In vitro activities of levofloxacin, gatifloxacin, moxifloxacin and garenoxacin against Bacteroides fragilis strains evaluated by kill kinetics.

This study was designed to investigate the killing activity of levofloxacin, gatifloxacin, moxifloxacin and garenoxacin against 12 Bacteroides fragilis strains by kill kinetics over time. MIC values were determined by Etest and by agar dilution. B. fragilis strains were divided according to their MIC values into two groups: one group with strains with MIC <8.0 µg ml(-1) and one group with strains with MIC ≥ 8.0 µg ml(-1). For kill kinetics over time, the strains with MIC <8.0 µg ml(-1) were incubated with the antibiotics at 0.5, 1, 2 and 4 times their MIC values. The strains with MIC ≥ 8.0 µg ml(-1) were incubated with 0.5, 1, 2, and 4 times the maximum achievable concentrations of the antibiotics in human plasma (Cmax). Among the strains with MIC <8.0 µg ml(-1) levofloxacin and gatifloxacin showed equal efficacy. The growth of the strains with MIC ≥ 8.0 µg ml(-1) was barely affected by levofloxacin, while gatifloxacin had bactericidal action when concentrations of 4 × C max were used. Moxifloxacin was more effective against both groups of strains compared with levofloxacin and gatifloxacin. Garenoxacin was the most active agent against all strains investigated. Due to the varying in vitro activity of the quinolones against obligate anaerobes the treatment with quinolones of patients with intra-abdominal infections needs intensive scrutiny.