taken into consideration. Studying in vitro activity of rifaximin against C. difficile, Ripa et al. (1987) previously postulated the occurrence of chromosomal resistance against rifaximin caused by mutation. In agreement with these authors, we found a good correlation between rifaxi- min and rifampicin susceptibility testing in vitro, under- pinning the data obtained by O’Connor et al. (2008). However, these results seem to be in contrast to the findings of Jiang et al. (2010), who could not predict rifaximin resistance by doing only rifampicin resistance testing. Comparison of our data and those of O’Connor et al. (2008) with the study by Jiang et al. (2010) suggests that the use of acetone as solvent for rifaximin and rifampicin by Jiang’s group is the reason for the differences. To answer this question future studies should be undertaken. However, in our opinion, in vitro susceptibil- ity testing of rifampicin can be used to predict resistance to rifaximin when done according to our method. Rifaximin exhibits high activity against C. difficile in vitro, with a very low MIC50 and a very high MIC90 (0.032 mg ml21 and 1024 mg ml21, respectively). By using our suggested breakpoints (Table 4) as criteria for in vitro resistance, the prevalence of resistance was 7.5 % for all clinical isolates tested. Among the so-called hypervirulent ribotype RT 027, the prevalence of resistance was as high as 26 %. These data suggest that the development of resistance is in some way dependent on the PCR ribotype investigated; this is in concordance with general findings that some strains can become resistant more easily than others, e.g. C. difficile PCR ribotype 027, Mycobacterium tuberculosis spoligotype Beijing. Susceptibility testing in the microbiology laboratory there- fore could have an impact on the care and outcome of patients with infection (Doern et al., 1994). The rifaximin agar dilution and broth microdilution test systems facilitate reading of MICs and are often considered as gold standards for susceptibility testing. However, susceptibility testing by the disc diffusion method has the advantages of simplicity, low cost and a high degree of flexibility in the selection of agents tested (Richter & Ferraro, 2007). Our results show that rifaximin – despite its water-insolubility – may be a suitable candidate for disc diffusion testing. Whether 40 mg per disc is the ideal concentration must be examined in further studies. Whether it was prudent to officially license a substance ‘for treatment of all enteric infections caused by susceptible organisms’ also remains to be answered. 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