JCM Accepts, published online ahead of print on 4 November 2009

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J. Clin. Microbiol. doi:10.1128/JCM.01507-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Detection of CTX-M type ESBLs by Testing with MicroScan Overnight and ESBL Confirmation Panels

James H. Jorgensen^*, M. L. McElmeel, L. C. Fulcher, and B. L. Zimmer

Department of Pathology, University of Texas Health Science Center, San Antonio, Texas 78229; and Siemens Healthcare Diagnostics, West Sacramento, CA 95691

^* To whom correspondence should be addressed. Email: jorgensen{at}uthscsa.edu.

CTX-M extended-spectrum beta-lactamases (ESBLs) have emerged as the most common type of ESBL globally, easily surpassing the incidence of SHV and TEM ESBLs in most locales. This study has 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 based upon PCR and sequencing of the bla gene, 17 isolates that produced a SHV or TEM ESBL, and 19 with both CTX-M and SHV ESBLs. Each isolate was tested by a frozen reference microdilution panel, MicroScan ESBL plus Confirmation, and a routine MicroScan dried panel containing streamlined ESBL confirmation dilutions (Neg MIC type 32) that included cefotaxime and ceftazidime tested alone or with a fixed concentration of 4 μg/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 could not be determined because of off-scale MICs that exceeded the clavulanate combination concentrations in the panel), the ESBL plus detected 98% (1 missed, 1 off-scale), and the Streamlined ESBL 95% (5 off-scale). Very high MICs with a few strains that produced SHV or both CTX-M and SHV ESBLs precluded noting the required three 2-fold dilution differences with clavulanate needed to confirm an ESBL primarily in the reference and Neg MIC type 32 panels.