Lack of in vitro and in vivo selection of bacterial resistance by roxithromycin

The in vitro and in vivo selective pressure exerted by roxithromycin on Lan cefield group A P-haemolytic streptococci (GABHS) was investigated. In vitr o antimicrobial activity on fifty GABHS strains was determined by the micro dilution method and by boundary concentration (BC) determination. Insorgenc e of resistance was evaluated by redetermining MIC and BC after exposure to 16 x MIC roxithromycin concentration. In vivo insorgence of resistance was evaluated by MIC and BC determinations on the GABHS strain recovered from infected mice peritoneum, after treatment with 20 mg/kg roxithromycin, The roxithromycin serum kinetics was established in healthy and infected mice. Neither significant changes in GABHS MIC or BC after in vitro or in vivo ex posure to roxithromycin nor a difference in roxithromycin serum levels betw een healthy and infected mice were detected, suggesting that the roxithromy cin selection of resistance in GABHS is low. Copyright (C) 2000 S. Karger A G, Basel.