Antimicrobial Susceptibilities of Anaerobic Bacteria: Recent Clinical Isolates

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Abstract
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 μg/ml. Only 58% of other clostridial species were inhibited by this concentration of clindamycin. Minocycline at 3.1 μg/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 μg/ml inhibited all bacteria tested. B. fragilis was resistant to both penicillin and carbenicillin at 6.2 μg/ml. Concentrations of 25 μg/ml for penicillin and 100 μg/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.