However, Voyich et al. (2009) reported that ssl11 and

the agr operon in a saeR/S mutant of MW2 strain is downregulated by ∼16- and 2-fold, respectively, at the early stationary growth phase. In concordance with our data, Liang et al. (2006) showed, by RT-PCR analysis, that agrA mRNA levels were significantly upregulated in the saeS null mutant compared with its wild-type strain, WCUH29, a virulent clinical isolate. Taken together, these data suggest that the influence of saeR/S on the transcriptional regulation of virulence genes is probably dependent on multiple factors including the genomic background of the strain studied (Liang et al., 2006; Rogasch et al., 2006). Interestingly, in the agr/sigB double mutant, the expressions of ssl5, ssl8, and sae was downregulated (Fig. Epacadostat price 2). However, in the agr mutant strain, these genes were upregulated, whereas the expression of either ssl5 or ssl8 did not change in

a Newman sigB mutant. This suggests that SigB probably acts synergistically with Agr, but not alone, to upregulate ssl5 and ssl8. This could very well be mediated by sae specifically in the Newman strain. An analogous phenomenon such as enhanced repression of exotoxin-encoding genes in double mutants of regulatory genes in S. aureus is not uncommon. For example, sar and agr double mutants are less virulent compared with the agr single mutant (Booth et al., 1997). Differences in the transcript levels of regulatory genes (agr, sarA, sigB, and saeR/S) have been reported between COL and Newman strains that correlate well with the expression of virulence-associated genes (Rogasch et al., 2006). In summary, ssl5 Thiazovivin nmr and ssl8 expression in S. aureus clinical isolates is strain dependent and not influenced by differences in their alleles. They are positively regulated by Sae and negatively

Elongation factor 2 kinase by Agr in the Newman strain. Furthermore, the ssl5 and ssl8 repression by Agr is probably achieved by the downregulation of Sae in the Newman strain. This is the first report of a negative regulation of an ssl gene by Agr. This study also highlights the potential challenges in managing infections due to S. aureus strains, which could potentially produce varying amounts of SSLs. Understanding the intricacy of global regulatory genes and their mode of regulation in different genetic backgrounds would provide an important insight into the molecular mechanisms of staphylococcal virulence. This may perhaps reveal specific targets, which would enable therapeutic intervention in S. aureus infections. This research was funded in part by research grant RO1 AI061385 from the National Institutes of Allergy and Infectious Diseases to S.K.S. The authors thank James Burmester and Joseph Mazza, Marshfield Clinic Research Foundation, for critically reviewing the manuscript. Table S1. List of SNPs identified in the ssl5 coding and upstream regions in Staphylococcus aureus strains.