[In vitro effect of piperacillin, amoxicillin, cefoxitin and metronidazole against obligate anaerobic bacteria].

The minimum inhibitory concentrations (MIC) of piperacillin and three other antibiotics (amoxicillin, cefoxitin, metronidazole) against 165 strains of obligate anaerobes isolated in hospitals in 1985 and 1986 were determined by the Wilkins-Chalgren agar medium dilution method. Among the 67 strains of Bacteroides fragilis, 98.5% were sensitive to piperacillin (MIC less than or equal to 128 mg/l), 100% were sensitive to metronidazole (MIC less than or equal to 4 mg/l), 11.9% were resistant to cefoxitin (MIC greater than 32 mg/l) and 46% were resistant to amoxicillin (MIC greater than 16 mg/l). The MIC50 was about 4 mg/l with piperacillin, 8 mg/l with cefoxitin and 0.5 mg/l with metronidazole. All 32 strains of Clostridium perfringens were sensitive to the 4 antibiotics tested, with an MIC50 of about 0.06 mg/l with piperacillin, 0.125 mg/l with amoxicillin and 0.5 mg/l with cefoxitin and metronidazole. All other strains of Clostridium spp. (33 in all) were sensitive to piperacillin and metronidazole; the MIC's were about 0.06 mg/l with piperacillin, 0.25 mg/l with amoxicillin and metronidazole and 0.5 mg/l with cefoxitin. Seven strains (C. innocuum 5, C. difficile 1, C. ramosum 1) were resistant to cefoxitin. All 33 strains of Peptostreptococcus and Peptococcus were sensitive to the 4 antibiotics tested. Although metronidazole still is the most effective of anti-anaerobes agents, piperacillin has proved to be effective against most anaerobic bacteria, including B. fragilis. It is clearly superior to amoxicillin and relatively superior to cefoxitin, notably against this species.