Investigation of whether CLSI broth microdilution method is applicable for MICs Determination of Enterococcus species

The broth microdilution (BMD) method is an antimicrobial susceptibility testing method defined as a guideline by the Clinical and Laboratory Standards Institute (CLSI). To date, the Japanese Veterinary Antimicrobial Resistance Monitoring System (JVARM), has adopted the agar dilution (AD) method for monitoring antimicrobial resistances targeting isolates of Enterococcus spp., found in the fecal flora of food-producing animals, as indicator bacteria. However, the AD method is tedious, and time-consuming. In order to examine whether it could be replaced with the BMD method, the two methods were compared in terms of the correlation of MICs. In this study, the BMD results agreed with the AD results within ±1 log(2) dilutions in 72.3% of cases, except for the antimicrobial feed additive, Nosiheptide (NHT). Similarly, except for NHT, the MIC(50) and MIC(90) values obtained by the two methods were well correlated. In conclusion, our results indicate that the BMD method might be suitable for antimicrobial susceptibility testing targeting Enterococcus spp..