Phylogenomic analysis of a methicillin-resistant Staphylococcus aureus ST764 isolate from Thailand in global context reveals limited cross-border transmission.

Methicillin-resistant Staphylococcus aureus (MRSA) ST764, a variant of ST5, has emerged and spread in Japan and China. We investigated the genome of a Thai isolate (SATU136) and the global phylogeny of ST764 to understand its global transmission history. The complete genome of SATU136 consists of a 2.89 Mb chromosome with a type II SCCmec element and a qacB-carrying plasmid but lacks the arginine catabolic mobile element (ACME). Based on the currently available global dataset, phylogenetic and phylodynamic analyses suggest that ST764 emerged in Japan in the early 1980s and was subsequently inferred to have disseminated to China and Thailand during the 1990s and early 2000s, coinciding with a peak in its overall effective population size. After this period, transmission was inferred to have become more geographically structured, with distinct clades forming in each country. Although cross-country transmission was inferred to be limited overall, onward dissemination to non-Asian countries was also detected within the Thailand-associated lineage. In contrast, the ACME element remained confined to a single Japanese subclade, with currently no evidence of spread beyond Japan. The relatively structured geographic distribution of ST764 offers a window for early detection upon introduction to new countries, which may facilitate control of its spread.