Effect of single and combination of fluoroquinolones in vitro on mutant selection window of staphylococcus aureus

Objective To determine the mutant prevention concentration(MPC)of 2 fluoroquinolones on staphylococcus aureus isolates,compare MPC and MPC/MIC(selective index,SI),and study the effect of the drug combination on the mutant selection window(MSW) in vitro.Methods The MIC for staphylococcus aureus isolates and quality control(ATCC25923) were determined by the standard 2 fold agar dilution method.The cells of 3×1010colony forming units per milliliter staphylococcus were enriched in broth.The MPC of fluoroquinolones against the strains were determined by agar dilution method.The value of MIC50,MIC90,MPC50,MPC90 and MPC/MIC was calculated.SPSS was used to analyze the data.Results The MPC90 and MPC90/MIC90 of levofloxacin were 16 mg·L-1 and 8.Moxifloxacin had lower MPC90 and MPC90/MIC90(2 mg·L-1 and 4 respectively).The Cmax/MPC90 and AUC/MPC90 values were 2.3 and 18.The Cmax/MPC90 and AUC/MPC90 values of levofloxacin 500 mg once daily were 0.5 and 2.4,higher than 300 mg once daily.Conclusion Moxifloxacin exhibits lower MPC,narrower MSW and better ability of resistant mutants than levofloxacin.The regimen like levofloxacin 500 mg once daily has advantage in resistance mutation prevention.