LAUSR

OBJECTIVES Many multidrug-resistant Gram-negative bacilli (MDR-GNB) harbor multiple beta-lactamases. The goal of this study was to assess the impact of multiple beta-lactamase carriage on the accuracy of susceptibility tests and extended-spectrum beta-lactamase (ESBL) and carbapenemase confirmation methods. METHODS Fifty MDR-GNB, 29 of which carried multiple beta-lactamases, underwent broth microdilution (BMD) and disk diffusion (DD) testing, and confirmation tests for ESBLs and carbapenemases. Whole genome sequencing (WGS) was used for beta-lactamase gene identification. RESULTS The category agreement of BMD and DD testing results ranged from 86.5% to 97.7% for 10 beta-lactam agents. BMD and DD algorithms for ESBL detection were highly variable; 6 of 8 positive strains carried an ESBL plus a carbapenemase or an AmpC enzyme, which may confound antimicrobial selection. The sensitivity and specificity of mCIM were both 100%, while mCIM and eCIM when used together to differentiate serine from metallo-beta-lactamase carriage, were both 96%. Xpert® CarbaR results (IVD. In vitro diagnostic) were consistent with WGS results. Predicting phenotypic carbapenem resistance from WGS data overall showed 100% specificity, but sensitivity of only 66.7% for Enterobacterales isolates that were non-susceptible to imipenem and meropenem. CONCLUSIONS Multiple beta-lactamases in MDR-GNB does not impact DD results, the utility of mCIM/eCIM tests, or Xpert CarbaR. However, ESBL algorithms produced inconsistent results and predicting carbapenem resistance from WGS data was problematic in such strains.