Spa-typing challenging the unique position of PFGE - A comparison of MRSA-isolates from Norway 2006-2007

  • Janne Fossum, Department of Medical Microbiology, St. Olavs Hospital, Trondheim University Hospital, Norway., Norway
  • Lillian Marstein, Department of Medical Microbiology, St. Olavs Hospital, Trondheim University Hospital, Norway., Norway
  • Anne Kilnes, Department of Medical Microbiology, St. Olavs Hospital, Trondheim University Hospital, Norway., Norway
  • Frode Gran, Department of Medical Microbiology, St. Olavs Hospital, Trondheim University Hospital, Norway., Norway
  • Trond Jacobsen, Department of Medical Microbiology, St. Olavs Hospital, Trondheim University Hospital, Norway., Norway

    • Objective: PFGE has been considered the gold standard for typing of S. aureus and hence MRSA, but lately there has been other faster and objective sequence based methods like spa-typing challenging this unique position of PFGE. The Norwegian reference laboratory for MRSA has been using PFGE and spa-typing in parallel during 2006 – 07. The question that needs answering is whether spa-typing is as applicable as PFGE in investigating outbreaks of MRSA in Norwegian settings, as the endemic occurrence of MRSA is low. Both methods focus on rapidly changing genetic events. Is the discrimination of strains similar when comparing results of the two methods?

    Methods: All MRSA strains submitted in 2006 and 2007 (n=876) to the National reference laboratory of Norway was the starting point of investigation. All spa-types represented by ≥ 15 strains, not belonging to only one outbreak, is selected in this comparison. A single person is only represented by one strain. Totally this includes: 418 strains (48 %). This selection of strains is compared to PFGE-clustering results using Bio-Rads fingerprinting II (UPGMA and Dice-coefficient, Position tolerance: 3,0 % and Optimization: 1,0 %).

    Results: Totally 10 spa-types are included when using above criteria. The PFGE-clustering of strains belonging to each of this spa-types show > 80 % similarity for 7 spa-types (t002, t008, t024, t032, t065, t067 and t437), and >85 % similarity for 2 spa-types (t019 and t044). Spa-type t037 seems to be the most heterogenic in PFGE-pattern of the Norwegian strains showing 64 % similarity. Some of the remaining PFGE-clusters are homogenous, and some can be subdivided in several subclusters.

    Conclusion: Among the MRSA strains investigated, PFGE gives a better discrimination compared to spa-typing. When investigating strains suspected to belong to the same outbreak, identical spa-results seems to be sufficient to confirm the relationship. When receiving identical spa-results missing this suspected outbreak link, PFGE can constitute a very useful supplemental investigation to strengthen the suspicion. Since Spa-typing is more convenient, of reasons listed above, it is a good choice as a primary typing-method.