Routine disc diffusion antimicrobial susceptibility testing of Clostridium difficile and association with PCR ribotype 027.

Reduced susceptibility to metronidazole and vancomycin in Clostridium difficile has been reported, which emphasises the need for simple antimicrobial susceptibility testing methods. The aim of this study was to apply a published disc diffusion method and zone diameter breakpoint correlates to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) epidemiological minimum inhibitory concentration (MIC) cut-off values in a routine setting. Metronidazole and vancomycin zone diameters from 2702 isolates were recorded. Fifteen isolates had a metronidazole zone diameter below the published breakpoint (<23 mm) and five isolates had a vancomycin zone diameter below the published breakpoint (<19 mm), most of which were polymerase chain reaction (PCR) ribotype 027. The total number of PCR ribotype 027 was 29 (1.1 %). Overall, C. difficile PCR ribotype 027 isolates had smaller zone diameters than non-027 isolates. The disc diffusion method is very simple and inexpensive, and the published zone diameter breakpoints will detect C. difficile isolates with reduced susceptibility to metronidazole and vancomycin.