Comparison of different phenotypic assays for the detection of extended-spectrum β-lactamase production by inducible AmpC-producing Gram-negative bacilli.

Routine detection of extended-spectrum β-lactamase (ESBL) production by AmpC-producing Enterobacteriaceae in microbiology laboratories is still a problem. The aim of this study was to compare the performance of four different phenotypic ESBL confirmation assays within this group of Enterobacteriaceae. A total of 83 AmpC-inducible Enterobacteriaceae were included in this study (58 clinical isolates with presumptive ESBL production and 25 molecularly characterized ESBL-producing isolates). Each isolate was tested for the presence of an ESBL enzyme by four phenotypic ESBL confirmation assays: ESBL Etests and combined double-disk synergy tests (CDDST), both on Mueller-Hinton (MH) agar with and without the use of cloxacillin, an AmpC inhibitor. Our study showed that performing a CDDST on MH agar with cefotaxime as the only indicator cephalosporin is not a reliable way to detect ESBL-encoding genes among chromosomal AmpC-producing Enterobacteriaceae due to its low sensitivity (52 %). The use of cloxacillin in this CDDST could only significantly increase the specificity of the CDDST when used with ceftazidime as the indicator [sensitivity (SN), 92 %; specificity (SP), 93 %]. Regarding ESBL Etest® strips, the sensitivity of the cefepime strip (80 %) was significantly higher compared to the cefotaxime and ceftazidime strips (16 % and 32 %, respectively). Adding cloxacillin to the MH agar improved the ESBL detection of each of these strips. We recommend the CDDST on MH agar supplemented with cloxacillin and ceftazidime or cefepime as the indicator cephalosporin as the most cost-efficient strategy to confirm ESBL production in inducible AmpC-producing Enterobacteriaceae.