Comparison of PFGE and spa for routine epidemiological typing to monitor and track spread of MRSA over time at a regional level

Abstract number: P1430

Welinder-Olsson C., Kjellin E., Florén-Johansson K., Larsson L., Berglund C., Söderquist B., Åhrén C.

Objectives: The epidemiology of meticillin-resistant Staphylococcus aureus (MRSA) is presently changing due to the worldwide emergence of community-associated MRSA (CA-MRSA). These clones are more diverse than the classical nosocomial MRSA clones. Pulsed-field gel electrophoresis (PFGE) of genomic macrorestriction fragments has been the gold standard method for epidemiological typing of MRSA. Recently, sequence-based typing methods such as sequencing of the polymorphic X region of the protein A gene (spa typing) have been commonly used due to their ease of interpretation, exportability and speed. For infection control purposes we have collected, typed and stored all MRSA isolates detected in the west part of Sweden together with clinical and epidemiological data since 1983.We here report a comparison of PFGE with spa-typing for monitoring and tracking spread of MRSA over time.

Methods: The first detected CA-MRSA isolate of each PFGE-type (n = 38) was spa-typed, as well as the first isolate of all subsequent PFGE-subtypes of the nine most prevalent groups (n= 73). In addition all consecutive MRSA isolates January 2006 to June 2007 were typed by both methods. PFGE patterns were determined both visually, according to the Tenover criteria, and by computer matching with BioNumerics 5.0 software. Spa-typing was determined with Ridom StaphType software 1.4.11.

Results: For the first 38 isolates of each PFGE-type 25 different spa-types were found. The nine most prevalent PFGE-groups were found within 5 spa-groups according to BURP algorithm. The number of PFGE-subtypes, closely or possible related according to the Tenover criteria, within a group always exceeded that of the spa-types, and the relationship varied from 18:12 to12:2. For PFGE-groups belonging to sequence type (ST) 45 and ST30 PFGE and spa-typing resulted in concordant epidemiological results, in contrast to those consisting of ST80 and ST5. For isolates detected in 2006/07 the previous PFGE- and spa-types usually corresponded, with the exception of two PFGE-groups.

Conclusion: For the current circulating clones in our region, PFGE has a higher discriminatory power than spa-typing with the exception of the PFGE-groups belonging to ST45 and ST30. If only spa-typing had been used isolates of spa-type t002 (ST5) and t044 (ST80) as well as those of t008, t015, t037 and t127 would have mimicked outbreak situations which however not was the case according to the epidemiological investigation.