Co-occurrence of bla NDM-5 and mcr-1 on Two Distinct, Independently Transferable Plasmids in Commensal Escherichia coli ST156 from a Healthy Companion Dog

The emergence of carbapenem- and colistin-resistant Enterobacterales mediated by transmissible plasmids poses a critical One Health threat by compromising the efficacy of last-resort antimicrobials. We characterized a commensal Escherichia coli ST156 isolate recovered from a clinically healthy companion dog in South Korea that carried bla NDM-5 and mcr-1 on distinct plasmids. Antimicrobial susceptibility testing revealed a multidrug-resistant phenotype, including resistance to carbapenems (imipenem MIC, ≥8 μg/mL) and colistin (MIC, 8 μg/mL). Hybrid long- and short-read genome assembly localized bla NDM-5 to an IncX3 plasmid and mcr-1 to an IncX4 plasmid. Broth-mating assays showed that each plasmid was independently transferable under the corresponding single-antimicrobial selection. Among 30 transconjugant colonies screened under each selection, both resistance determinants were detected in five colistin-selected and two meropenem-selected colonies, indicating co-detection of both resistance determinants in a minority of the screened colonies. Comparative genomic analysis showed that the IncX4- mcr-1 plasmid shared a highly conserved backbone with globally distributed IncX4 plasmids from diverse sources. These findings demonstrate asymptomatic carriage of independently transferable carbapenem- and colistin-resistance plasmids in a companion animal and support the inclusion of companion animals in integrated antimicrobial resistance surveillance.