori.

Azithromycin generally is less active than erythromycin against gram-positive organisms and slightly more active than either erythromycin or clarithromycin against H. influenzae and Campylobacter spp. Azithromycin is very active against M. catarrhalis, P. multocida, Chlamydia spp., M. pneumoniae, L. pneumophila, B. burgdorferi, Fusobacterium spp., and N. gonorrhoeae.

Azithromycin and clarithromycin have enhanced activity against M. avium-intracellulare, as well as against some protozoa (e.g., Toxoplasma gondii, Cryptosporidium, and Plasmodium spp.). Clarithromycin has good activity against Mycobacterium leprae (see Chapter 47).

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