Case Report: Transferable IncX4 plasmid carrying mcr-1 in colistin-resistant Escherichia coli from a healthy pet dog in South Korea

Colistin resistance mediated by the plasmid-borne <i>mcr-1</i> gene presents a significant challenge in both human and veterinary medicine. While colistin-resistant bacteria have been reported in food-producing animals and humans, <i>mcr-1</i>-harboring strains remain relatively underreported in companion animals, particularly in South Korea. In this study, a colistin-resistant <i>Escherichia coli</i> strain Z1324PEC0026 was isolated from a clinically healthy companion dog owned by a veterinary nurse, which exhibited resistance to multiple antimicrobials, including colistin, β-lactams, aminoglycosides, tetracyclines, and phenicols. Its genome harbored two plasmids: a 33.9 kb IncX4 plasmid pEC027-3 carrying <i>mcr-1</i> gene with a high conjugation frequency, 4.64 × 10^-2 per recipient cell, and another plasmid pEC027-2 carrying additional resistance genes such as <i>bla</i> _CTX-M-55, <i>bla</i> _OXA-10, <i>qnrS1</i>, <i>dfrA14</i>, <i>aph(3")-Ia</i>, <i>aadA1</i>, <i>cmlA1</i>, <i>arr-2</i>, and <i>tet(A)</i>. The genetic structure of pEC027-3 exhibited high synteny with global IncX4 plasmids but showed divergence from <i>mcr-1</i>-carrying plasmids previously reported in South Korea, suggesting an independent origin. The presence of a transferable <i>mcr-1</i>-harboring IncX4 plasmid in a healthy dog with no prior colistin exposure implies the risk of silent dissemination of antimicrobial resistance.