Objective
Since 2003 a major increase in MRSA-positive persons has been observed in Denmark. More than every third of the MRSA-cases in Copenhagen has been caused by MRSA with spa type t024. We wanted to address the qquestion: has t024 in Copenhagen arisen by clonal spread of one successful staphylococcus or have several clones of t024-MRSA spread independently of each other as part of the general spread of MRSA?

Methods
All MRSA t024 isolates and derivates of t024 from 2003-6 were included, a total of 302 isolates. In addition, four t024-isolates from other countries were analyzed. The MRSA-collection was characterized by spa-typing, SCCmec-typing and IV-subtyping, PVL-detection, sequence–based dru-typing, and on some isolates MLST.

Results
One isolate was PVL-positive, six were dru-negative. Most had SCCmec IVa with 10 dru-repeats (dru-type dst10A). Two clones were identified. The major clone (ST8-IVa-dst10A) was found in central Copenhagen, a minor clone (ST8-V-dst11B) in an area approximately 70 km from Copenhagen. Eight sequence variants of the dru-region were found, all derived from dst10A.

Conclusions
MRSA of spa-type t024 is primarily found in residents of nursing homes and in elderly people receiving home care and has in some cases spread to staff members and in hospitals. Two t024-clones were identified, one in Copenhagen and one outside the city, differing by their SCCmec cassette. The t024-ST8-MRSA-IVa-dst10A clone has successfully spread in the health care associated setting and several dru-type variants have evolved.