Role of Lrg pyruvate uptake system in Streptococcus mutans environmental adaptation

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The ability of Streptococcus mutans to efficiently and rapidly adjust to the changing environment is an essential element of the persistence and cariogenicity of the organism. During the previous funding period of our R01 grant, we revealed that the S. mutans LrgAB system, hypothesized to mediate cell death and lysis with its partner proteins CidAB, functions as a stationary-phase pyruvate uptake system and its activity is tightly regulated in response to both the bacteria’s extracellular and intracellular metabolic status. Pyruvate directs key metabolic fluxes for growth and energy generation, and is rapidly recycled into the cell upon nutrient limitation as an overflow metabolite. The population distribution of lrgAB expression at stationary phase also takes on a bimodal character, suggesting that pyruvate uptake may be limited to a lrgAB-expressing subpopulation of stationary phase cells. Therefore, this renewal proposal tests the hypothesis that pyruvate may initiate a cellular and/or metabolic response through LrgAB to deal with unfavorable conditions, consequently providing a selective advantage to a subpopulation of cells to overcome the limited resources and environmental fluctuations, experienced in the oral cavity, consequently promoting the resilience of a biofilm community. This hypothesis will be tested by dissecting the response of S. mutans to key environmental parameters such as external pyruvate and oxygen, critical for lrgAB expression and function, in the context of underlying biochemical, transcriptomic, and metabolomic processes. The fact that diverse members of the microbiome engage in the response of lrg to environmental cues also has profound implications for the effect of microbially dynamic environments in lrg-mediated phenotypes. Study of the contribution of the Lrg system to the integration of complex environmental signals into the regulatory networks modulating S. mutans virulence and homeostasis will be undertaken as described in the following aims: Aim 1 addresses the metabolic coordination by lrgAB expression and in response to key environmental components, external pyruvate and oxygen, in the context of bacterial community and during growth phase transitions (targeted metabolomics and transcriptomics). Given that the dynamic regulation of pyruvate uptake is primarily driven by Plrg activation, Aim 2 addresses how the lrgAB promoter is differentially activated in response to the cell’s external- and internal metabolic status. For this, we examine the potential interplays among CcpA, CodY, and LytT on the lrgAB promoter, and the process by which LytST senses external pyruvate and subsequently activates the lrgAB promoter. And Aim 3 tests the hypothesis that taxonomic composition and their metabolic activity may shape local environment, and have an effect on the pyruvate-mediated community development of S. mutans in oral cavity, by using an ex vivo saliva biofilm model containing oral commensal bacteria.