Susceptibility, resistance development, and synergy of antimicrobial combinations againstClostridium difficile

Studies of susceptibility, development of resistance, and synergy were performed with 32 antimicrobics againstClostridium difficile strains. A microtiter technique was used to demonstrate the in vitro activities of 40C. difficile isolates. Rifampin inhibited 73% of the strains at a concentration of ≤0.06 μg/ml, and metronidazole inhibited 90% at 0.5 μg/ml. Penicillin, vancomycin, cephaloridine, and chloramphenicol were also active. Development of resistance to penicillin or chloramphenicol was not found, but vancomycin, methicillin, and minocycline showed development of low levels of resistance. Rifampin in combination with erythromycin, methicillin, or penicillin was synergistic against more than 90% of theC. difficile isolates. Rifampin-ampicillin was synergistic against 85%, and vancomycin-metronidazole was synergistic against 68% of the isolates.