Development of a EUCAST disk diffusion method for the susceptibility testing of rapidly growing anaerobic bacteria using Fastidious Anaerobe Agar (FAA): a development study using Bacteroides species.

Objectives: Antimicrobial resistance among anaerobic bacteria is increasing, leading to a growing demand for inexpensive and reliable susceptibility testing methods. The aim of this study was to determine the suitability of Fastidious Anaerobe Agar (FAA) as a medium for disk diffusion for rapidly growing anaerobic bacteria.

Methods: Reproducibility of zone diameters and quality of growth were tested using six quality control (QC) strains. We compared four anaerobic incubation systems, two incubation temperatures (35°C and 37°C), and FAA from four manufacturers. The effect of incubation for 16-20 hours instead of 24 hours was tested on ten randomly selected isolates of the Bacteroides fragilis group. The final method was tested on 170 clinical B. fragilis-group isolates and compared to agar dilution MICs.

Results: After 24 hours' incubation, all QC strains demonstrated confluent growth. The different anaerobic incubation systems were equal regarding quality of growth and zone diameters. Incubation at 35°C resulted in slightly larger zones (1-2 mm) than at 37°C. Except for Acumedia FAA, the different manufacturers showed good agreement in zone diameters. All B. fragilis-group isolates displayed confluent growth after 16-20 hours. Metronidazole inhibition zones correlated well with the reference MICs. There was an area of poorer separation for meropenem and piperacillin-tazobactam between 19-27 and 14-23 mm respectively. Prolonged incubation (40-44 h) of clindamycin resulted in better separation and the area of overlap was reduced from 13 to 8 mm compared with 16-20 hours' incubation.

Conclusion: FAA is a suitable medium for disk diffusion of these rapidly growing anaerobic bacteria.