Molecular epidemiology of invasive methicillin-susceptible Staphylococcus aureus strains circulating at a Swiss University Hospital.

In contrast to methicillin-resistant Staphylococcus aureus, little is known of the distribution of spa types among methicillin-susceptible S. aureus (MSSA). We have analyzed 101 nonrepetitive invasive MSSA isolates from infected patients, consecutively isolated during 14 months between 2006 and 2007 at University Hospital Basel. They were genetically characterized according to S. aureus protein A (spa) types and important virulence-associated genes. Sixty-five different spa types corresponding to nine different spa clonal complexes were observed. Analysis of different virulence genes showed a frequency of 17% for toxic-shock syndrome toxin and 5% for exfoliative toxin D. In conclusion, spa typing revealed a great genetic diversity without predominant spa type, not providing evidence for clonal spreading.