Aminoglycosides and Virulence Determinants of Methicillin-Resistant Staphylococcus aureus from Cow Milk, Faeces, and Teat Swabs Associated with Subclinical Mastitis in Dairy Cows

Staphylococcus aureus infections, including methicillin-resistant S. aureus (MRSA), inflict considerable economic burdens upon the dairy industry through bovine subclinical mastitis. Here, S. aureus isolates were recovered from milk, cow dung, and teat swabs of dairy cows (n = 68) on three commercial farms in the Free State Province, South Africa, and screened for antibiotic resistance determinants and virulence-associated genes. Out of 204 samples collected (68 milk samples, 68 teat swabs, and 68 cow dung samples), S. aureus isolates were recovered from 50/68 (73.5%) milk samples, 42/68 (61.7%) teat swab samples, and 15/68 (22.0%) cow dung samples. The mecA-targeted PCR screening identified genotypic MRSA in 14.0% (7/50) milk, 11.9% (5/42) teat swabs, and 40.0% (6/15) cow dung isolates. Screening for aminoglycoside resistance genes identified a high prevalence of strB among all isolates, often concomitant with strA, aadA, and aac(6′)-Ib. Successful amplification of virulence determinants (spa, coa, hla, hlb) identified the highest allelic diversity among milk isolates. Phenotypic–genotypic discrepancy was observed with regard to cefoxitin, where, despite all isolates carrying mecA, only 14.3% (1/7) milk, 50.0% (3/6) cow dung, and 0.0% (0/5) teat swab isolates were phenotypically resistant, while the remaining isolates were either intermediate or susceptible. Low resistance was detected toward streptomycin, tobramycin, gentamicin, and amoxicillin/ac. clavulanique. The co-occurrence of these antimicrobial resistance and virulence determinants represents possible risk trajectories that are important for both human health and dairy herd health. These findings indicate the need for farm-specific hygiene practices, enhanced mastitis prevention, and continued molecular epidemiologic surveillance to elucidate staphylococcal ecology within the “One Health” paradigm.