Occurrence of Plasmid- and Chromosome-Carried <i>mcr-1</i> in Waterborne Enterobacteriaceae in China

Abstract

The aim of this study was to investigate the prevalence of the polymyxin resistance gene <i>mcr-1</i> in <i>Enterobacteriaceae</i> from environmental water sources in Hangzhou, China. Colistin-resistant bacteria were isolated from environmental water samples using an enrichment broth culture method, were screened for <i>mcr-1</i>, and then were analyzed for the location and transferability of <i>mcr-1</i> Isolates positive for <i>mcr-1</i> were further examined to determine their susceptibility profiles and were screened for the presence of additional resistance genes. Twenty-three <i>mcr-1</i>-positive isolates (16 <i>Escherichia coli</i>, two <i>Citrobacter freundii</i>, two <i>Klebsiella oxytoca</i>, two <i>Citrobacter braakii</i>, and one <i>Enterobacter cloacae</i>) were isolated from 7/9 sampling locations; of those, eight <i>mcr-1</i>-positive isolates also contained β-lactamase-resistance genes, eight contained <i>qnrS</i>, and 10 contained <i>oqx</i> No <i>mcr-2</i>-positive isolates were identified. The majority of isolates demonstrated a low to moderate level of colistin resistance. Transconjugation was successfully conducted from 14 of the 23 <i>mcr-1</i>-positive isolates, and <i>mcr-1</i> was identified on plasmids ranging from 60 to 220 kb in these isolates. Conjugation and hybridization experiments revealed that <i>mcr-1</i> was chromosome-borne in only three isolates. Pulsed-field gel electrophoresis showed that the majority of <i>E. coli</i> isolates belonged to different clonal lineages. Multilocus sequence typing analysis revealed that sequence type 10 (ST10) was the most prevalent, followed by ST181 and ST206. This study demonstrates the utility of enrichment broth culture for identifying environmental <i>mcr-1</i>-positive isolates. Furthermore, it highlights the importance of responsible agriculture and clinical use of polymyxins to prevent further widespread dissemination of polymyxin-resistant pathogens.