Susceptibility of anaerobic bacteria to five antimicrobial agents and demonstration of resistance of Bacteroides fragilis to clindamycin.

Minimal inhibitory concentrations (MICs) of penicillin G, cefamandole, tetracycline, clindamycin and metronidazole were determined against 42 isolates of anaerobic bacteria. metronidazole was the most active antimicrobial agent against all anaerobic bacteria tested and especially against the Bacteroides fragilis group (MICs for all strains ranged from less than equal to 0.5 to 4 micrograms/ml). Seventeen out of 20 strains of B. fragilis subsp. fragilis were resistant to clindamycin with an MIC ranging between 8 and 128 micrograms/ml. Such resistance of B. fragilis strains should alarm physicians, after the widespread use of clindamycin, since this group of organisms is the most frequently isolated from anaerobic infections.