Potentially synergistic antimicrobial combinations with metronidazole against Bacteroides fragilis

Synergy studies were performed with metronidazole in combination with clindamycin, rifampin, nalidixic acid, ticarcillin, erythromycin, and spectinomycin against Bacteroides fragilis strains. An agar dilution technique was used with two inoculum sizes (10(6) to 10(7) or 10(3) to 10(4) colony-forming units per ml). None of the drugs showed synergy with metronidazole against greater than 50% of the strains, and the synergistic effect was generally less at the larger inoculum. Of the six drugs tested, nalidixic acid, clindamycin, and rifampin showed the most synergy with metronidazole. A partially synergistic or additive effect was the usual interaction occurring between metronidazole and each of the other drugs and no antagonism was seen.