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

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Thomas Büdel1, Mathieu Clément1,2, Odette J Bernasconi1,2

  • 11 Institute for Infectious Diseases, University of Bern, Bern, Switzerland.

Microbial Drug Resistance (Larchmont, N.Y.)
|November 16, 2018
PubMed

Insights

The dipicolinic acid (DPA)-based broth microdilution test, or colistin-MAC test, accurately detects mcr-producing Escherichia coli. This method is valuable for clinical labs lacking molecular testing capabilities to prevent the spread of colistin-resistant bacteria.

Area of Science:

  • Antimicrobial Resistance
  • Molecular Biology
  • Clinical Microbiology

Background:

  • Colistin-resistant Enterobacteriaceae pose a significant public health threat.
  • Plasmid-mediated mcr genes are responsible for colistin resistance in a subset of these strains.
  • Accurate detection of mcr gene carriers is crucial for infection control.

Purpose of the Study:

  • To evaluate the efficacy of ethylenediaminetetraacetic acid (EDTA)-based and dipicolinic acid (DPA)-based broth microdilution (BMD) tests for detecting mcr-1 to mcr-5 producing Enterobacteriaceae.
  • To assess the diagnostic accuracy (sensitivity and specificity) of these assays.
  • To determine the utility of the DPA-based assay (colistin-MAC test) for clinical laboratories.

Main Methods:

  • Tested 92 Enterobacteriaceae strains (85 colistin-resistant) for mcr genes.
  • Utilized EDTA (100 μg/mL) in Mueller-Hinton broth (MHB) and DPA (900 μg/mL) in cation-adjusted MHB for BMD assays.
  • Defined detection as ≥3 two-fold MIC decrease in the presence of chelator compared to colistin alone.

Main Results:

  • The EDTA-based BMD assay showed 93.2% sensitivity but 54.2% specificity, with numerous false positives.
  • The DPA-based BMD assay exhibited 84.1% sensitivity and 100% specificity overall, with 7 false negatives.
  • The DPA-based BMD test demonstrated high accuracy (94.9% sensitivity, 100% specificity) specifically for mcr-producing Escherichia coli.

Conclusions:

  • The EDTA-based BMD assay is not reliable for detecting mcr producers.
  • The DPA-based BMD test (colistin-MAC test) is accurate for detecting mcr-producing E. coli.
  • The colistin-MAC test is a viable alternative for clinical laboratories unable to perform molecular tests, aiding in preventing the dissemination of mcr-carrying E. coli.
Keywords:
DPAEDTAEnterobacteriaceaecolistindiagnosticsmicrodilutionphenotypeplasmid-mediatedtests

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