Original Research

The accuracy of extended-spectrum beta-lactamase detection in Escherichia coli and Klebsiella pneumoniae in South African laboratories using the Vitek 2 Gram-negative susceptibility card AST-N255

Andrea L. Young, Mark P. Nicol, Clinton Moodley, Colleen M. Bamford
Southern African Journal of Infectious Diseases | Vol 34, No 1 | a114 | DOI: https://doi.org/10.4102/sajid.v34i1.114 | © 2019 Colleen Bamford, Andrea L. Young, Clinton Moodley, Mark Nicol | This work is licensed under CC Attribution 4.0
Submitted: 28 May 2019 | Published: 07 August 2019

About the author(s)

Andrea L. Young, Division of Medical Microbiology, Department of Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa
Mark P. Nicol, Division of Medical Microbiology, Department of Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Groote Schuur Hospital, Cape Town, South Africa
Clinton Moodley, Division of Medical Microbiology, Department of Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Groote Schuur Hospital, Cape Town, South Africa
Colleen M. Bamford, Division of Medical Microbiology, Department of Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Groote Schuur Hospital, Cape Town, South Africa


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Abstract

Background: Phenotypic detection of extended-spectrum beta-lactamases (ESBLs) is based on the inhibition of ESBL enzymes by β-lactamase inhibitors and on the comparison of cephalosporin activity with or without a β-lactamase inhibitor. Many South African diagnostic laboratories rely on the Vitek 2 for automated susceptibility testing and for ESBL detection. However, the Gram-negative susceptibility card currently used locally (AST-N255) has been modified and its accuracy for ESBL detection is not known.

Methods: We randomly selected 50 isolates of Klebsiella pneumoniae and Escherichia coli from a collection of clinical bloodstream isolates from Groote Schuur Hospital from 2015 to 2016, including ESBL-producing and non-ESBL-producing strains. We used standardised phenotypic (disc diffusion and broth microdilution) and genotypic (conventional polymerase chain reaction (PCR) for blaCTX-M, blaSHV and blaTEM) methods for detection of ESBLs. We compared ESBL detection by Vitek 2 to a composite reference standard comprising ESBL detection either by both phenotypic methods or by one phenotypic method together with genotypic detection.

Results: The sensitivity of Vitek 2 system for detection of ESBLs was 33/36 or 92% (78% – 97%) for E. coli, and 40/40 or 100% (91% – 100%) for K. pneumoniae, whilst specificity was 10/10 or 100% (72% – 100%) and 9/10 or 90% (60% – 98%), respectively. This is comparable with previous studies.

Conclusion: Using a composite reference standard of the phenotypic and genotypic methods employed in this study, no Vitek-categorised ESBL E. coli or K. pneumoniae was found to be a non-ESBL with the exception of possible misinterpretation with K. pneumoniae SHV-hyper-producing isolates.


Keywords

Antimicrobial susceptibility testing; Extended-spectrum beta-lactamase (ESBL) Detection; Automated systems for ESBL Detection; Vitek 2 ESBL detection; Gram-negative susceptibility card AST-N255

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