Susceptibility of Anaerobic Bacteria to Ten Antimicrobial Agents

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Abstract
The susceptibility pattern of 265 anaerobic bacteria from clinical isolates to 10 antimicrobial agents was investigated by the agar dilution technique. Penicillin G, in a concentration of 16 μg/ml, was active against most organisms, important exceptions being 12% of Bacteroides melaninogenicus and 24% of B. fragilis strains. The susceptibility of strains to ampicillin was similar to their susceptibility to penicillin G. Carbenicillin, at ≤128 μg/ml, inhibited all but a few strains. Cefamandole was less active than the penicillins; 82% of B. melaninogenicus , 32% of B. fragilis , and 75% of Fusobacterium strains were inhibited by ≤16 μg/ml. A trend towards tetracycline resistance was seen in many bacterial groups, especially Bacteroides, Fusobacterium , and Clostridium . All organisms were susceptible to chloramphenicol and clindamycin in concentrations of ≤16 μg/ml and ≤4 μg/ml, respectively. Erythromycin was less active than clindamycin against all strains tested. Metronidazole and tinidazole were active against most anaerobes, but resistance of a few strains in each group was encountered. The increased resistance of B. melaninogenicus strains to penicillin, and emergence of anaerobes resistant to >16 μg of imidazole per ml may have therapeutic implications.