Susceptibility of Clostridium difficile to metronidazole assessed using the E-test:: influence of the culture medium.

The treatment of intestinal Clostridium difficile infections rests on administration of either a glycopeptide or metronidazole. Given the current shifts in resistance patterns of anaerobes to antimicrobials, a study of the susceptibility of C. difficile to metronidazole was timely. The objective of this study was to evaluate the influence of the culture medium on the Minimal Inhibitory Concentration (MIC) of metronidazole as determined using the E-test. Thirty-one strains were grown on three different media supplemented with 5% horse blood namely Columbia agar, Wilkens Chalgren agar, and Brucella agar. Results were compared to those obtained using the reference agar dilution method (ADM). As recommended by the French Society for Microbiology, susceptibility was defined as an MIC less than or equal to 4 mg/L. When used on strains susceptible by the ADM, the E-test yielded lower values than the ADM with all three media. Furthermore, findings suggest that E-test results obtained with strains whose MIC is in the 4 to 8 mg/L range by the ADM should be interpreted with caution and, in some cases, tested using the ADM.