Fastidious anaerobe agar (FAA) as a suitable medium for antimicrobial susceptibility testing (AST) of anaerobes by agar dilution

Agar dilution is the reference method for antimicrobial susceptibility testing (AST) of anaerobes, but currently the only verified and published method utilises supplemented brucella agar, which is not readily available in the UK.

As FAA is often the medium of choice for primary culture of anaerobes in clinical laboratories, the aim of this study was to investigate the suitability of this medium as an alternative to brucella agar for the reference method.

One hundred isolates submitted to the UKARU, all with pre-determined MICs to: clindamycin; meropenem; metronidazole; penicillin and doxycycline, were tested.

MICs were obtained by agar dilution using both in-house produced FAA, supplemented with 5% defibrinated horse blood, and supplemented brucella blood agar (BRU).

End points were selected according to CLSI guidance and interpreted using EUCAST clinical breakpoints for anaerobes (FDA guidance for doxycycline).

For the majority of isolates the correlation between FAA and BRU MICs was good.

Errors were found for each antimicrobial, with the highest numbers recorded for metronidazole and doxycycline. The majority of errors were within 1-2 dilutions, but spanned the clinical breakpoints. Others were single replicate errors, which were anomalous against a minimum of three additional results.

Several species such as P. distasonis and C. ramosum demonstrated raised numbers of errors/variable results and will require further investigation.

Overall the reproducibility of agar dilution testing on FAA compared to BRU was good, suggesting that FAA is a suitable media for AST of anaerobes. Further assessment of several species is required.