Project Summary Oral bacteria exacerbate a number of systemic diseases, although the mechanisms influencing such processes are unclear. Mononuclear phagocytic cells, front-line managers in the removal of invading oral bacteria are often unwitting associates in the dissemination of microbes. However, many details of the cellular interactions of oral microorganisms with phagocytes remain poorly characterized, including how changes in these interactions resulting from oral microbiome dysbiosis can allow commensal organisms to become pathogenic. The overall aim of this research is to increase our understanding of the dynamic phagocyte interactions with oral bacteria. The long-term goal is to use insights regarding the molecular mechanisms by which oral microorganisms evade the immune system to reduce the contributions of these organisms to disease. Oral streptococci are associated with several systemic diseases, including infective endocarditis. An important virulence determinant of these bacteria is their ability to evade destruction by phagocytic cells, yet the molecular mechanisms allowing for this subversion are mostly unknown. This proposal will use Streptococcus gordonii as a model oral streptococcus to define the mechanisms by which resistance to destruction by phagocytosis occurs. The central hypothesis is that S. gordonii combines resistance to reactive oxygen species (ROS) with an active ability to damage phagosomes to avoid destruction by these immune cells. The hypothesis stems from published reports showing oral streptococci can survive within phagocytes and preliminary data indicating S. gordonii with virulence potential have increased ROS resistance, an ability to prevent phagosome maturation, and a concomitant ability to survive within macrophages. The hypothesis will be tested by pursuing the following two specific aims: 1) Identify genes responsible for increased ROS resistance and survival in phagocytes by virulent S. gordonii strains. It's hypothesized that ROS resistance imparts an important initial resistance to killing of S. gordonii within phagocytes, allowing for increased virulence potential. In this Aim we will confirm our preliminary data regarding the roles of the specific S. gordonii genes. 2) Understand the mechanics of phagosomal disruption induced by S. gordonii. Preliminary data indicates that live virulent S. gordonii alters the ability of phagosomes to fully mature to phagolysosomes. It's hypothesized that the bacterium directly disrupts phagosome integrity. This aim will test for bacterial-induced damage to phagosomes, bacterial-induced alteration of phagosomal maturation, and phagosomal ROS production. With these aims we will verify essential genes accounting for ROS resistance and detail the basic mechanisms underlying the ability of pathogenic streptococci to alter phagosome maturation kinetics. Taken together, the proposed studies will provide essential insights regarding the mechanisms through which normally commensal oral bacteria can contribute to both local and systemic disease, which may be exploited for treatment.