Periodontal diseases are biofilm-induced pathologies mediated by out-growth of commensal pathogens that live within a complex microbial community in the oral cavity. The development and function of this community depends on an array of cell-to-cell interactions and communication networks that rely on small molecules. There is little known regarding the nature of the signals, and the mechanisms as to how they are sensed and transmitted. This represents a major gap in knowledge. Determining the signals that promote colonization and growth (biofilm formation) of oral pathogens is fundamental to the development of therapeutic strategies. We have identified an extracellular arginine deiminase (ADI) produced by oral streptococci that acts on the oral pathogen Porphyromonas gingivalis, inhibiting its biofilm formation and expression of virulence determinants. The goal of this application is to determine the mechanism through which ADI signals P. gingivalis. A comprehensive set of experiments has been outline to test the central hypothesis that in addition to its key role in anaerobic growth and pH homeostasis of the oral biofilm, ADI acts as an effector protein. The aims are to characterize the action of ADI on P. gingivalis biofilm development and to determine if ADI interacts directly with the surface of P. gingivalis cells. In addition, the molecular mechanism by which P. gingivalis senses ADI will be explored. Specifically, the studies will determine the transcriptional and physiological affect of ADI signaling on P. gingivalis through whole genome transcriptional studies, genetic screening, and direct visualization of biofilms using confocal microscopy. The overall goal of the proposal is to synthesize the data and develop a model as to how ADI signaling plays a role in vivo. This will advance the long-term goal to provide a basic understanding of the communication mechanisms that control colonization and out-growth of P. gingivalis, with the ultimate goal being to develop therapeutic strategies to prevent or treat periodontal disease.