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Journal of Clinical Microbiology, February 2009, p. 358-361, Vol. 47, No. 2
0095-1137/09/$08.00+0     doi:10.1128/JCM.01687-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Unreliable Extended-Spectrum β-Lactamase Detection in the Presence of Plasmid-Mediated AmpC in Escherichia coli Clinical Isolates{triangledown}

F. J. L. Robberts,1 P. C. Kohner,1 and R. Patel1,2*

Division of Clinical Microbiology, Department of Laboratory Medicine and Pathology,1 Division of Infectious Diseases, Department of Medicine, Mayo Clinic, Rochester, Minnesota2

Received 29 August 2008/ Returned for modification 12 November 2008/ Accepted 12 December 2008

The emergence of extended-spectrum β-lactamase (ESBL) and plasmid-mediated AmpC (pAmpC) enzymes in Escherichia coli raises concern regarding accurate laboratory detection and interpretation of susceptibility testing results. Twenty-six cefpodoxime ESBL screen-positive, cefoxitin-resistant E. coli clinical isolates were subjected to clavulanate ESBL confirmatory testing employing disk augmentation, Etest, and the BD Phoenix NMC/ID-132 panel. Phenotypic pAmpC production was assessed by boronic acid disk augmentation. ESBL and pAmpC genes were detected by gene amplification and sequencing. ESBL genes (SHV and/or CTX-M-type genes) were detected in only 7/26 ESBL screen-positive isolates. Of 23 aminophenylboronic acid screen-positive isolates, pAmpC genes were detected in 20 (CMY-2 or FOX-5 genes). High incidences of false-positive ESBL confirmatory results were observed for both clavulanate disk augmentation (9/19) and BD Phoenix (5/19). All were associated with the presence of pAmpC genes with or without TEM-1. Etest performed poorly, as the majority of interpretations were nondeterminable. In addition, false-negative ESBL confirmatory results were observed in isolates possessing concomitant ESBL and pAmpC genes for Etest (four of five), BD Phoenix (three of five), and disk augmentation (one of five). The results indicate poor performance of currently employed ESBL confirmatory methods in the setting of concomitant pAmpC. Some isolates with pAmpC and ESBL genes fell within the susceptible category to extended-spectrum cephalosporins, raising concern over currently employed breakpoints.

* Corresponding author. Mailing address: Division of Clinical Microbiology, Mayo Clinic, 200 First St. Southwest, Rochester, MN 55905. Phone: (507) 538-0579. Fax: (507) 284-4272. E-mail: patel.robin{at}mayo.edu

{triangledown} Published ahead of print on 24 December 2008.

Journal of Clinical Microbiology, February 2009, p. 358-361, Vol. 47, No. 2
0095-1137/09/$08.00+0     doi:10.1128/JCM.01687-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.



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  • Jeong, S. H., Song, W., Kim, J.-S., Kim, H.-S., Lee, K. M. (2009). Broth Microdilution Method To Detect Extended-Spectrum {beta}-Lactamases and AmpC {beta}-Lactamases in Enterobacteriaceae Isolates by Use of Clavulanic Acid and Boronic Acid as Inhibitors. J. Clin. Microbiol. 47: 3409-3412 [Abstract] [Full Text]  


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