Multidrug-resistant livestock-associated ST9-MRSA causing severe human infection without animal exposure: the food chain as a potential transmission route in China.

ST9 methicillin-resistant Staphylococcus aureus (MRSA) is a typical livestock-associated lineage in Asia and represents an important reservoir of antimicrobial resistance (AMR). However, its contribution to human infection and its transmission pathways remain incompletely understood. Here, we characterized a multidrug-resistant ST9-MRSA isolate (SAH77) recovered from a severe skin infection in a patient without occupational animal exposure, aiming to assess its antimicrobial resistance, genomic features, and pathogenic potential. Phylogenetic analysis based on a dataset of 218 Chinese ST9 S. aureus genomes revealed that SAH77 clustered within a "pig-food-human" mixed clade. SAH77 exhibited resistance to multiple antibiotics, including oxacillin, erythromycin, clindamycin, tetracycline, gentamicin, penicillin, and cefoxitin, and harbored 16 resistance genes, including mecA, spw, and tet(L), confirming a multidrug-resistant phenotype. Notably, highly similar antimicrobial resistance gene profiles were observed across animal-, food-, and human-derived isolates, suggesting that AMR may spread along the farm‑to‑table continuum. The isolate also carried multiple virulence-associated genes and displayed strong biofilm formation, high hemolytic activity, and significant lethality in Galleria mellonella. In a murine skin infection model, SAH77 caused more severe lesions than the USA300 reference strain (p<0.05), indicating substantial pathogenicity. Collectively, these findings suggest that multidrug-resistant ST9-MRSA can cause severe human infection without direct animal contact, and they are consistent with the hypothesis of food-chain-mediated transmission. Importantly, this study highlights the need for integrated surveillance strategies across the food chain.