Comparison of antimicrobial susceptibility testing of <i>Clostridium perfringens</i> by agar dilution and broth microdilution methods

In the poultry industry, antimicrobials are commonly used to prevent and treat bacterial infections. However, antimicrobial use in food-animal production has been implicated in the emergence of drug resistance in both pathogenic and non-pathogenic microorganisms. The Clinical and Laboratory Standards Institute (CLSI) suggests using the agar dilution method when testing for the antimicrobial susceptibility (AMS) of <i>Clostridium perfringens</i>, the causative agent of necrotic enteritis. Yet multiple studies on <i>C. perfringens</i> use the microdilution method, which is faster. Using the standard agar dilution and the microdilution methods, we obtained and compared the minimal inhibitory concentrations values of 64 field isolates of <i>C. perfringens</i> for 11 antimicrobials (amoxicillin, bacitracin, ceftiofur, erythromycin, monensin, narasin, penicillin, salinomycin, tetracycline, tylosin, and virginiamycin). Given the high variability in the values obtained for most of the antimicrobials by the microdilution method, we could not calculate the correlation coefficient between the 2 antimicrobial susceptibility techniques. Our study highlights the importance of following CLSI protocols when testing the AMS of <i>C. perfringens.</i>