, 2003; Novick & Jiang, 2003), suggesting that the sae transcription could be influenced by Agr in some strains, but acts independent of Agr in other strains (Ross & Novick, 2001). In the present study, we describe the expression pattern of ssl5 and ssl8 in the early stationary phase in several S. aureus strains belonging to different clones. It appears that the regulation of ssl5 and ssl8 expression in S. aureus is strain specific as they varied even within an ST and gene haplotype (Fig. 1). Staphylococcus aureus is known to show a differential expression of genes implicated in virulence. Harraghy et al.

(2005) observed marked differences in the expression of staphylococcal adhesins, eap and emp between Newman and NCTC8325 derivative strains, SH1000 (8325-4 rsbU+) and 8325-4 (rsbU−). Our data show that the ssl5 and ssl8 expression is downregulated Torin 1 in vitro in the sae PD-0332991 chemical structure mutant strain and upregulated in the agr mutant strain, suggesting that Sae and Agr are possible inducers and repressors, respectively, for ssl5 and ssl8 in the Newman strain (Fig. 4). Indeed, downregulation of several proteins including SSL7 and SSL11 has been observed in a Newman sae mutant strain (Rogasch et al., 2006). The Newman strain is characterized by unusually high sae levels, which have been confirmed in this study as well. The high sae

expression in this strain can be attributed to a point mutation in the sensor histidine kinase of the SaeR/S two-component regulatory system (Steinhuber et al., 2003; Geiger et al., 2008). Proteomics and microarray analyses have revealed that most of the genes influenced by Sae are involved in bacterial adhesion, immune evasion, immune modulation, or toxicity (Foster, 2005; Liang et al., 2006; Rogasch et al., 2006). Non-specific serine/threonine protein kinase More importantly, it has been shown that sae is essential for virulence gene expression in vivo (Goerke et al., 2001). It was interesting to observe the suppressive effect of Agr on ssl5 and

ssl8 expression, suggesting that Agr does not always act as a positive regulator for virulence gene expression in S. aureus, and inhibiting the Agr function to reduce virulence could have other consequences (Otto, 2001). Loss of Agr increases the bacterial colonization, biofilm formation, and attachment to polystyrene, suggesting that the agr mutant strain may have a greater capacity to cause chronic infections than agr-positive strains (McNamara & Bayer, 2005). We speculated that the lack of Agr could have caused the enhanced expression of some proteins that aid in the upregulation of ssl5 and ssl8. Surprisingly, we found that the agr mutation caused increased sae transcript levels and vice versa, which indicated that the sae and agr could have an inhibitory effect on each other, and repression of ssl5 and ssl8 genes by Agr is dependent on Sae in the Newman background.