Abstract: In today's microbiome era, it is well-recognized that dental caries, one of the most prevalent and costly chronic infectious diseases world-wide, results from dysbiosis of the oral microbiota and the oral environmental changes that cause tooth damage. Specifically, frequent intake of fermentable carbohydrates promotes a progressive shift in microbial composition toward acidogenic and acid-tolerant species. The continual acid- induced demineralization eventually overcomes the buffering capacity and anti-microbial properties of saliva, leading to irreversible tooth destruction. The goal of this proposed research is to prevent dental caries through targeted treatment of acid-producing bacteria (t-TAB). t-TAB will promote a healthy microbial community that is vital for modulating pH and preventing acid-induced teeth damage. The t-TAB will be achieved by selectively inhibiting the growth of cariogenic bacteria through enhanced antimicrobial (AM) efficacy in response to the accelerated acid production by these bacteria in comparison to commensal species. We propose four specific aims to develop, identify and assess effective t-TAB candidates. In Specific Aim 1, we will synthesize and characterize six new pH-sensitive quaternary pyridinium salts (pH-QPSs). We expect to identify compounds or combinations of compounds that provide t-TAB in aqueous mixtures. We will enhance our understanding of the chemical structure/AM efficacy relationship and optimize the AM efficacy and solubility of pH-QPS(s) to obtain safe and effective t-TAB treatments. In Specific Aim 2, we will transform a clinically tested AM agent, chlorhexidine (CHX), into a t-TAB agent which provides pH-responsive AM efficacy. We will achieve acid enhanced CHX release through encapsulated CHX in QPS-functionalized mesoporous silica nanoparticles. We will also identify the synergistic pH-AM-E induced by interactions of CHX and pH-QPSs. In Specific Aim 3, we will assess and compare the t-TAB efficacy of lead candidates from Aim 1 and Aim 2 by employing a multispecies biofilm model that simulates human oral microbial community (named O-mix). The t-TAB efficacy will be assessed in the presence and absence of sucrose—the cariogenic dietary carbohydrate. Strategy will entail evaluating biomass, analyzing microbial profiles and determining environmental pH. Finally, the most effective t-TAB candidates that successfully inhibit the growth of cariogenic acid-producing bacteria without affecting the functions of commensal species will be further assessed in Specific Aim 4 in vitro using a microbial-caries model on human enamel and in vivo employing a well-developed mouse caries model. Successful completion of the proposed aims will provide new materials for oral rinse in dental clinics to prevent/treat dental caries. Knowledge gained from this study will also advance material development to prevent infection and erosion.

Narrative: The proposed research focuses on prevention of dental caries through targeted treatment of acid- producing bacteria (t-TAB). Its goal will be achieved by formulating and developing a series of pH-sensitive quaternary pyridinium salts and transforming the traditional, non-pH-sensitive chlorhexidine into antimicrobial materials with t-TAB capability. The successful candidates capable of preventing acid-induced tooth damage through achieving a healthy microbial community and maintaining environmental pH above 5.5 will be identified using a multispecies biofilm model that simulates human oral microbial community and further assessed for caries prevention in vitro using a microbial-caries model on human enamel and in vivo through a well-developed mouse caries model .