Susceptibility of the Bacteroides fragilis group to 10 antibiotics. Results of 4 laboratories in Austria

The susceptibility of 197 strains of the Bacteroides fragilis group (109 B. fragilis, 27 B. thetaiotamicron, 23 B. vulgatus, 1 B. distasonis, 11 B. ovatus and others) to penicillin G, cefoxitin, latamoxef, piperacillin, mezlocillin, clindamycin, metronidazole, tetracycline, doxycycline and chloramphenicol was tested by an agar dilution method. 4 microbiological laboratories in Austria participated in this investigation. In all laboratories the same method was used and a joint quality control carried out. The most active substances were metronidazole and clindamycin (MIC50 1.0 and 0.5 respectively; MIC90 4 mg/l for both). Tetracycline showed only little activity, whereas doxycycline inhibited a major portion of the strains at therapeutically relevant concentrations (MIC50 16 and 2 mg/l, MIC90 32 and 16 mg/l, respectively). The MIC90 of chloramphenicol was 8 mg/l. Piperacillin and mezlocillin were about equally active (MIC50 both 16, MIC90 32 and 64 mg/l, respectively). For penicillin G, the MIC values were also favourable, the MIC90 being 32 mg/l. The possible causes for the discrepancy between the good in vitro activity and the clinical ineffectiveness of penicillin G are discussed. The cumulative MIC values (50, 90%) for cefoxitin were 8 and 32 mg/l, respectively. The MIC's of latamoxef showed a wide distribution resulting in a low MIC50 (4 mg/l) and a high MIC90 (128 mg/l). This distribution was due to the differing susceptibility of the individual species within the Bacteroides fragilis group. For B. thetaiotaomicron the highest MIC values were determined. This species as a whole was less susceptible to the beta-lactam antibiotics tested. The non beta-lactam antibiotics showed no substantial species related differences. No relevant differences between the individual laboratories were observed.