Susceptibility testing of Clostridium difficile against metronidazole and vancomycin by disk diffusion and Etest

A prospective study on the susceptibility of Clostridium difficile to metronidazole and vancomycin using the Etest and disk diffusion test was performed over a 6-month period. One hundred strains were tested; one strain was highly resistant to metronidazole (MIC = 64 micrograms/mL). The zone size of inhibition by the disk diffusion test correlated with the MIC as determined by the Etest (regression coefficient = -0.043 for metronidazole and -0.044 for vancomycin, p < 0.001 for both antibiotics). However, the correlation coefficient was low for both metronidazole (r = 0.574) and vancomycin (r = 0.473); hence the zone of inhibition by disk diffusion test could not predict the MIC satisfactorily. Metronidazole is still the first-line antibiotic for the treatment of C. difficile-associated diarrhea because the incidence of metronidazole resistant strains remains very low. However, the efficacy of metronidazole in the treatment of infections attributable to isolates with high-level metronidazole resistance may be compromised because the fecal concentration of metronidazole is relatively low when compared with the MIC values of the less susceptible strains. Oral vancomycin is the drug of choice under such circumstances, as its fecal concentration is much higher than that of metronidazole.