In vitro susceptibility of Clostridium difficile to rifaximin and rifampin in 359 consecutive isolates at a university hospital in Houston, Texas

Aim This was an in vitro study to analyse the susceptibility of Clostridium difficile isolates to rifampin and rifaximin. Methods Stool samples from patients who had nosocomial diarrhoea and C difficile toxin B at a university hospital between August 2006 and December 2007 were cultured for C difficile. Susceptibility of C difficile isolates to rifaximin and rifampin was determined by agar dilution and E strips, respectively. C difficile isolates were analysed via PCR for genes encoding toxins A and B, for binary toxin (BT), and for partial deletions of the tcdC gene (tcdC-del). Results Rifaximin exhibited high-level activity against 359 C difficile isolates, with MIC50 < 0.01 mu g/ml and MIC90 0.25 mu g/ml; rifampin had MIC50 < 0.002 mu g/ml and MIC90 4 mu g/ml. Among isolates analysed, 55 (15%) were positive for BT and tcdC-del. 28 (8% of 359) isolates were resistant to rifampin (>= 32 mu g/ml), of which 6 (2% of 359) were resistant to rifaximin and rifampin with MIC values >= 32 mu g/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. Conclusions The study demonstrates that rifaximin has high-level activity against C difficile in vitro. Determination of resistance to rifampin by E strip did not predict rifaximin resistance.