In vitro postantibiotic effect of roxithromycin and erythromycin against gram-positive cocci

A persistent suppression of bacterial growth following a brief exposure to an antibiotic (postantibiotic effect [PAE]) has been described for a variety of antibiotics and microorganisms. Data concerning PAE have not yet been demonstrated in Taiwan. In this study, the PAEs of erythromycin (EM) and roxithromycin (RXM) against 4 strains of Staphylococcus aureus, 3 of Streptococcus pyogenes and 1 of Streptococcus pneumoniae were tested. Dilution method was used for removal of drugs. Two controls, one free of drug and one in 1/1000 of the original drug concentrations were used. The results showed that the PAE lasted longer in strains of S. aureus (RXM 1.7-2.1 h; EM 1.7-2.1 h) and S. pyogenes (RXM 1.7-3.0 h; EM 1.9-2.9 h) than those in S. pneumoniae (RXM 0.6-1.6 h; EM 0.8-1.4 h). There was no difference of PAE between RXM and EM against each bacterial strain tested.