Emergence of Transferable Plasmid Coharboring mcr-1.1 and blaCTX-M-55 Genes in Multidrug-Resistant Escherichia coli From Mink.

Multidrug-resistant (MDR) bacteria have reduced the effectiveness of antimicrobial agents and limited available treatment options in both human and animal settings. Antimicrobial resistance (AMR) in animal-derived Escherichia coli (E. coli) has become increasingly common, with many isolates showing resistance to multiple classes of antimicrobials. However, compared with other animal sources, resistance characteristics of E. coli from mink remain less well understood, particularly in terms of genomic features. In this study, we identified a MDR E. coli strain, EC0D1, isolated from the lung tissue of a farmed mink that succumbed to hemorrhagic pneumonia (HP) in China. Antimicrobial susceptibility testing indicated that EC0D1 exhibited resistance to β-lactams, fluoroquinolones, aminoglycosides, tetracyclines, and polymyxins but was sensitive to carbapenems and tigecycline. Whole genome sequencing showed that EC0D1 contains a single chromosome and eight plasmids. Among these, plasmid pEC0D1-3 coharbored blaCTX-M-55 and mcr-1.1, while pEC0D1-2 carried tet(A) and floR, together explaining the observed MDR phenotype. Phylogenetic analysis classified EC0D1 as sequence type ST457 and demonstrated a close genetic relationship with several human clinical isolates. Comparative genomic analysis further revealed that plasmids pEC0D1-2 and pEC0D1-3 shared high sequence similarity with plasmids previously identified in avian-derived E. coli and human-derived Klebsiella pneumoniae isolates. Conjugation assays confirmed that plasmids carrying blaCTX-M-55, mcr-1.1, tet(A), and floR were transferable to different bacterial recipients. The emergence of transferable resistance determinants in mink-associated E. coli suggests a potential role in the transmission of resistance genes between human and animal hosts.