Divergent rifamycin susceptibilities of Clostridium difficile strains in Canada and Italy and predictive accuracy of rifampin Etest for rifamycin resistance.

We tested the activities of rifampin (RIF) and rifaximin (RFX) against 180 Clostridium difficile clinical isolates selected from Canadian and Italian culture collections. MICs were determined by CLSI agar dilution for both drugs and by Etest for RIF. Sixteen of 85 Italian isolates (18.8%) showed high-level resistance to both rifamycins (MICs, >16 μg/ml), compared to 2 of 95 (2.1%) Canadian isolates. Two new rpoB mutations were identified in rifamycin-resistant isolates. RIF susceptibility by Etest correlated completely with susceptibility to both rifamycins determined by agar dilution.