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

Abstract

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, the phage open- reading frame (ORF) typing (POT) method, is a rapid PCR-based technique, in which the ORFs of the phage genomes lysogenized in MRSA are amplified. The aim of this study was to evaluate the utility of the POT method for the epidemiological analysis of nosocomial MRSA outbreaks. Forty-four strains from 12 episodes of three or more nosocomial MRSA infections in one ward, within a 4-week period, were characterized with the PFGE and POT methods. The strains were classified into 16 distinct POT types and 19 PFGE subtypes. We defined an outbreak as three or more new MRSA infections caused by strains with the indistinguishable genetic pattern. The identification of 11 (91.7%) episodes with PFGE, including four outbreaks and seven sporadic events, was consistent with the results of the POT analysis. These results suggest that POT is a useful epidemiological tool for evaluating nosocomial MRSA outbreaks.