Evaluation of susceptibility testing methods for Burkholderia cepacia complex: a comparison of broth microdilution, agar dilution, gradient strip and EUCAST disk diffusion.

Objectives To evaluate the accuracy and reproducibility of antimicrobial susceptibility testing methods in Burkholderia cepacia complex (BCC). Methods Minocycline, ciprofloxacin, trimethoprim/sulphamethoxazole, meropenem, ceftazidime, and chloramphenicol were tested against 155 BCC strains using broth microdilution at 35±1°C (BMD35) in triplicate, then BMD at 30±1°C (BMD30), agar dilution at 30°C and 35°C (AD30 & AD35), gradient strip (GS) and EUCAST standardised disk diffusion (DD) testing methods once. Results BMD35reproducibility ranged from 70-84.5% for all agents. Correlations of MICs from BMD35 with BMD30 ranged from 63-85%, with AD35 from 32.9-87% and with GS methods from 36-83.9%. Essential Agreement (EA) of MICs by GS with BMD35 ranged from 62.6% (trimethoprim-sulphamethoxazole) to 83.9% (minocycline). EA of EUCAST DD zone diameters using CLSI breakpoint criteria was between 85.8-97.4%, however Very Major Errors (VMEs) for trimethoprim/sulphamethoxazole were 31%. Conclusions BMD at 35±1°C was poorly reproducible for most agents and no method showed acceptable performance. Of particular concern were GS results. Whilst this is the most commonly used method for determining MICs in laboratories, there was poor correlation to BMD35 for meropenem and trimethoprim/sulphamethoxazole. EUCAST DD correlated poorly with BMD35 MICs. This study confirms that no susceptibility method is capable of providing reproducible and accurate MICs when testing Burkholderia cepacia complex.