3 years ago

A simple phenotypic method for screening of MCR-1-mediated colistin resistance

To evaluate a novel method, named Colistin-MAC test, for phenotypic screening of acquired colistin-resistance mediated by transferable mcr-1 resistance determinants, based on colistin MIC reduction in the presence of dipicolinic acid (DPA). Methods The Colistin-MAC test consists in a broth microdilution method, in which colistin MIC is tested in absence or presence of DPA (900 μg/mL). Overall, 74 colistin-resistant strains of Enterobacteriaceae (65 Escherichia coli and 9 of other species), including 61 strains carrying mcr-1-like genes and 13 strains negative for mcr genes, were evaluated with the Colistin-MAC test.The presence of mcr-1-like and mcr-2-like genes was assessed by Real-Time-PCR and end-point PCR. For 20 strains, Whole-Genome Sequencing data were also available. Results A ≥8-fold reduction of colistin MIC in the presence of DPA was observed with 59 mcr-1-positive strains, including 53 E. coli of clinical origin, three E. coli transconjugants carrying MCR-1-encoding plasmids, one Enterobacter cloacae complex, and two Citrobacter spp. Colistin MICs were unchanged, increased, or at most reduced by two-fold with the 13 mcr-negative colistin-resistant strains (nine E. coli and four Klebsiella pneumoniae), but also with two mcr-1-like-positive K. pneumoniae strains. Conclusions The Colistin-MAC test could be a simple phenotypic test for presumptive identification of mcr-1-positive strains among isolates of colistin-resistant E. coli, based on a ≥8-fold reduction of colistin MIC in the presence of DPA. Evaluation of the test with a larger number of strains, species, and mcr-type resistance determinants would be of interest.

Publisher URL: www.sciencedirect.com/science

DOI: S1198743X17304573

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