France - MCR-1 Gene (Colistin Resistance) detected in 4 Salmonella Isolates (2012-2013)

[Gert van der Hoek]

In response to the Yi-Yun Liu and colleagues' finding of a mobile genetic element responsible for colistin resistance, mcr-1,1 and the accompanying Comment asking &#147;is plasmid-mediated colistin resistance a purely Chinese phenomenon?&#148;2, we, and others,3, 4 can now reply no. As part of routine surveillance, we screened 8684 salmonella isolates collected during 2012&#150;13 from the French agricultural food sector for colistin resistance using disk diffusion.5 Between October and December, 2013, 27 isolates that showed a reduced susceptibility to colistin (ie, zone of inhibition <15 mm) were further assessed using a colistin concentration gradient assay. Five isolates (0?06%) had a distinctly different minimum inhibitory concentration (≥4 mg/mL) and were defined as colistin-resistant. In 2014, whole-genome sequencing of the five isolates was done and resultant sequences were assembled and interrogated for mutations and genetic elements associated with colistin resistance.
We identified mcr-1 in four of five phenotypically colistin-resistant isolates.

Our findings, and those of others,1 reinforce the need to reconsider the use of in-feed colistin in veterinary medicine at a worldwide level. Stewardship efforts are needed to preserve the efficacy of antimicrobial drugs for society&#151;both now and into the future. Effective strategies that limit selection and further dissemination of plasmid-associated mcr-1 are clearly needed.