spa Typing and Antimicrobial Susceptibility of Bovine Mastitis-Associated Staphylococcus aureus Isolates from Dairy Farms in South Korea

Background: Staphylococcus aureus is a major contagious pathogen responsible for persistent bovine mastitis, resulting in substantial economic losses to the dairy industry.This study investigated the molecular epidemiology and antimicrobial susceptibility of S. aureus isolates recovered from clinical and subclinical bovine mastitis in South Korea.Methods: A total of 41 S. aureus isolates (11 clinical and 30 subclinical) were analyzed for antimicrobial susceptibility and spa types based on the Staphylococcal protein A-encoding gene.Clinical isolates were recovered from mastitis cases submitted by dairy farms nationwide, whereas subclinical isolates were collected from six commercial dairy farms, including a certified organic farm.Results: Resistance among clinical isolates was observed to benzylpenicillin (45.5%), gentamicin (18.2%), clindamycin (9.1%), erythromycin (9.1%), and minocycline (100%), whereas subclinical isolates showed resistance only to trimethoprim/sulfamethoxazole (16.7%).Intermediate susceptibility to cefpodoxime was detected in 54.5% of clinical isolates and 6.7% of subclinical isolates.All isolates remained susceptible to oxacillin.Of the 41 isolates, 33 (80.5%) were assigned to known spa types, including t189 (n = 13), t164 (n = 10), t9580 (n = 6), t2883 (n = 1), t034 (n = 1), t127 (n = 1), and t518 (n = 1).One isolate (2.4%) was identified as a potential novel spa type, and seven (17.1%) were untyped.Conclusion: These findings provide updated insights into the molecular epidemiology of bovine S. aureus and demonstrate the predominance of farm-specific spa types in Korean dairy herds, emphasizing the importance of continued molecular surveillance for monitoring the dissemination of circulating lineages and improving mastitis control.