Comparative in vitro activities of ciprofloxacin, enoxacin, norfloxacin, ofloxacin and pefloxacin against Bacteroides fragilis and Clostridium difficile.

The in vitro activity of ciprofloxacin, enoxacin, norfloxacin, ofloxacin and pefloxacin against 99 Bacteroides fragilis strains and 105 Clostridium difficile strains were determined by the agar dilution method. Ofloxacin was the most potent agent. The MIC for 90% of the B. fragilis and C. difficile strains was 8 mg/l. Ciprofloxacin had MIC90's of 16 mg/l for B. fragilis and C. difficile. The MIC90's of pefloxacin against B. fragilis and C. difficile were 32 mg/l. 90% of the B. fragilis strains were inhibited by 32 mg/l enoxacin, while the MIC90's of enoxacin as well as of norfloxacin against C. difficile were 64 mg/l. The lowest activity against B. fragilis was obtained with norfloxacin (MIC90's of 128 mg/l). Since serum levels of these agents are reported to be between 1 and 3 mg/l after orally administered doses, B. fragilis and C. difficile should be regarded as insusceptible to these quinolone compounds.