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

doi:10.4102/sajid.v34i1.114

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

S Afr J Infect Dis. 2019 Aug 7;34(1):114. doi: 10.4102/sajid.v34i1.114. eCollection 2019.

Authors

Andrea L Young¹, Mark P Nicol^{1

2}, Clinton Moodley^{1

2}, Colleen M Bamford^{1

2}

Affiliations

¹ Division of Medical Microbiology, Department of Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa.
² National Health Laboratory Service, Groote Schuur Hospital, Cape Town, South Africa.

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 bla_CTX-M, bla_SHV and bla_TEM ) 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; Automated systems for ESBL Detection; Extended-spectrum beta-lactamase (ESBL) Detection; Gram-negative susceptibility card AST-N255; Vitek 2 ESBL detection.