Quantification of virulence-associated gene transcripts in epidemic methicillin resistant Staphylococcus aureus by real-time PCR.

The control of Staphylococcus aureus virulence gene expression is complex and few data are available for the epidemic methicillin resistant S. aureus clones circulating in the UK. Quantitative real-time PCRs were developed for key gene transcripts involved in S. aureus infection (RNAIII, hla and spa) and for the 16S rRNA. These assays were applied to log and stationary phase cultures of the important EMRSA strains. To correct for inconsistencies in extract yield, results were calculated as ratios using the 16S rRNA values as denominator. The quantitative assays were sensitive and reproducible. The number of copies of each transcript present differed greatly between the EMRSA strains tested. Strains within an EMRSA clone or type gave similar results. High levels of RNAIII transcripts were not consistently linked to elevated levels of hla transcripts or to low levels of the spa transcript. In addition, strains showed significant variations in their patterns of induction (or repression) of all three transcripts. A complex interplay exists between the regulatory factors that control the expression of proteins required for colonisation and survival in the host. The transcript level data suggest that this pattern shows great diversity among the currently important EMRSA strains.