| Summary: | A number of studies from different countries have characterized <i>mcr-1</i>-harboring plasmids isolated from food; however, nothing has been reported about it in South Korea. In this study, we report the characterization of <i>mcr-1</i> plasmids from pan drug-resistant (PDR) <i>Escherichia coli</i> strains isolated from retail food in the country. Colistin-resistant <i>E. coli</i> strains were isolated from retail raw chicken, and PCR was carried out to detect the <i>mcr-1</i> gene. Whole genome sequencing of the <i>mcr-1</i>-positive strains was performed for further characterization. The results of whole genome sequencing revealed that all <i>mcr-1</i> plasmids belonged to the IncI2 type. In addition to the <i>mcr-1</i> plasmids, all of the isolates also carried additional plasmids possessing multiple antibiotic resistance genes, and the PDR was mediated by resistant plasmids except for fluoroquinolone resistance resulting from mutations in <i>gyrA</i> and <i>parC.</i> Interestingly, the <i>mcr-1</i> plasmids were transferred by conjugation to other pathogenic strains including enterohemorrhagic <i>E. coli</i> (EHEC), enterotoxigenic <i>E. coli</i> (ETEC), enteroaggregative <i>E. coli</i> (EAEC), <i>Salmonella</i>, and <i>Klebsiella</i> at the frequencies of 10<sup>−3</sup>−10<sup>−6</sup>, 10<sup>−2</sup>−10<sup>−5</sup>, 10<sup>−4</sup>−10<sup>−5</sup>, 10<sup>−4</sup>−10<sup>−6</sup>, and 10<sup>−5</sup>−10<sup>−6</sup>, respectively. The results showed that <i>mcr-1</i> plasmids can be easily transmitted to pathogenic bacteria by conjugation.
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