POSTANTIBIOTIC EFFECT OF ROXITHROMYCIN ON STREPTOLYSIN-O PRODUCTION, HYDROPHOBICITY, AND BACTERICIDAL ACTIVITY OF PMNL BY STREPTOCOCCUS-PYOGENES

Exposure of Streptococcus pyogenes to 5 x minimum inhibitory concentra tion of roxithromycin for 1 h produced a significant postantibiotic ef fect. More than 2.5 h was necessary for roxithromycin-treated bacteria to increase by 1 log(10) in colony-forming units after drug removal, compared with the unexposed cells. After exposure to and removal of th e drug, treated cells failed to exhibit normal hemolytic activity for at least 4 h. The inhibitory effect persisted for 20 h after drug remo val, 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 m ore susceptible to the bactericidal activity of human PMNL than untrea ted bacteria. The data indicate that some of the metabolic activity th at contributes to the virulence of S. pyogenes is affected by postexpo sure to roxithromycin, and its minimum inhibitory concentration and se rum level might not be the best indicators of efficacy in this class o f drugs.