Comparison of Clostridium difficile minimum inhibitory concentrations obtained using agar dilution vs broth microdilution methods

Due to increasing antibiotic resistance among anaerobic bacteria, routine antimicrobial susceptibility testing is recommended by the Clinical and Laboratory Standards Institute (CLSI). This study compared the minimum inhibitory concentrations (MICs) from 920 Clostridium difficile isolates tested against seven antimicrobial agents using the two current CLSI reference methodologies, agar dilution method, vs broth microdilution method. A subset of isolate testing was performed independently by two laboratories to evaluate reproducibility. A negative bias was noted for MICs generated from broth microdilution compared to agar dilution and the reproducibility was variable and drug dependent. Therefore, broth microdilution is not recommended as an alternative to agar dilution for C. difficile antimicrobial susceptibility testing.