Usefulness of mec-associated direct repeat unit (dru) typing in the epidemiological analysis of highly clonal methicillin-resistant Staphylococcus aureus in Scotland

The incidence of the epidemic methicillin-resistant Staphylococcus aureus (EMRSA) strains EMRSA-15 and EMRSA-16 in Scotland has increased dramatically, now accounting for c. 70% and c. 20% of isolates, respectively. Epidemiological tracking of these EMRSA strains is difficult, as c. 50% of EMRSA-15 and c. 35% of EMRSA-16 isolates are indistinguishable using pulsed-field gel electrophoresis (PFGE) and other typing methods. The usefulness of mec-associated direct repeat unit (dru) sequence analysis as a more sensitive approach to tracking the persistence and spread of these 'clonal' EMRSA strains in Scotland was evaluated. Analysis of 47 EMRSA-15 and 57 EMRSA-16 isolates (including two separately cultured isolates of the Harmony collection type strain) obtained from 22 hospital laboratories over an 8-year period (1997-2005) revealed 13 and 12 different dru types, respectively. Whereas some types appeared to be endemic in multiple hospitals, subtypes that may represent specific strain movement among hospitals in a given geographical region were identified in other instances. These results suggest that mec-associated dru typing may have potential for identifying and tracking specific subtypes of otherwise indistinguishable epidemic MRSA isolates such as those in Scotland.