Study on mutant prevention concentrations of roxithromycin and azithromycin for Staphylococcus aureus

AIM:To determine the mutant prevention concentrations (MPC) of roxithromycin and azithromycin for Staphylococcus aureus to amplify the resistance gene of Staphylococcus aureus mutants selected in the mutant selection window (MSW) so as to understand the mechanism of Staphylococcus aureus resistance to macrolides. METHODS:The Staphylococcus aureus strain LATCC29213 was enriched in broth and the bacterial concentrations were adjusted to 1010 colony forming units per milliliter. The minimal inhibitory concentration (MIC),MIC for 99% of input cells (MIC99),provisional MPC (MPCpr) and MPC of roxithromycin and azithromycin for Staphylococcus aureus were determined by agar plates dilution method. The ermA,ermB and ermC of Staphylococcus aureus mutants selected in the MSW were obtained by PCR method and sequencing. RESULTS:The MPC of roxithromycin and azithromycin for ATCC29213 was 0.24 μg/mL and 0.18 μg/mL and the MPC/MIC99 was 106 and 114. The ermA and ermC gene were detected in Staphylococcus aureus mutants selected in the MSW. CONCLUSION:Owing to the tendency of resistant mutant selection,roxithromycin and azithromycin should be administered in combination with other antibiotic agents.