Development of a rapid strain differentiation method for methicillin-resistant Staphylococcus aureus isolated in Japan by detecting phage-derived open-reading frames

Aims: To develop a rapid genotyping method for investigating outbreaks of methicillin-resistant strains of Staphylococcus aureus (MRSA) isolated in Japan. Methods and Results: Isolates were genotyped by detecting the keeping pattern of 16 open-reading frames (ORFs), a process we call phage ORF typing (POT). Thirteen of the ORFs were selected from phage genomes and one from a genomic island SaGIm in the genome of strain Mu50. The other two ORFs, one from Tn554 and one from staphylococcal cassette chromosome mec (SCCmec) type II, were used as strain markers. Three hundred and sixty-eight isolates from five hospitals were classified into 133 types by POT, whereas they were classified into 139 types by pulsed-field gel electrophoresis (PFGE) subtyping. The discriminatory power of POT (D = 0.989) was equal to that of PFGE subtyping (D = 0.986). Conclusions: MRSA isolates collected in Japan can be genotyped by detecting the keeping pattern of phage-derived ORFs with a discriminatory power equal to that of PFGE subtyping. Significance and Impact of the Study: MRSA isolates can be genotyped rapidly by detecting phage-derived ORFs. As particular pandemic clones can be found in a specific region, a typing method localized to a pandemic clone may be effective for the rapid genotyping of MRSA during outbreaks.