PROJECT SUMMARY/ABSTRACT Dental caries is the most common childhood disease in today’s society. Untreated dental caries contributes to oral pain, abscess development, tooth loss and poor esthetics affecting both the health and self-esteem of children. Restorative dental procedures are the standard of care in the treatment of dental caries yet often are not feasible for those children of low socioeconomic status. Long lasting preventive methods are the treatment of choice in such patients that cannot routinely see dental professionals. The etiology of dental caries in pediatric patients is attributed to tooth-borne biofilms comprising Streptococcus mutans and Streptococcus sobrinus. This bacteria-based etiology has led clinicians to turn to tooth-applied, bactericidal silver (Ag) agents as a cost- effective method to arrest dental caries. However, Ag use is associated with tooth staining, tissue toxicity and disruption of the microbiota calling into question their repeated application and long-term use. This proposal sets forth the first example of cerium oxide nanoparticles (CeO2-NP) as non-bactericidal biofilm inhibitors of oral Streptococci. Preliminary results demonstrate in vitro biofilm inhibition of S. mutans and S. sobrinus by CeO2- NP prepared by Ce(IV) ammonium salt hydrolysis. CeO2-NP prepared by the current methodology have exhibited a higher efficacy in limiting in vitro biofilm formation as compared to AgNO3, the current standard for topical treatment in pediatric dental caries arrest. Importantly, the mechanism of biofilm inhibition by CeO2-NP is non- bactericidal as opposed to AgNO3. A significant challenge of tooth-applied agents is maintaining a clinically effective concentration of the agent at the tooth surface. This proposal is unique in that it sets forth not only non- bactericidal biofilm inhibitors for tooth application, but a method of retaining them at the enamel surface for an extended period of time. Cerium salts have a well-known affinity for hydroxyapatite and have been shown to limit demineralization (erosion) of the enamel surface with acidic challenge. Hydroxyapatite is a dynamic structure that facilitates surface chemical exchange of ions and nanoparticles of varying size with simple topical administration. Given the known affinity of Ce-agents for the enamel surface, we propose the incorporation of the novel biofilm inhibiting CeO2-NP agents of this proposal onto the enamel surface via adsorption. The objectives of this proposal are to investigate potential extracellular mechanisms of biofilm inhibition by CeO2-NP as well as the chemical interaction of CeO2-NP with hydroxyapatite surfaces and its efficacy in translation model studies.