Abstract / Summary
Abstract Staphylococcus aureus ( S. aureus ), a major cause of bovine mastitis, represents a significant challenge for the dairy industry due to its economic and zoonotic importance. This study aimed to investigate the distribution of major virulence-associated genes (lukE, seA, sei), the methicillin-resistance gene (mecA), antimicrobial susceptibility, and biofilm formation in S. aureus isolates obtained from cases of bovine clinical mastitis in dairy farms of East and West Azerbaijan provinces, Iran. A total of 138 milk samples were collected from East Azerbaijan (67; 48.6%) and West Azerbaijan (71; 51.4%) provinces. S. aureus isolates were identified by culture, biochemical tests, and polymerase chain reaction (PCR) targeting the nuc gene for confirmation. Target genes were detected using conventional PCR. Antimicrobial susceptibility was evaluated by the disk diffusion method, and biofilm formation was assessed using the crystal violet microtiter plate assay. Of the 138 samples, 42.0% were positive for S. aureus . The prevalence of lukE, seA, sei, and mecA genes was 39.7%, 36.2%, 17.2%, and 6.9%, respectively. Resistance was highest to penicillin (94.8%) and lowest to gentamicin (22.4%). Overall, 39 isolates (67.2%) were classified as multidrug-resistant (MDR), of which 25.8% were resistant to four or five of the six antimicrobial classes tested and 10.3% were resistant to all six tested antimicrobial classes, with a mean multiple antibiotic resistance (MAR) index of 0.53 ± 0.23. Resistance profiles did not differ significantly between provinces, whereas biofilm formation showed a significant interprovincial difference ( P = 0.002), with a moderate degree of herd-level clustering (ICC = 0.158). These results indicate a high isolation proportion of S. aureus with substantial multidrug resistance and regional variation in biofilm-forming capacity, emphasizing the necessity of targeted antimicrobial stewardship and improved surveillance.