In vitro susceptibility of genotypically distinct and clonal Clostridium difficile strains to oritavancin

Objectives: Clostridium difficile infection is a nosocomial disease of increasing importance. First-line treatment is limited to metronidazole or vancomycin. Oritavancin is a lipoglycopeptide with activity against Gram-positive bacteria, including drug-resistant pathogens. MICs of oritavancin, metronidazole and vancomycin for genotypically distinct C. difficile strains, including epidemic C. difficile PCR ribotypes 001 and 027, were determined by agar incorporation and broth macrodilution methods. In agar incorporation methods, the impact of supplements on oritavancin MICs was tested to address oritavancin binding to surfaces. Methods: Thirty-three genotypically distinct C. difficile strains were identified by PCR ribotyping. Wilkins Chalgren agar incorporation plates containing oritavancin, metronidazole and vancomycin were prepared with and without 0.002% polysorbate-80 (P80) and lysed horse blood (2%). Broth macrodilution MICs of oritavancin, metronidazole and vancomycin were determined in Brucella broth. Inoculated agar incorporation plates and broth macrodilution tubes were cultured anaerobically at 37 degrees C for 48 h. Results: Broth macrodilution MICs were lower than agar incorporation MICs for oritavancin, but not for metronidazole and vancomycin. Oritavancin broth macrodilution MIC(90)s were 2-to 4-fold lower than the corresponding agar incorporation MIC90s, while geometric mean MICs were > 5-fold lower. Oritavancin broth macrodilution MIC90s were similar to 2-and 5-fold lower than those for metronidazole and vancomycin. Metronidazole was the most active antimicrobial agent against C. difficile using agar incorporation methods. Oritavancin agar incorporation MIC90s were unaffected by 0.002% P80 and/or 2% lysed horse blood. Conclusions: Oritavancin was at least 4-fold more potent than vancomycin against the majority (25/33) of C. difficile strains tested by broth macrodilution. Oritavancin activity may be underestimated by agar incorporation methods, regardless of the use of P80 or lysed horse blood.