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Hiroki Takahashi,1,* Masafumi Seki,2,4,* Norihisa Yamamoto,2 Shigeto Hamaguchi,2 Masahiro Ojima,1 Tomoya Hirose,1 Kazuhisa Yoshiya,1 Masahiro Toyokawa,3 Isao Nishi,3 Hiroshi Ogura,1 Takeshi Shimazu,1 Kazunori Tomono21Department of Traumatology and Acute Critical Medicine, Osaka University Hospital, Suita, Osaka, Japan; 2Division of Infection Control and Prevention, Osaka University Hospital, Suita, Osaka, Japan; 3Laboratory for Clinical Investigation, Osaka University Hospital, Suita, Osaka, Japan; 4Division of Infection Control and Prevention, Tohoku Pharmaceutical University Hospital, Sendai, Japan *These authors contributed equally to this work Abstract: Surveillance is very important to prevent the nosocomial spread of methicillin-resistant Staphylococcus aureus (MRSA), and infection sources and routes have historically been identified using molecular and epidemiological genotyping with pulsed-field gel electrophoresis. However, phage-open reading frame typing (POT) has recently been developed. Here, we investigated whether POT would be useful to survey MRSA outbreaks and transmission. We therefore applied POT to 91 MRSA isolates detected in cultures from inpatients at our hospital between May and October 2014. Among the 91 isolates, 12 POT types comprising 38 isolated MRSA strains were considered as overlapping. Five of them were detected in different wards, whereas the remaining seven were found in the same ward, including the emergency department. Three of seven POT number 93-155-111 strains were detected in the surgical ward, and all of four POT number 93-157-61 strains were detected in the cardiosurgical ward. These data suggested that transmission of the MRSA strains with the same POT-types from the same wards was nosocomial, and that POT accurately and rapidly identified MRSA strains, which allowed effective control of infection and transmission. Keywords: MRSA, active surveillance, POT, nosocomial transmission