Phenotypic and Molecular Characterization of Staphylococcus aureus in Dairy Farms from Henan Province and the Inner Mongolia Autonomous Region, China

<i>Staphylococcus aureus</i>, a prevalent pathogen associated with infectious and foodborne diseases, is also a significant cause of intramammary infections in dairy farms. This study aimed to determine the phenotypic and molecular characterization of <i>S. aureus</i> in two different stock sizes of dairy farms in Henan province (HN) and the Inner Mongolia autonomous region (IM), China, through biofilm formation, antimicrobial resistance, virulence, and molecular type of <i>S. aureus</i> isolates. In HN, 74 <i>S. aureus</i> isolates (60.7%) were recovered from 122 bulk tank milk samples, while in IM, 24 <i>S. aureus</i> isolates (17.4%) were detected from 161 samples soured from various origins. Notably, 25.7% (19/74) of isolates in HN and 20.8% (5/24) in IM exhibited multidrug-resistant (MDR) phenotypes. Molecular typing revealed distinct patterns: ST97 (n = 32) and <i>spa</i> type t189 (n = 20) predominated in HN, whereas ST50 (n = 13) and <i>spa</i> type t518 (n = 11) were prevalent in IM. Additionally, three isolates harbored both <i>tsst-1</i> and <i>lukF-PV</i> genes, and two MRSA strains displayed a MDR phenotype in raw milk samples from HN. Biofilm formation was observed in 91.8% strains. Phylogenetic analysis identified two subpopulations (lineages 1 and 2). Among them, cluster 6 in lineage 2 comprised <i>S. aureus</i> strains from three sources within a farm, suggesting potential cross contamination during different stages in IM. Remarkably, among 19 MDR isolates in HN, ST398 MSSA strains exhibited a higher multidrug resistance compared to non-ST398 MSSA strains. This study underscores the high prevalence and diverse characteristics of <i>S. aureus</i> in raw milk, necessitating enhanced surveillance and control measures to mitigate associated risk.