Staphylococcus aureus is a human commensal colonizing about 30 per cent of the population. Besides, it is a frequent cause of infections such as skin, wound and deep tissue infections and also more life-threatening conditions such as pneumonia, endocarditis and septicaemia. S. aureus may also cause different toxicoses. Moreover, this bacterium is one of the most common causes of nosocomial infections worldwide and an increase in antibiotic resistance, especially against methicillin, is seen. This underlines the importance to prevent and control outbreaks of S. aureus. The aims of this thesis were to increase the knowledge of S. aureus virulence and pathogenesis as well as to understand pattern of colonization and transmission.

Various virulence factors operate together in the pathogenic process of S. aureus. The virulence of S. aureus was studied by the interaction with human umbilical vein endothelial cells (HUVEC) as a model. In paper I, we found that one bacterial isolate survived intracellularly and that 156 genes were differentially regulated in microarray analysis of HUVEC. The major part of these genes coded for proteins involved in innate immunity. In paper II, we wanted to explore possible differences in global gene expression patterns in HUVEC induced by invasive compared to colonizing isolates of S. aureus. We also used microarray to investigate possible differences in the presence of virulence genes between the two groups. The main finding was that virulent and commensal S. aureus did not differ in interaction with HUVEC and in the presence of virulence genes. All isolates survived intracellularly for days.

Since no obvious differences in virulence between the two groups of isolates were found, we focused on epidemiology and transmission patterns. Colonization with S. aureus is an important risk factor for subsequent S. aureus infection. In paper III, we investigated S. aureus colonization and transmission among nursing home residents in three regions in the south of Sweden and used staphylococcal protein A (spa) typing as an epidemiological tool. A diverse distribution of different spa types was found and a majority of types were unique to one individual. Interestingly, we found a local accumulation of one spa type in one nursing home. Also common spa types were equally distributed in the different regions. We also noted that some individuals were colonized with two different spa types of S. aureus and in five of these cases there was one resistant and one non-resistant strain.

The issue of multiclonal colonization and infection is highly important and clinical diagnostic laboratories do not routinely address this problem. Therefore, in paper IV a novel method to assess multiclonality of S. aureus was developed. It was based on denaturing gradient gel electrophoresis with the amplification of the spa gene. The method simultaneously separated eight different spa types. It also detected two spa types in an outbreak.

In conclusion, we found no differences in virulence genes and in the interaction with HUVEC between commensal and invasive isolates. This indicates that any isolate of S. aureus might have a pathogenic potential. We also confirmed that some spa types are more successful colonizers with a potential to nosocomial spread. The method for detection of multiclonality of S. aureus is of importance in future epidemiological and clinical studies.