Molecular Epidemiology of Methicillin-Resistant Staphylococcus aureus in Zürich, Switzerland (2003): Prevalence of Type IV SCC mec and a New SCC mec Element Associated with Isolates from Intravenous Drug Users

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Abstract
The majority of methicillin-resistant Staphylococcus aureus (MRSA) isolates, recovered in 2003 at the Department of Medical Microbiology in Zürich, Switzerland, belonged to major clones that are circulating worldwide. Staphylococcal cassette chromosome mec type IV (SCC mec -IV), harbored by half of the isolates, was found in sequence type 217 (ST217), which is an allelic variant of epidemic MRSA-15 (designated EMRSA-15), in a new local ST617 descending from clonal complex CC8 and in low-level oxacillin-resistant strains of multiple genetic lineages characteristic of community-onset MRSA. SCC mec -I, SCC mec- II, and SCC mec -III were in the minority, and four MRSA isolates had complex, rearranged SCC mec elements. A novel SCC mec -N1 of approximately 30 kb, associated with a dfrA gene and a ccr4 -related recombinase complex, was identified in a large number of low-level oxacillin-resistant isolates, which descended from the successful clonal complex CC45 and are spreading among intraveneous drug users. In contrast, the SCC mec types of oxacillin-resistant coagulase-negative staphylococci (MRCNS) were of completely different composition. SCC mec type I (SCC mec -I) and SCC mec -II were more frequent than in the MRSA, while fewer contained SCC mec -IV. The other MRCNS displayed 11 different, complex patterns, suggesting frequent recombination between different SCC mec elements. With one ccr -negative exception, these strains amplified between one and three different ccr products, indicating either new varied complexes or multiple ccr loci. This suggests the presence of novel SCC mec types in MRCNS and no extensive interspecies SCC mec transfer between MRSA and MRCNS.

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