Genomic analysis of Enterococcus faecium co-carrying optrA and poxtA from a swine farm: dissemination across the human–animal–environment interface

BACKGROUND: The transferable resistance genes optrA and poxtA mediate cross-resistance to florfenicol and linezolid, posing serious challenges to both veterinary and human healthcare. Swine farms serve as critical ecological niches for the development and dissemination of multidrug-resistant (MDR) Enterococcus faecium (E. faecium) strains. However, the mechanisms by which E. faecium harboring optrA and poxtA disseminates and persists across the human-animal-environment interface remain unclear. RESULTS: In this study, 61 multidrug-resistant E. faecium isolates carrying optrA and/or poxtA were recovered from swine, farm workers, and surrounding environments. Antimicrobial susceptibility testing, conjugation assays, whole-genome sequencing, and phylogenomic analysis were performed. The predominant resistance genes were optrA (78.7%), poxtA (28.5%), and fexA (74.9%). Phylogenetic analysis of 18 representative isolates identified six distinct clades, including a novel sequence type (ST2514) shared across all three sources, suggesting potential inter-host transmission. One representative strain (RX23) harbored optrA and poxtA on two distinct multi-replicon plasmids. Experimental exposure to florfenicol increased plasmid stability (> 90% retention) and resistance levels (2-4-fold MIC elevation), indicating adaptive persistence under antibiotic pressure. Although co-transfer imposed an initial fitness cost, this burden was mitigated over serial passages, enabling long-term plasmid retention. CONCLUSIONS: Our findings provide evidence that both plasmid-mediated transfer and ecological selection contribute to the dissemination and persistence of optrA/poxtA-positive E. faecium in swine farms. The presence of shared lineages across humans, animals, and environmental niches highlights a potential public health threat. Integrated surveillance and antimicrobial stewardship under the One Health framework are essential to prevent further dissemination along the food production chain.