Detection of CTX-M-type extended-spectrum beta-lactamase (ESBLs) by testing with MicroScan overnight and ESBL...

CTX-M extended-spectrum beta-lactamases (ESBLs) have emerged as the most common type of ESBL globally, their incidence easily surpassing those of SHV and TEM ESBLs in most locales. This study compared the performance of two MicroScan dried panels with CLSI reference broth microdilution and disk diffusion methods on a collection of genetically characterized ESBL-producing isolates. These included 64 Enterobacteriaceae isolates that produced CTX-M8, -14, -15, or -16 according to PCR and sequencing of the bla gene, 17 isolates that produced a SHV or TEM ESBL, and 19 that produced both CTX-M and SHV ESBLs. Each isolate was tested by a frozen reference microdilution panel, the MicroScan ESbetaL plus confirmation panel, and a routine dried panel containing streamlined ESBL confirmation dilutions (MicroScan Neg MIC panel type 32) that included cefotaxime and ceftazidime tested alone or with a fixed concentration of 4 microg/ml of clavulanate. Each isolate was also tested by the standard CLSI double-disk confirmation tests. The disk diffusion method detected all ESBL-producing isolates, the frozen reference panel detected 90% of isolates (10 out of 100 could not be analyzed because of off-scale MICs that exceeded the clavulanate combination concentrations in the panel), the ESbetaL plus panel detected 98% (1 missed and 1 off scale), and the streamlined ESBL panel detected 95% (5 off scale). Very high MICs for a few strains that produced SHV or both CTX-M and SHV ESBLs precluded noting the required three twofold-dilution differences with clavulanate needed to confirm an ESBL primarily in the reference panel and the Neg type 32 panel.