Plasmid-mediated colistin resistance (mcr-1 gene): three months later, the story unfolds.

On 18 November 2015, Liu et al. reported the first description of plasmid-mediated colistin resistance (mcr-1 gene) in food animals, food and humans in China [1]. In this issue, Kluytmans-van den Bergh et al. report on their finding of the mcr-1 gene in Escherichia coli isolates from three (1.5%) of 196 samples of chicken meat collected at Dutch supermarkets, one in 2009 and two in 2014 [2]. This was done by whole genome sequencing of all E. coli isolates and then screening for the presence of the mcr-1 gene by comparing the assembled sequences with sequence data from two databases. The same study did not find any mcr-1-positive isolate among 2,275 extended-spectrum beta-lactamase-positive Escherichia coli (screening and clinical isolates) sampled in humans between 2009 and 2015. The exact origin of the sampled chicken meat was not known, with the two samples from 2014 being labelled ‘non-Dutch, European’. The fact that the genomes of the two isolates from 2014 differed by only three loci and were from the same lot of chicken meat strongly suggest cross-contamination from a common source.