Comparison of mutant prevention concentrations of fluoroquinolones against methicillin-resistant and susceptible Staphylococcus aureus isolated from animal

In order to compare the selection capacity on resistant mutant strains of 4 kinds of fluoroquinolones against methicillin-susceptible(MSSA) and methicillin-resistant(MRSA) S.aureus isolated from animal,the minimum inhibitory concentration(MIC) and mutant prevention concentration(MPC) of ciprofloxacin,levofloxacin,gatifloxacin,and gemifloxacin for MSSA and MRSA were determined by agar plate dilution methods.The results showed that for 85 MSSA strains,the rank order of potency based on MPC90 values were ciprofloxacin(4 mg/L) levofloxacin(2 mg/L) gatifloxacin(0.5 mg/L) =gemifloxacin(0.5 mg/L).The selective index(MPC90/MIC90) of gatifloxacin was minimal.For 21 MRSA strains,the MPC90 of ciprofloxacin,levofloxacin,gatifloxacin and gemifloxacin were 128 mg/L,64 mg/L,16 mg/L and 8 mg/L respectively and the selective index of gemifloxacin was minimal.The ability of gatifloxacin for restricting the selection of MSSA resistant mutants was stronger than that of other fluoroquinolones.Gatifloxacin and gemifloxacin may restrict the selective enrichment of resistant mutants among MSSA whenever they are administered as mono-therapy.But for MRSA strains,four fluoroquinolones will select resistant mutants whenever they are administered as mono-therapy.