PROJECT SUMMARY/ABSTRACT Periodontitis, or gum disease, affects approximately 50% of the adult population in the United States and can result in tissue damage and bone loss [1,2]. This disease is unusual in that it is not caused by a specific organism, but by a consortia of microbes including opportunistic pathogens and commensals [2]. The complex interactions between oral microbes influence the progression and severity of disease, as well as its resistance to treatment [3–7]. Studying these interactions in vivo is challenging, hindering our ability to understand periodontal disease. Productive approaches to characterize polymicrobial interactions have included the use of model oral communities and animal models of periodontitis [5,8–10]. Previously, the Whiteley lab used these approaches to identify cross-feeding and cross-respiration mediated interactions between the opportunistic pathogen Aggregatibacter actinomycetemcomitans (Aa) and the oral commensal Streptococcus gordonii (Sg) [6,11,12]. In addition, these interactions increase Aa’s resistance to certain components of the innate immune system [8]. In this study, the overarching hypothesis is that metabolic interactions impact the survival of pathogens in the oral cavity. This proposal aims to expand our understanding of oral polymicrobial interactions in two significant ways. First, it proposes to characterize how interactions between Aa and Sg alter the ability of Aa to resist the innate immune system, antimicrobial compounds, and the invasion of other oral microbes using a combination of in vitro manipulations and transposon sequencing (Aim 1). Second, this proposal aims to use the framework developed in characterizing the interactions between Aa and Sg as a model to identify metabolically-driven interactions between Aa and a diverse set of microbes (Aim 2). The metabolic impact of interactions will be characterized by comparing the genes necessary for Aa fitness in co-culture in a murine abscess model of infection with those necessary in vitro in chemically defined manipulations. This Aim will also serve to improve the functional annotation of the Aa genome, and potentially genomes of other closely-related clinically relevant microbes in the Pasteurellaceae family, including Haemophilus. Overall, the proposed research will greatly expand our understanding of how metabolic interactions in the oral cavity influence the persistence of pathogens during periodontitis.