| Summary: | National audience The old antibiotic colistin is used since recently in human medicine as a last resort to treat infections caused by antibioticresistant Gram negative bacteria. In 2015, a Chinese team reported a disturbing discovery: horizontally transferable colistin resistance mediated by the gene mcr-1 localized in a plasmid (a mobile genetic element). The testing of archived isolates of Enterobacteriaceae has evidenced the worldwide unnoticed dissemination of mcr-1 in humans and animals for years. Cabello and Godfrey (2017a) suggest that aquaculture may be involved in the origin and selection of mcr-1. In this retrospective study, we aimed at detecting resistance to colistin in a historic collection of Yersinia ruckeri causing disease in farmed fish in NW France (Table 1). We tested 104 Y. ruckeri clinical isolates for susceptibility to colistin by the broth microdilution method following the guidelines by the Clinical and Laboratory Standards Institute (2014). Because of the lack of a cut-off value for Y. ruckeri, we used the one reported by the European Committee on Antibiotic Susceptibility Testing for another Enterobacteriaceae, Escherichia coli (Minimum Inhibitory Concentration >2μg/ml). One isolate (0.09%) showed phenotypic resistance to colistin (MIC=128 μg/ml). Its plasmidic DNA was investigated for the presence of mcr-1 by the Polymerase Chain Reaction, being positive. This isolate corresponded to a case of yersiniosis in an Atlantic salmon from 1985. Pending sequencing results will shed light on the genetic relatedness between this mcr-1 and those carried by other isolates. To our knowledge this is the first report of mcr-1 in Y. ruckeri and the oldest bacterial isolate discovered in Europe that is carrying mcr-1, with the previous oldest isolate dating from 2004. Our findings are in agreement with the hypothesis of an origin and mobilization of mcr-1 in the aquatic or aquaculture environment (Cabello et al., 2017b), and also suggest that Y. ruckeri does not act as a reservoir of mcr-1. The ...
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