Usefulness of phage open-reading frame typing method in an epidemiological study of an outbreak of methicillin-resistant Staphylococcus aureus infections

A novel genotyping method for methicillin-resistant Staphylococcus aureus (MRSA), the phage open-reading frames typing (POT) method, was evaluated using 92 MRSA isolates collected from blood cultures between 1991 and 2003 at Nagoya University Hospital. These strains were divided into 64 distinct POT types, classified into 21 genotypes by pulsed-field gel electrophoresis (PFGE) using SmaI, and analyzed with the DICE coefficient of 80% in dendrogram analysis, with 48 subtypes analyzed with the DICE coefficient of 100%. The discriminatory indices of these three methods were 0.988, 0.719, and 0.953, respectively. The first and second prevalent PFGE subtypes A1 and A2, which comprised 16 and 13 isolates recovered serially during the study period, were both divided into 11 distinct POT types. Six isolates belonging to PFGE subtype A1 were indistinguishable with POT. The six isolates were probably involved in an outbreak. Phenotypic analysis suggested that these isolates were the siblings of the New York/Japan clone which are prevalent in many Japanese hospitals. In conclusion, in the strain population studied, POT is a more rapid and discriminatory method than PFGE, and is a useful epidemiological tool for evaluating the available clinical information.