OBJECTIVES: The EUCAST disk diffusion method on Fastidious Anaerobe Agar with horse blood (FAA-HB) was developed for antimicrobial susceptibility testing (AST) of common anaerobic bacteria. This study evaluated its performance in routine clinical microbiology laboratories using FAA-agars and disks from various manufacturers. METHODS: Thirty isolates of Bacteroides spp., Prevotella spp., Fusobacterium necrophorum, and Clostridium perfringens were tested using FAA-HB agar manufactured by Liofilchem, EO Labs, and Xebios Diagnostics according to EUCAST manual. Cutibacterium acnes, another common anaerobic bacterium for which EUCAST provides breakpoints, was not included due to insufficient growth on the agar in preliminary testing. Six laboratories participated, each selecting at least one manufacturer. Agar dilution MICs, interpreted with EUCAST breakpoints, served as the reference. Categorical agreement (CA), very major (VMD) and major discrepancy (MD) between disk diffusion and reference method were calculated. RESULTS: For F. necrophorum and C. perfringens, the CA for all antibiotics across FAA-HB agars was above 90% (range between agar manufacturers 91.2%-100%). For Bacteroides spp., CA for clindamycin, metronidazole, and meropenem, exceeded 85% across all agars, with VMD for clindamycin range between 3.2 to 7.9.%. CA for piperacillin/ tazobactam and amoxicillin/ clavulanic acid for Bacteroides spp. was between 55.5% and 61.8%, and 84.9% to 93.1%, respectively, with VMD for amoxicillin/ clavulanic acid for Bacteroides spp. varied between 5.6% and 8.1%%. . For Prevotella spp., 30-40% of disk diffusion inhibition zones were uninterpretable (i.e. no inhibition zone could be read) across tested antimicrobial-organism combinations. After exclusion of these unreadable results, FAA-HB performance was > 90% (range 91.7%-100%). CONCLUSION: In general, FAA-HB demonstrated acceptable performance for disk diffusion AST of frequently isolated anaerobic species. However, its performance was not acceptable for beta-lactam/ beta-lactamase inhbitors, especially for piperacillin/tazobactam in Bacteroides spp., and its use is limited in Prevotella spp. due to frequently insufficient growth.
Yusuf et al. (Fri,) studied this question.