Antimicrobial Susceptibilities of Anaerobic Bacteria: Recent Clinical Isolates

Minimal inhibitory concentrations of clindamycin, minocycline, metronidazole, penicillin, and carbenicillin were determined by agar dilution against 150 recent clinical isolates of anaerobic bacteria. Ninety-nine percent of Bacteroides fragilis and all B. melaninogenicus, Clostridium perfringens, and Fusobacterium were inhibited by clindamycin at 3.1 mug/ml. Only 58% of other clostridial species were inhibited by this concentration of clindamycin. Minocycline at 3.1 mug/ml inhibited 72% of C. perfringens, 81% of other Clostridium species, and 66, 75, and 100% of B. fragilis, B. melaninogenicus, and Fusobacterium, respectively. Metronidazole at 12.5 mug/ml inhibited all bacteria tested. B. fragilis was resistant to both penicillin and carbenicillin at 6.2 mug/ml. Concentrations of 25 mug/ml for penicillin and 100 mug/ml for carbenicillin were needed to inhibit more than 90% of B. fragilis. Organisms other than B. fragilis were moderately or extremely susceptible to the penicillins.