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14 membered ring
Azalides
16 membered ring
Ketolides

Macrolides

 The first macrolide, erythromycin, was isolated in 1952 from Streptomyces erythreus. Macrolide antibiotics share a similar molecular structure characterized by a 14-, 15-, or 16-membered macrocyclic lactone ring substituted with some particular sugars (Fig. 1). More recently, removal of sugar at position 3 and replacement with a 3-keto function together with introduction of a cyclic carbamate ring between positions 11 and 12, gave rise to the ketolides which have improved antibacterial activity, particularly against inducibly resistant strains. All macrolides are believed to act by causing the growing peptide chain to dissociate from the ribosome during the translocation step in bacterial protein synthesis.

Macrolides are most notable for their antistaphylococcal and antistreptococcal activity, though the spectrum encompasses other important pathogens, including chlamydiae, Mycoplasma pneumoniae, legionellae, and some mycobacteria; they lack useful activity against enterobacteria and P. aeruginosa. Resistance is common among staphylococci, but less so in streptococci. However, resistant Streptococcus pyogenes strains have a significant prevalence.

Figure 1. Chemical structure of macrolides

Macrolides have many attractive properties, are well-tolerated oral compounds and display good tissue penetration. Their spectrum of activity is particularly suitable for the treatment of respiratory and soft-tissue disease and for infections caused by susceptible intracellular bacteria. They are also used in campylobacter enteritis if the severity of infection warrants antimicrobial treatment, and in Legionella pneumophila pneumonia.
 


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