Objective
Staphylococcus aureus isolates belonging to clonal complex 398 (CC398) have emerged worldwide over the last few years, and has been associated with livestock, particularly pigs. Denmark is a high producer of pigs with a production of ~25 mill/year. Isolates of CC398 has been found in Denmark since 2003. The aim of this study was to characterize S. aureus belonging to CC398 isolated in Denmark.

Methods
In Denmark, all MRSA and most Staphylococcus aureus isolates from blood are referred to the Staphylococcus reference laboratory at Statens Serum Institut for typing. Isolates belonging to CC398 were included in this study. All isolates were spa typed and susceptibility tested, and MRSA isolates were furthermore confirmed to be carrying the mecA gene as well as SCCmec typed.

Results
In the period 2003 - May 2008, 53 new cases of either infection or colonisation with MRSA CC398 were detected. Of these, 50 isolates (94%) belonged to spatype t034, while two were t108 and one was t1793. Furthermore, 41 of the t034 isolates could be divided into three main groups. One group of 30 contained an SCCmec cassette type V, were Panton-Valentine Leukocidin (PVL) negative and tetracycline resistant. The majority of these isolates were in persons with connection to pig farming. Another group of five isolates were also carrying SSCmec type V but were PVL positive and tetracycline sensitive. These cases had an association with children adopted from China, where PVL positive CC398 cases recently have been reported. A third group of six isolates was carrying an SCCmec cassette containing ccrAB2 (SCCmec type II or IV) but a mec class could not be determined. These isolates were PVL negative and tetracycline resistant, In addition, one isolate was carrying an SCCmec IV cassette and were tetracycline sensitive. The SCCmec types for the remaining eight isolates have not been fully determined.
Four methicillin-sensitive S. aureus (MSSA) bacteraemia isolates were found to belong to CC398 and two of the four isolates were t034 and tetracycline resistant.

Conclusion
This study shows that CC398 is emerging in Denmark, and although the variation in spatypes is small, there are still several subtypes of this clone with an identified diversity in SCCmec types, resistance patterns and harbouring of virulence genes.