Epidemiological Analysis of Nosocomial MRSA Outbreaks Using Phage Open-Reading Frame Typing in a Tertiary-Care Hospital.

Pulsed-field gel electrophoresis (PFGE) is a reliable method for analyzing outbreaks of methicillin-resistant Staphylococcus aureus (MRSA); however, it is time-consuming and technically demanding. A new strain-differentiation method for MRSA, namely phage open reading frame (ORF) typing (POT), is a rapid PCR-based technique, in which the ORFs of lysogenized phage genomes in MRSA are amplified. The aim of this study was to evaluate the utility of the POT method for epidemiological analysis of nosocomial MRSA outbreaks. Forty-four strains from 12 episodes of 3 or more nosocomial MRSA infections in 1 ward within a 4-week period were characterized using PFGE and POT methods. The strains were classified into 16 distinct types using POT and 19 subtypes using PFGE. We defined an outbreak as 3 or more new MRSA infections caused by strains with indistinguishable genetic patterns. The identification of 11 (91.7%) episodes by PFGE, including 4 outbreaks and 7 sporadic events, was consistent with the results of POT analysis. These results suggest that POT is a useful epidemiological tool for evaluating nosocomial MRSA outbreaks.