Prevalence of the colistin resistance gene mcr-1 in colistin-resistant Escherichia coli and Klebsiella pneumoniae isolated from humans in Thailand

Objectives: Historically, colistin has been considered a last-line therapeutic option against multidrug-resistant Gram-negative bacterial infections. However, chromosomally-encoded and plasmid-mediated colistin resistance is increasingly being reported worldwide. Spread of the plasmid-borne colistin resistance gene mcr-1 is of great concern since it can be transferred between bacteria. The aim of this study was to investigate the prevalence of mcr-1 in Escherichia coli and Klebsiella pneumoniae collected from human clinical specimens in Thailand during 2014-2017. Methods: Minimum inhibitory concentrations (MICs) of colistin were determined by the broth microdilution method for 317 non-duplicate Enterobacteriaceae clinical isolates (37 E. coli and 280 K. pneumoniae). All isolates were screened for the mcr-1 gene by PCR. Results: The colistin MIC50, MIC90 and MIC range for the 37 E. coli isolates were 0.5, 8 and 0.5-32 mg/L, respectively. The mcr-1 gene was detected in 11 E. coli isolates (29.7%). Escherichia coli harbouring the mcr-1 gene had a colistin MIC range of 4-32 mg/L. The colistin MIC50, MIC90, and MIC range for the 280 K. pneumoniae isolates were 32, >128, and 0.25 to >128 mg/L, respectively. The mcr-1 gene was detected in 4 K. pneumoniae isolates (1.4%). Klebsiella pneumoniae harbouring the mcr-1 gene had a colistin MIC range of 4-64 mg/L. Conclusions: This is the first report on the prevalence of the mcr-1 gene in colistin-resistant E. coli and K. pneumoniae isolated from humans in Thailand. These data provide added insight into the mechanism of colistin resistance among Enterobacteriaceae pathogens. (C) 2018 International Society for Chemotherapy of Infection and Cancer. Published by Elsevier Ltd. All rights reserved.