Abstract

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 μg/ml and MIC₉₀ 0.25 μg/ml; rifampin had MIC₅₀ <0.002 μg/ml and MIC₉₀ 4 μg/ml. Among isolates analysed, 55 (15%) were positive for BT and tcdC-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.

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.