The macrolides, erythromycin, azithromycin, and clarithromycin, differ in the exact composition of a large 14- or 15-member ring structure. They affect protein synthesis at the ribosomal level by binding to the 50S subunit and blocking the translocation reaction. Their effect is primarily bacteriostatic. Macrolides, which are concentrated in phagocytes and other cells, are effective against some intracellular pathogens.
Erythromycin, the first and still the most commonly used macrolide, has a spectrum of activity that includes most of the pathogenic Gram-positive bacteria and some Gram-negative organisms. The Gram-negative spectrum includes Neisseria, Bordetella, Campy-lobacter, and Legionella, but not the Enterobacteriaceae. Erythromycin is also effectiveagainst Chlamydia and Mycoplasma.
Bacteria that have developed resistance to erythromycin are usually resistant to the newer macrolides azithromycin and clarithromycin as well. These newer agents have the same spectrum as erythromycin, with some significant additions. Azithromycin has quantitatively greater activity (lower MICs) against most of the same Gram-negative bacteria. Clarithromycin is the most active of the three against both Gram-positive and Gram-negative pathogens. Clarithromycin is also active against mycobacteria. In addi-tion, both azithromycin and clarithromycin have demonstrated efficacy against Borreliaburgdorferi, the causal agent of Lyme disease and the protozoan parasite Toxoplasma gondii, which causes toxoplasmosis.
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