Differences in expression of virulence genes amongst invasive and colonizing isolates of meticillin-resistant Staphylococcus aureus.
The prevalence of meticillin-resistant Staphylococcus aureus (MRSA) is an important global concern in healthcare due to potentially life-threatening infections and difficulties in treatment. The organism can colonize the mucosa, e.g. anterior nares or result in invasive infections, e.g. bloodstream infections, through the coordinated expression of extracellular and cell-bound virulence factors (Goerke et al., 2000; Novick et al., 1993). The transcription profiles of virulence genes vary in-vivo depending on the dynamic interaction between the host environment and the pathogen. Since the dynamic environment may be differentially altered in MRSA carriage compared to infection, the expression of S. aureus virulence genes may also vary. We previously found no significant correlation between the presence of 17 virulence genes and invasiveness of MRSA isolates (O'Donnell et al., 2008) and this non-association of specific genes or combinations of genes with invasive isolates is in agreement with other larger studies (Lindsay et al., 2006). The lack of a definitive correlation between invasive S. aureus strains and the carriage of virulence genes, suggests that the expression, rather than carriage of virulence determinants in-vivo, may mediate pathogenicity.