First report of sasX-positive methicillin-resistant Staphylococcus aureus in Japan

SasX is a known virulence factor of Staphylococcus aureus involved in colonisation and immune evasion of the bacterium. The sasX gene, which is located on the fSPß prophage, is frequently found in the sequence type (ST) 239 S. aureus lineage, which is the predominant healthcare-associated clone in Asian countries. In Japan, ST239 clones have rarely been identified, and sasX-positive strains have not been reported to date. Here, we report the first identification of 18 sasX-positive methicillin-resistant S. aureus (MRSA) strains in Japanese hospitals between 2009 and 2011. All sasX-positive isolates belonged to an ST239-staphylococcal cassette chromosome mec type III (ST239-III) lineage. However, we were unable to identify additional sasX-positive MRSA strains from 2012 to 2016, indicating that the small epidemic of sasX-positive isolates observed in this study was temporary. The sequence surrounding sasX in the strain TOHH628 lacked 51 genes that encode phage packaging and structural proteins, and no bacteriophage was induced by mitomycin C. Additionally, in the TOHH628 strain, the region (64.6 kb) containing sasX showed high identity to the fSPß-like element (71.3 kb) of the Taiwanese MRSA strain Z172. The data strongly suggest that the present sasX-positive isolates found in Japanese hospitals were transmitted incidentally from other countries.