Effects of inoculum, pH, and cations on the in vitro activity of fidaxomicin (OPT-80, PAR-101) against Clostridium difficile.

The effects of the inoculum, pH, cation concentrations, and different lots of commercial media on the in vitro susceptibility of Clostridium difficile to fidaxomicin were examined. Of the factors evaluated, only pH alterations influenced the activity of fidaxomicin against C. difficile, noticeably reducing its activity at higher pH (>= 7.9).