Research on Mutant Prevention Concentration of Vancomycin and Linezolid Against Methicillin-resistant Staphylococcus Aureus

Objective To determine the antibacterial activity and the mutant prevention concentrations (MPCs) of vancomycin and linezolid to clinical methicilin-resistant Staphylococcus aureus (MRSA) isolates,and to compare the capacity of two antibiotics for preventing the resistant mutant strains.Methods The minimal inhibitory concentrations (MICs) of vancomycin and linezolid against 122 MRSA clinical isolates were measured by 2-fold agar dilution method according to the criteria from the Clinical and Laboratory Standard Institute (CLSI).The MPCs of the two antibiotics against MRSA were measured by 2-fold increments agar plate dilution method.Results The MICs for 90% of the isolates (MIC90) of vancomycin and linezolid to 122 MRSA clinical isolates were both 2.0 μg/ml,the MICs for 50% of the isolates (MIC50) were 1.0 μg/ml and 2.0μg/ml,respectively,and the ranges of MICs were 0.5～4.0 μg/ml and 1.0～4.0 μg/ml,respectively.The MPCs for 90% of the isolates (MPC90) of vancomycin and linezolid for 122 MRSA clinical isolates were 22.4 μg/ml and 5.6 μg/ml,and the selective indexes (SI=MPC90/MIC90) were 11.2 and 2.8,respectively.Conclusions Both vancomycin and linezolid exhibit excellent activity against the MRSA isolates,the two antibiotics are not easy to select resistant mutants of MRSA.The ability of linezolid for restricting the selection of resistant mutants of MRSA is stronger than that of vancomycin.