Prevalence and Characterization of Staphylococcus aureus and Methicillin-Resistant Staphylococcus aureus Isolated from Guangxi Dairy Farms

<i>Staphylococcus aureus</i> (<i>S. aureus</i>) is a major pathogen responsible for mastitis in dairy cows and can contaminate raw milk, thereby posing significant health risks to consumers. The emergence of methicillin-resistant <i>S. aureus</i> (MRSA) has further heightened public health concerns due to its antibiotic resistance and infectious potential. In this study, we examined the prevalence, virulence genes, antimicrobial resistance, <i>spa</i> types, and biofilm formation of <i>S. aureus</i> isolates from dairy farms in Guangxi Province, China. Among 242 randomly selected samples, 37 <i>S. aureus</i> strains were identified (15.3% infection rate), including 67.5% MRSA. Antibiotic resistance was observed in 78.4% of isolates, with 35.1% exhibiting multidrug resistance (MDR). Enterotoxin gene analysis showed <i>sea</i> as the most common (67.6%), followed by <i>ser</i> (54.1%) and <i>seh</i> (51.4%), whereas <i>seb</i> and <i>selj</i> were absent. All isolates formed biofilms in vitro, with 64.8% showing strong biofilm-forming ability. Staphylococcal protein A (<i>spa</i>) typing classified the 37 <i>S. aureus</i> strains into 11 <i>spa</i> types, with t030 being the most prevalent (43.2%). These findings indicate that <i>S. aureus</i> is moderately prevalent in raw milk, often carrying multiple virulence genes, forming robust biofilms, and showing antimicrobial resistance. The MRSA that is "latent" in raw milk reminds us of the need for monitoring at the farm level.