Transmission of Similar Mcr-1 Carrying Plasmids among Different Escherichia coli Lineages Isolated from Livestock and the Farmer

Abstract

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.