Detection of the plasmid-mediated mcr-1 gene conferring colistin resistance in human and food isolates of Salmonella enterica and Escherichia coli in England and Wales

In response to the first report of transmissible colistin resistance mediated by the mcr-1 gene in Escherichia coli and Klebsiella spp. from animals and humans in China, we sought to determine its presence in Enterobacteriaceae isolated in the UK. The PHE archive of whole-genome sequences of isolates from surveillance collections, submissions to reference services and research projects was retrospectively analysed for the presence of mcr-1 using Genefinder. The genetic environment of the gene was also analysed. Rapid screening of the genomes of similar to 24aEuroS000 Salmonella enterica, E. coli, Klebsiella spp., Enterobacter spp., Campylobacter spp. and Shigella spp. isolated from food or humans identified 15 mcr-1-positive isolates. These comprised: 10 human S. enterica isolates submitted between 2012 and 2015 (8 Salmonella Typhimurium, 1 Salmonella Paratyphi B var Java and 1 Salmonella Virchow) from 10 patients; 3 isolates of E. coli from 2 patients; and 2 isolates of Salmonella Paratyphi B var Java from poultry meat imported from the EU. The mcr-1 gene was located on diverse plasmids belonging to the IncHI2, IncI2 and IncX4 replicon types and its association with ISApl1 varied. Six mcr-1-positive S. enterica isolates were from patients who had recently travelled to Asia. Analysis of WGS data allowed rapid confirmation of the presence of the plasmid-mediated colistin resistance gene mcr-1 in diverse genetic environments and plasmids. It has been present in E. coli and Salmonella spp. harboured by humans in England and Wales since at least 2012.