Comparative Susceptibilities of Anaerobic Bacteria to Metronidazole, Ornidazole, and SC-28538

The susceptibilities of 284 anaerobic bacteria, including 55 strains of the Bacteroides fragilis group, were determined by an agar dilution technique to metronidazole and two newer nitroimidazoles, ornidazole and SC-28538. All three agents showed marked in vitro activity against virtually all anaerobic bacteria tested. At concentrations </=1 mug/ml, SC-28538 was significantly more active than either metronidazole or ornidazole. At concentrations of >1 mug/ml, the activities of all three agents were comparable. Propionibacterium and Actinomyces showed significant resistance to all three agents. Anaerobic and microaerophilic members of the genus Streptococcus were also often resistant, in contrast to Peptococcus and Peptostreptococcus strains. In addition, the bactericidal activities of ornidazole and SC-28538 were determined against 27 strains of the B. fragilis group by a broth dilution technique. The minimal inhibitory and minimal bactericidal concentrations of each agent were very close. At concentrations of </=0.5 mug/ml, SC-28538 showed greater bactericidal activity; at concentrations of >/=2 mug/ml, the activies of both agents were similar.