text: Staphylococcus aureus (S. aureus) is a leading cause of human infections worldwide and causes a variety of diseases ranging in severity from mild to life-threatening. The ability of S. aureus to cause disease is based in part on its ability to subvert the innate immune system. Recent studies suggest S. aureus evades killing by human polymorphonuclear leukocytes (PMNs or neutrophils). However, relatively little is known about how pathogens detect components of the innate immune system to respond and survive within the host. Therefore, the long term goal of this study is to gain a better understanding of S. aureus pathogenesis. The specific hypothesis is that saeR/S facilitates evasion of innate host defenses by S. aureus and is therefore important for pathogenesis. This hypothesis is based on a few key observations: 1) saeR/S impacts expression of virulence factors; 2) saeR/S appears to play a role in integrating cell density signaling with cues from the environmental stimuli; and 3) saeR/S was highly up-regulated during interaction with human PMNs in five virulent strains of S. aureus. Based on these observations (from our group and others), the focus of this proposal is to determine the function of saeR/S in S. aureus pathogenesis. This will be achieved by studies directed at two specific aims. The first aim will identify the role of saeR/S during interaction with PMNs using an isogenic saeR/S deletion mutant in S. aureus strain MW2 (pulse-field type USA400). This aim will be accomplished by creating a saeR/S deletion mutant and investigating PMN phagocytosis, ROS production, and bactericidal activity toward the saeR/S mutant and wild-type strains. In addition, this aim will determine if saeR/S is induced in wild-type S. aureus by specific PMN components. Oligonucleotide microarrays will also be used to gain a comprehensive analysis of the influence of saeR/S on the S. aureus virulon. The second aim will determine if saeR/S is important for S. aureus virulence using murine models of S. aureus infection. Mouse models will consist of soft-tissue and bacteremia and will be analyzed by histopathology of infected tissues. In summary, we anticipate these studies will provide valuable insight into the role of saeR/S in S. aureus pathogenesis. Perhaps more importantly, the proposed research should identify pathogenic strategies used by S. aureus to initiate infection. [unreadable] [unreadable] [unreadable] [unreadable]