Serum Bactericidal Activity of the Methoxyfluoroquinolones Gatifloxacin and Moxifloxacin against Clinical Isolates of Staphylococcus Species: Are the Susceptibility Breakpoints Too High?

Healthy volunteers received a single dose of gatifloxacin and moxifloxacin (400 mg each), and serum samples were obtained from these volunteers over a 24-h period. Prolonged (> or =12 h) serum bactericidal activity (SBA) was observed for both agents against staphylococcal isolates with minimum inhibitory concentrations (MICs) of gatifloxacin of < or =0.5 mug/mL. In strains with gatifloxacin MICs of 1.0 mug/mL, SBA was observed for < or =6 h, and, for isolates with gatifloxacin MICs of 2.0 mug/mL, little or no SBA was observed for either drug. The relative lack of SBA against less susceptible strains of staphylococci suggests that the current susceptibility breakpoint concentration (MIC, 2.0 mug/mL) for these methoxyfluoroquinolones against Staphylococcus is too high.