Isolation, antimicrobial resistance and biofilm gene analysis of MRSA in clinical and sub-clinical bovine mastitis.

Methicillin-resistant Staphylococcus aureus (MRSA) is a significant public health concern due to its zoonotic potential, multidrug resistance, and persistence in livestock environments. This study aimed to isolate and characterize MRSA from clinical and subclinical mastitis cases in dairy cows in the rural regions of North Bengaluru, India. A total of 133 milk samples from mastitis-affected cows were analysed, revealing that 48.8 % (65/133) exhibited coagulase-positive colonies on Mannitol Salt Agar. Of these, 28 isolates were confirmed as S. aureus via detection of the nuc gene and among them, 16 isolates (12 %) were positive for the mecA gene, confirming MRSA. Subclinical mastitis accounted for the majority of MRSA isolates (87.5 %) suggesting that asymptomatic carrier of the pathogens. Antimicrobial susceptibility testing of isolates showed high resistance to cefoxitin (75 %) and penicillin (68.75 %), with highest sensitivity observed against chloramphenicol (93.75 %), trimethoprim-sulfamethoxazole (87.5 %), and linezolid (87.5 %). Among MRSA isolates, 62.5 % exhibited multidrug resistance. Molecular screening for biofilm and adhesion-associated virulence genes revealed that all MRSA isolates harboured cna and clfA, while 93.75 % were positive for fnbpA and eno, and 81.25 % for icaA and ebps. The agrI gene was present in 56.25 % of isolates, indicating regulatory variability in biofilm-associated gene expression. These findings highlight the emerging role of MRSA in bovine mastitis, especially in subclinical infections, and emphasize the need for enhanced surveillance, responsible antimicrobial usage, and biosecurity interventions in dairy farming to mitigate the spread of drug-resistant pathogens.