spa typing alone is not sufficient to demonstrate endemic establishment of meticillin-resistant Staphylococcus aureus in a low-prevalence country

Background: The prevalence of meticillin-resistant Staphylococcus aureus (MRSA) in Norway is low but increasing. Over the last decade, numerous nursing homes have experienced MRSA outbreaks. One genetic lineage, spa type t304, has been identified at multiple nursing homes and has caused large outbreaks lasting for several years.

Aim: To evaluate whether spa typing is sufficient for the detection of MRSA spread and endemic establishment in a low-prevalence area, using spa type t304 as the test organism.

Methods: All spa type t304 isolates detected in 1991-2010 in the most densely populated area of Norway were included. Time and place of bacterial sampling were recorded. The isolates were analysed using multi-locus sequence typing, staphylococcal cassette chromosome mec typing, detection of lukS/F-PV and pulsed-field gel electrophoresis (PFGE).

Findings: In total, 181 spa type t304 isolates were identified in three of 23 municipalities. Most (91%) of the isolates could be linked to 13 nursing homes, eight of which experienced outbreaks. PFGE analysis revealed three PFGE types, consisting of 19 PFGE patterns; 95% of the isolates were PFGE type 2. In total, PFGE types 2 and 3 accounted for 99% of all nursing home isolates, and included isolates from different nursing homes, different outbreaks and different time periods. Additional genetic analyses did not further differentiate between the spa type t304 isolates.

Conclusion: MRSA spa type t304 appears to have established itself as an endemic genetic lineage in the study area. spa typing does not provide sufficient resolution when investigating the spread of an endemic-like genetic lineage in a low-prevalence area, and should be supplemented by additional typing techniques.