In vitro susceptibility of anaerobic bacteria to nitroimidazoles

The in vitro susceptibility of 355 Swedish clinical isolates of anaerobic bacteria to three nitroimidazole drugs was determined by the agar dilution method. The drugs were metronidazole, tinidazole and ornidazole. In addition the in vitro susceptibility of 32 strains of clostridium difficile to metronidazole was determined. Gram-negative rods and clostridia were inhibited by 4 microgram/ml or less of all three compounds except one strain of C. difficile, which was not inhibited by 16 microgram/ml of metronidazole. Against strains of the Bacteroides fragilis group, tinidazole was slightly more active than metronidazole and ornidazole. Only about 70% of anaerobic cocci (peptococci and peptostreptococci) were inhibited by 4 microgram/ml of the three drugs, whereas the remaining isolates were resistant (minimum inhibitory concentration greater than or equal to 8 microgram/ml). Anaerobic streptococci were not included in this material. Actinomyces, arachnia and propionibacteria were resistant to the nitroimidazole drugs. At achievable serum levels, nitroimidazoles inhibited the majority of anaerobic bacteria found with greatest frequency in anaerobic infections.