PROJECT SUMMARY Type 2 diabetes (T2D) is a significant and increasingly prevalent disease in the US population. It substantially increases the risk for chronic periodontitis (PD), another important public health problem affecting nearly half of the American adults. Conversely, PD adversely affects glycemic control in T2D patients, supporting a bi-directional relationship between these two major chronic diseases. Currently, the mechanisms underlying this two-way relationship are not well understood. PD is an inflammatory disease associated with the alterations of the subgingival microbiome. In T2D, the host immune response is altered, which could affect the host-biofilm interaction of the subgingival microbiome and thus disease etiology. However, comprehensive analysis of the subgingival microbiome and its changes upon treatment, and the associated functions including host responses in T2D with PD is lacking. To address this knowledge gap, we propose to investigate the subgingival microbiome and host responses in T2D patients with different levels of glycemic control and systemically healthy, non-diabetic individuals (ND) with PD. Specifically, we hypothesize that the response of the subgingival microbiome to PD treatment in T2D patients with different levels of glycemic control and ND individuals differ at the levels of transcriptional activities and spatial organization of key community members. We also hypothesize that the host immune responses including neutrophil behavior differ significantly among the groups and play an important role in shaping the microbiome. To reveal whether microbiome composition and activities differ between T2D and ND in response to treatment, in Aim 1, we will characterize the composition and transcriptional activities of the subgingival microbiome in T2D patients with well, moderately, or poorly controlled glycemic level, in comparison to ND subjects, prior to and after periodontal treatment. In Aim 2, we will focus on the spatial organization of key PD-associated taxa to reveal their relationship in vivo in T2D and ND patient groups before and after treatment. In Aim 3, we will investigate host cytokine production and neutrophil responses to microbial challenge with core PD-associated species and compare the differences among the T2D and ND patient groups. This research will address a fundamental gap in our knowledge of the subgingival microbiome in a population at high risk for PD, which may lead to further studies for the development of innovative clinical approaches to PD diagnosis, prevention and management in T2D population. This study can also provide a model system for future investigations of the interplay between a localized microbiome and a systemic disease. The success of this project will not only shed light on the oral microbiome and PD pathogenesis in relation to T2D but also have a significant impact on future investigations of host - microbiome interactions.