| Summary: | Colistin use has mostly been stopped in human medicine, due to its toxicity. However, nowadays, it still is used as a last-resort antibiotic to treat hospital infections caused by multi-drug resistant Enterobacteriaceae. On the contrary, colistin has been used in veterinary medicine until recently. In this study, 210 fecal samples from pigs (<i>n</i> = 57), calves (<i>n</i> = 152), and the farmer (<i>n</i> = 1) were collected from a farm where <i>E. coli</i> harboring <i>mcr</i>-1-<i>mcr</i>-3 was previously detected. Samples were plated, and <i>mcr</i>-genes presence was confirmed by multiplex-PCR. Hybrid sequencing which determined the presence and location of <i>mcr-1</i>, other antibiotic resistance genes, and virulence factors. Eighteen colistin resistant isolates (13 from calves, four from pigs, and one from the farmer) contained <i>mcr</i>-1 associated with plasmids (IncX4, IncI2, and IncHI2), except for two that yielded <i>mcr</i>-1 in the chromosome. Similar plasmids were distributed in different <i>E. coli</i> lineages. Transmission of <i>mcr</i>-1 to the farmer most likely occurred by horizontal gene transfer from <i>E. coli</i> of calf origin, since plasmids were highly similar (99% coverage, 99.97% identity). Moreover, 33 virulence factors, including <i>stx2</i> for Shiga toxin <i>E. coli</i> (STEC) were detected, highlighting the role of livestock as a reservoir of pathotypes with zoonotic potential.
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