| Summary: | We examined the prevalence and molecular characteristics of <i>mcr-3</i> carrying colistin-resistant <i>Escherichia coli</i> among cattle, pig, and chicken isolates in South Korea. Among a total of 185 colistin-resistant <i>E. coli</i> isolates determined in this study (47 from cattle, 90 from pigs, and 48 from chicken), PCR amplification detected <i>mcr</i>-<i>3</i> genes in 17 isolates predominantly from diseased pigs. The <i>mcr</i>-<i>3</i> genes were characterized as <i>mcr-3.1</i> in 15 isolates and <i>mcr</i>-<i>3.5</i> in 2 isolates. The <i>mcr-3</i> gene was transferred to the <i>E. coli</i> J53 recipient strain from more than 50% of the <i>mcr</i>-<i>3</i>-carrying isolates. The <i>mcr</i>-<i>3.1</i> and <i>mcr</i>-<i>3.5</i> genes were identified predominantly in IncHI2 and IncP plasmids, respectively. Multi-locus sequence typing analysis revealed eight previously reported sequence types (ST), including ST1, ST10, and ST42. We identified isolates with similar pulsed-field gel electrophoresis patterns from diseased pigs in three farms. Besides, the isolates carried various virulence factors and demonstrated resistance to multiple antimicrobials, including β-lactams and quinolones. Further, the <i>mcr</i>-<i>3.5</i> encodes three amino acid substitutions compared with <i>mcr</i>-<i>3.1</i>. To the best of our knowledge, this is the first report of pathogenic <i>E. coli</i> carrying <i>mcr</i>-<i>3.5</i> in South Korea, which implies that <i>mcr</i>-<i>3</i> variants may have already been widely spread in the pig industry.
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