Genomic analyses of Staphylococcus aureus isolated from yaks in Ganzi Tibetan Autonomous Prefecture, China

Objectives To investigate the transmission and origination of MRSA in livestock with limited antimicrobial use. Yak (Bos grunniens) herds in Ganzi Tibetan Autonomous Prefecture, China were chosen for sampling. Methods The yaks from all 18 districts of Ganzi were sampled (anal swabs, n = 657; nasal swabs, n = 634). Based on the WGS data of 83 Staphylococcus aureus isolates, the novel structure of the yak S. aureus population was described. Phylogenetic analyses were utilized for determining the origin of the MRSA lineage in yaks. Results The yak S. aureus population consisted of 11 STs, 6 of which were previously undescribed, with ST6267 being the predominant novel ST. These isolates were generally susceptible to most of the tested antibiotics and lacked the associated antimicrobial resistance genes (ARGs) but showed high penicillin MIC values (MIC90 = 32 mg/L), which were consistent with the high positivity rate for blaZ (61/83). The MRSA identified in yaks were all ST59 and most likely of human origin. The yak ST59 MRSA each carried the human immune evasion cluster (IEC) while lacking the ARGs that are identified in the majority of reported Chinese human ST59 MRSA isolates [erm(B), ant6-Ia and aph(3 '')-III]. Conclusions The yak herds living on the Qinghai-Tibet Plateau are important livestock and follow the traditional free-grazing farming model. We surveyed the yak S. aureus population and found that all the yak MRSA isolates belonged to the lineage that might originate from the prevalent community-acquired MRSA ST59 in China. From a 'One Health' perspective, the transmission of human MRSA to farming animals with limited antimicrobial exposure highlights the multiple roles of animals in the expansion of MRSA.