Characteristics of methicillin-resistant Staphylococcus aureus isolates from bovine mastitis milk in South Korea: molecular characteristics, biofilm, virulence, and antimicrobial resistance

Methicillin-resistant Staphylococcus aureus (MRSA) is a drug-resistant pathogen causing subclinical and clinical bovine mastitis. This study examined the molecular properties, biofilm formation, virulence genes, and antimicrobial susceptibility of MRSA isolates from mastitis-infected dairy cow milk in South Korea. Whole-genome sequencing of an ST22-SCCmec IV MRSA strain positive for Panton-Valentine leukocidin (PVL) and toxic shock syndrome toxin-1 (TSST-1) was also performed. Of the 488 S. aureus isolates, 30 (6.1%) were identified as MRSA, harboring the mecA gene. MRSA exhibited the highest resistance to kanamycin (66.7%) among non-beta-lactam antibiotics. Multidrug resistance was observed in 83.3% of MRSA isolates. All MRSA strains had the capacity to form biofilm and harbored biofilm-related genes. The primary virulence genes included hla, hlb, lukED, seg, sei, sem, sen, and seo. The ST72-t324-SCCmec IV genotype was the most prevalent. Of note, three ST22-SCCmec IV isolates were positive for PVL and TSST-1. Our findings suggest that the majority of MRSA isolates from milk obtained from dairy cows with mastitis are multidrug-resistant, capable of forming robust biofilms, and harbor multiple virulence genes. The presence of PVL- and TSST-1-positive ST22-SCCmec IV isolates in mastitis-infected bovine milk highlights the need for persistent monitoring to mitigate possible public health risks. IMPORTANCE This study reports on the presence and characteristics of methicillin-resistant Staphylococcus aureus strains in milk from mastitis-infected cows. To our knowledge, this is the first report of a Panton-Valentine leukocidin- and toxic shock syndrome toxin-1-positive ST22-SCCmec IV strain in South Korea.