Lineage-Associated Genomic and Phenotypic Features of Bovine Mastitis-Associated Staphylococcus aureus from Northwestern China

Staphylococcus aureus is a major contagious pathogen causing bovine mastitis, with persistence and antimicrobial resistance posing ongoing challenges in dairy herds. From July 2022 to July 2023, 942 mastitis milk samples (267 clinical, 675 subclinical) from 10 farms in northwestern China yielded 98 S. aureus isolates (10.4%), which were characterized by whole-genome sequencing, antimicrobial susceptibility testing, biofilm assays, and virulence profiling. Ten sequence types, seven clonal complexes, and 16 spa types were identified, dominated by ST1 (42.9%) and ST97 (24.5%). ST1 was mainly associated with subclinical mastitis, whereas ST97 was enriched in clinical cases; a novel ST9955 (13.3%) was exclusively detected in subclinical isolates. No methicillin-resistant S. aureus was detected. Resistance was most frequently observed to penicillin (42.9%), erythromycin (28.6%), clindamycin (22.4%), and tetracycline (10.2%), with higher rates in clinical isolates. 65.3% of isolates exhibited strong biofilm-forming ability, which was significantly more frequent in subclinical cases (70.3% vs. 55.9%, p < 0.05). Virulence genes showed conserved adhesion/biofilm determinants but lineage-dependent distribution of immune evasion and toxin genes, with stable co-occurrence among capsule, biofilm, T7SS, iron acquisition, and immune evasion loci. Overall, the population was diverse but strongly lineage-structured, dominated by a few successful clones, highlighting the value of lineage-resolved surveillance for understanding and controlling bovine mastitis.