K ZURFLUH¹, L POIREL¹, P NORDMANN², M NÜESCH-INDERBINEN¹, H HÄCHLER¹, R STEPHAN<sup>1

1</sup>Institute for Food Safety and Hygiene, Vetsuisse Faculty University of Zurich Winterthurerstrasse 272, 8057 Zurich, Switzerland , ²Emerging Antibiotic Resistance Unit, Department of Medicine, Faculty of Science, University of Fribourg, 1701 Fribourg, Switzerland

The recent identification of Enterobacteriaceae harbouring the plasmid-mediated transferable colistin resistance mcr-1 gene is of great concern to public health. Here, we report on the occurrence of mcr-1 harbouring extended-spectrum ß-lactamase (ESBL)-producing Enterobacteriaceae from river water in Switzerland and from ready-to-eat imported vegetables.
Seventyfour ESBL-producing Enterobacteriaceae isolated from 21 rivers and lakes sampled in 2012 in Switzerland and 60 ESBL-producing Enterobacteriaceae isolated from 42 imported vegetable samples from the Dominican Republic, India, Thailand, and Vietnam were screened by PCR for the presence of the mcr-1 gene.
The mcr-1 gene was detected in one out of 74 water strains, and 2 out of 60 vegetable strains (products from Thailand and Vietnam). All 3 genes showed 100% homology with the published mcr-1 sequence. Colistin resistance was transferable by transformation experiments into Escherichia. coli DH5-alpha. All 3 original isolates were identified as E. coli and belonged to different multi locus sequence types. They harboured different blaESBL genes and showed a multiresistance phenotype.
The results prove that mcr-1 has arrived in Switzerland as in other countries. This is worrisome, because polymyxins are currently being chemically modified to become less toxic in order to replace currently used antimicrobials that are increasingly corrupted by multiple resistance.