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Published Online: 8 May 2019

Evaluation of EDTA- and DPA-Based Microdilution Phenotypic Tests for the Detection of MCR-Mediated Colistin Resistance in Enterobacteriaceae

Publication: Microbial Drug Resistance
Volume 25, Issue Number 4


The emergence of the colistin-resistant (COL-R) Enterobacteriaceae represents a worrying health issue. However, only a portion of these strains may carry the plasmid-mediated mcr colistin resistance genes. We evaluated the ability of both ethylenediaminetetraacetic acid (EDTA)-based and dipicolinic acid (DPA)-based broth microdilution (BMD) tests to detect mcr-1 to mcr-5 producers. Of 92 Enterobacteriaceae (85 COL-R), 44 mcr-positive strains (39 Escherichia coli, 3 Klebsiella pneumoniae, and 2 Salmonella spp.) were tested. EDTA (100 μg/mL) was tested in Mueller–Hinton broth (MHB), whereas the DPA (900 μg/mL) was used in cation-adjusted MHB. Results were categorized as positive if in presence of chelator strains exhibited ≥3 two fold MIC decrease compared to the COL MIC alone. The EDTA-based BMD assay detected 41 mcr-positive strains, but 22 false-positive strains (including 12 E. coli and 4 K. pneumoniae) were recorded (sensitivity [SN], 93.2%; specificity [SP], 54.2%). The DPA-based BMD assay detected 37 mcr-positive strains, with 7 false-negative (2 E. coli, 3 K. pneumoniae, 2 Salmonella spp.) strains (SN, 84.1%; SP, 100%). Overall, the EDTA-based BMD assay is not accurate to detect mcr producers, whereas the DPA-based BMD test (“colistin-MAC test”) demonstrated good accuracy, but only when implemented for E. coli strains (SN, 94.9%; SP, 100%). With the aim to prevent the dissemination of mcr-possessing E. coli strains, the COL-MAC test could be implemented by clinical laboratories that are unable to perform molecular tests. Moreover, this assay could be applied to screen large collections of isolates to reveal the expression of new mcr-like genes not yet targeted by the current molecular assays.

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Published In

cover image Microbial Drug Resistance
Microbial Drug Resistance
Volume 25Issue Number 4May 2019
Pages: 494 - 500
PubMed: 30431401


Published online: 8 May 2019
Published in print: May 2019
Published ahead of print: 15 November 2018


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Thomas Büdel*
Institute for Infectious Diseases, University of Bern, Bern, Switzerland.
Mathieu Clément*
Institute for Infectious Diseases, University of Bern, Bern, Switzerland.
Graduate School of Cellular and Biomedical Sciences, University of Bern, Bern, Switzerland.
Odette J. Bernasconi
Institute for Infectious Diseases, University of Bern, Bern, Switzerland.
Graduate School of Cellular and Biomedical Sciences, University of Bern, Bern, Switzerland.
Luigi Principe
Clinical Microbiology and Virology Unit, A. Manzoni Hospital, Lecco, Italy.
Vincent Perreten
Institute of Veterinary Bacteriology, University of Bern, Bern, Switzerland.
Francesco Luzzaro
Clinical Microbiology and Virology Unit, A. Manzoni Hospital, Lecco, Italy.
Andrea Endimiani [email protected]
Institute for Infectious Diseases, University of Bern, Bern, Switzerland.


These authors' contributed equally to this work.
Address correspondence to: Andrea Endimiani, MD, PhD, Institute for Infectious Diseases, University of Bern, Friedbühlstrasse 51, Bern CH-3001, Switzerland [email protected]; [email protected]

Disclosure Statement

No competing financial interests exist.

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