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

Büdel, Thomas; Clément, Mathieu; Bernasconi, Odette J.; Principe, Luigi; Perreten, Vincent; Luzzaro, Francesco; Endimiani, Andrea (2019). Evaluation of EDTA- and DPA-Based Microdilution Phenotypic Tests for the Detection of MCR-Mediated Colistin Resistance in Enterobacteriaceae. Microbial drug resistance, 25(4), pp. 494-500. Mary Ann Liebert 10.1089/mdr.2018.0275

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The emergence of the colistin-resistant 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 EDTA-based and dipicolinic acid (DPA)-based broth microdilution (BMD) tests to detect mcr-1 to mcr-5 producers. Ninety-two Enterobacteriaceae (85 colistin-resistant) of which 44 mcr-positive strains (39 E. coli, three K. pneumoniae, and two Salmonella spp.) were tested. EDTA (100 µg/mL) was tested in Mueller-Hinton broth (MHB), while 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 colistin MIC alone. The EDTA-based BMD assay detected 41 mcr-positives, but 22 false-positive strains (including 12 E. coli and 4 K. pneumoniae) were recorded (sensitivity, 93.2%; specificity, 54.2%). The DPA-based BMD assay detected 37 mcr-positives, with seven false-negative (two E. coli, three K. pneumoniae, two Salmonella spp.) strains (sensitivity, 84.1%; specificity, 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 (sensitivity, 94.9%; specificity, 100%). With the aim to prevent the dissemination of mcr-possessing E. coli strains, the colistin-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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