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S. aureus populations consist of about ten dominant human lineages and many minor lineages. The dominant human lineages of S. aureus are often referred to by their clonal complex (CC) numbers, although they can be detected by MLST, spa typing or microarray analysis. They are CC1, CC5, CC8, CC12, CC15, CC22, CC25, CC30, CC45 and CC51 (Feil et al., 2003). MRSA isolates are those S. aureus that have acquired SCCmecelements and the most common MRSA in hospitals belong to CC1, CC5, CC8, CC22, CC30 and CC45. However, some uncommon S. aureus lineages have become more successful when they have acquired SCCmec, especially CC59, CC80 and CC239, the latter appears to be a hybrid of CC8 and CC30 (Robinson and Enright, 2004). Each MRSA has a unique geographical distribution (Cockfield et al., 2007). As we do not know enough about methicillin-sensitive S. aureus (MSSA), it could be that there are also geographical differences for MSSA. Indeed, a recent study in Mali, Africa, identified a novel dominant clone, CC152 (Ruimy et al., 2008).Several studies of S. aureus isolates from different host species indicated that there is a high degree of host specialization 33, 47. Studies of the phylogeny of bovine and human isolates have indicated that strains (identified by PFGE, spa-type or MLST) that are isolated from one host species tend to be uncommon in the other species. For example, there are about 10 major human lineages or clonal complexes that are distributed globally (CC1, CC5, CC8, CC12, CC15, CC22, CC25, CC30, CC45 and CC51) 22. Of these lineages CC1, CC5, CC8, CC22, CC30 and CC45 contain the most common MRSA strains. Each S. aureus lineage also has a characteristic geographical distribution with some clones being broadly distributed while others have a more limited distribution 52. Staphylococcus aureus is one of the most important human pathogens, causing life-threatening infection in the community and hospital setting. The population genetics of S. aureus and the evolution of virulence is the focus of this review. We describe the various techniques in determining S. aureus population structure and discuss the insights gained from whole genome sequencing of various S. aureus strains. The emergence of community-acquired, methicillin-resistant S. aureus provides a framework for the discussion on evolution of virulence, and the role of horizontal gene transfer in the development of virulence and antibiotic resistance is explored. The knowledge generated from population genetics has the potential to inform strategies to assist in the prevention or treatment of this highly successful human pathogen.s. aureus

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