Postantibiotic effect of roxithromycin on streptolysin O production, hydrophobicity, and bactericidal activity of...

Exposure of Streptococcus pyogenes to 5 x minimum inhibitory concentration of roxithromycin for 1 h produced a significant postantibiotic effect. More than 2.5 h was necessary for roxithromycin-treated bacteria to increase by 1 log10 in colony-forming units after drug removal, compared with the unexposed cells. After exposure to and removal of the drug, treated cells failed to exhibit normal hemolytic activity for at least 4 h. The inhibitory effect persisted for 20 h after drug removal, although the extent of growth for treated and untreated cells was almost the same. Hydrophobicity of treated cells, studied throughout the logarithmic growth phase with a water-hexadecan two-phase system, was markedly decreased by 40%, compared with untreated cells 4 h after drug removal. Cells that had been treated with roxithromycin became more susceptible to the bactericidal activity of human PMNL than untreated bacteria. The data indicate that some of the metabolic activity that contributes to the virulence of S. pyogenes is affected by postexposure to roxithromycin, and its minimum inhibitory concentration and serum level might not be the best indicators of efficacy in this class of drugs.