Molecular epidemiology of methicillin-resistant Staphylococcus aureus isolated from pigs in Japan

Methicillin-resistant Staphylococcus aureus (MRSA) is the leading cause of infection in hospitalized patients and can be prevalent in humans and various animal species. In European countries, MRSA isolates belonging to clonal complex 398 have been detected at high rates in pigs. However, the prevalence of MRSA in pigs and farm environments in Japan remains unclear. MRSA isolates were obtained from pigs in slaughterhouses, diseased pigs on farms, imported breeding pigs, and farm dust. We conducted whole-genome sequencing (WGS) and analyzed the molecular epidemiological relationship between these MRSA isolates using core genome multilocus sequence typing (cgMLST). The prevalence rates of MRSA among pigs in slaughterhouses, diseased pigs on farms, imported breeding pigs, and farm dust were 5.2 %, 3.4 %, 28.8 %, and 0.06 %, respectively. ST 398 isolates that classified as ST398/t034 were isolated from pigs from all sources. The results of cgMLST showed that ST398/t034 isolates originating from domestic pigs clustered into the same cluster as the isolates from imported breeding pigs. However, some clusters only included isolates of domestic pig origin. Most MRSA isolates in this study carried resistance genes for aminoglycosides, β-lactams, macrolides, tetracyclines, and zinc. None of the MRSA isolates in this study harbored Panton-Valentine leukocidin toxin genes. Molecular epidemiological analysis suggested a relationship between isolates from slaughter pigs and imported breeding pigs and the presence of MRSA isolates of domestic origin. However, more data are needed for elucidation of the origin of these MRSA variants in the pig industry in Japan.