Antibacterial activities of nemonoxacin against clinical isolates of Staphylococcus aureus: an in vitro comparison with three fluoroquinolones.

In comparison with ciprofloxacin, levofloxacin and moxifloxacin, antimicrobial activity of nemonoxacin against ciprofloxacin-susceptible/-resistant methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) was determined with the availability to select resistant mutants evaluated. Minimum inhibitory concentrations and mutant prevention concentrations of quinolones were determined by agar dilution method, that concentrated bacterial cells were spread onto Mueller-Hinton agar plates containing antibacterials at different concentrations. Selection index (SI) was calculated. Minimum inhibitory concentration and mutant prevention concentration of nemonoxacin were 0.063 and 0.25 μg/mL for ciprofloxacin-susceptible MSSA and those were 0.5 and 4.0 μg/mL for ciprofloxacin-resistant MSSA, lower than observations of three fluoroquinolones distinctly. SI of nemonoxacin and moxifloxacin were similar, with narrower mutant selective window than levofloxacin and ciprofloxacin. Minimum inhibitory concentration and mutant prevention concentration of nemonoxacin were 0.25 and 2.0 μg/mL for ciprofloxacin-susceptible MRSA, which were 0.5 and 16.0 μg/mL for ciprofloxacin-resistant MRSA. Values were lower than those determined from fluoroquinolones. Nemonoxacin presents good antimicrobial activity against clinical isolates of S. aureus, especially for ciprofloxacin-resistant strains. But stepwise mutant accumulation of ciprofloxacin-resistant MRSA can be hardly inhibited by nemonoxacin with pharmacokinetic parameters considered.