Purpose: The aim of this study was to identify the subtype, characterize the antimicrobial resistance, determine the virulence gene distribution, and analyze the biofilm production of Staphylococcus aureus  isolates from bovine mastitis milk samples in the Liaoning Province of China.
Materials and Methods: In total, 56 Staph. aureus  isolates were collected and identified in this study; the isolates were divided into different spa types based on the sequence of the polymorphic X region of the spa  gene. Additionally, antimicrobial susceptibility was investigated using the broth microdilution method, and 18 virulence genes were detected using PCR. Biofilm formation was measured by spectrophotometry with crystal violet staining and observed using confocal laser scanning microscopy.
Results: There were 12.12% (56/462) milk samples that were positive for Staph. aureus . These isolates were nonsusceptible to sulfamethoxazole (100%), penicillin (76.9%), daptomycin (76.79%), clindamycin (69.64%), and oxacillin (60.71%); however, the majority of the isolates (80.4%) were susceptible to amoxicillin/clavulanate. The predominant virulence genes encoded the cytotoxins, hla  (94.64%) and hlb  (89.29%), and the adhesion factors clfA  (89.29%), clfB  (89.29%), and fnbB  (80.36%). Comparatively, virulence genes related to other adhesion factors such as cna  (8.93%) and enterotoxins, such as seg  (26.79%), sea  (16.07%), seb  (7.14%), and sec  (7.14%) were detected at relatively lower rates. The following eight spa types were identified: t267 (35.84%), t730 (22.64%), t518 (15.09%), t1190 (11.32%), t1456 (9.43%), t224 (1.88%), t9129 (1.88%), and t177 (1.88%). The highest biofilm production was observed for t267. Staph. aureus  exhibited various patterns of biofilm formation, with the biofilm often being associated with a tower-shaped structure or a thicker biofilm.
Conclusion: Our results indicated that Staph. aureus  isolates from dairy cows with mastitis in the Liaoning Province of China were non-susceptible to sulfamethoxazole, penicillin, daptomycin, oxacillin, and clindamycin. Additionally, the most prevalent subtype was t267, which displayed resistance to multiple antimicrobial agents and harbored several virulence genes, including clfA, clfB, fnbB, hla , and hlb .
Keywords: Staph. aureus , antimicrobial resistance, virulence factors, biofilm formation, mastitic dairy