In vitro susceptibility ofClostridium difficileto rifaximin and rifampin in 359 consecutive isolates at a university hospital in Houston, Texas

Abstract

AimThis was an in vitro study to analyse the susceptibility ofClostridium difficileisolates to rifampin and rifaximin.MethodsStool samples from patients who had nosocomial diarrhoea andC difficiletoxin B at a university hospital between August 2006 and December 2007 were cultured forC difficile. Susceptibility ofC difficileisolates to rifaximin and rifampin was determined by agar dilution and E strips, respectively.C difficileisolates were analysed via PCR for genes encoding toxins A and B, for binary toxin (BT), and for partial deletions of thetcdCgene (tcdC-del).ResultsRifaximin exhibited high-level activity against 359C difficileisolates, with MIC50<0.01 μg/ml and MIC900.25 μg/ml; rifampin had MIC50<0.002 μg/ml and MIC904 μg/ml. Among isolates analysed, 55 (15%) were positive for BT andtcdC-del. 28 (8% of 359) isolates were resistant to rifampin (≥32 μg/ml), of which 6 (2% of 359) were resistant to rifaximin and rifampin with MIC values ≥32 μg/ml. 2 of the 28 isolates resistant to rifampin were A+/B+/BT+/tcdC-del+, 5 were A+/B+/BT−/tcdC-del+, 4 were A+/B+/BT+/tcdC-del−, 13 were A+/B+/BT−/tcdC-del−, and 4 had no detectable toxin genes. Of the 11 isolates resistant to rifaximin alone, 1 was A+/B+/BT−/tcdC-del+, 2 were A+/B+/BT+/tcdC-del−, 6 were A+/B+/BT−/tcdC-del−, and 2 had no detectable toxin genes.ConclusionsThe study demonstrates that rifaximin has high-level activity againstC difficilein vitro. Determination of resistance to rifampin by E strip did not predict rifaximin resistance.