The project goal is to investigate a discordant, preschool child population whose caries status contradicts what is expected based on their S. mutans level in comparison to the accordance counterpart. Integrating innovative study design and novel tools, the research plan aims to 1) characterize S. mutans strain individuality and its ecological relationship within different microbial communities; 2) explore salivary host biomarkers related to childhood caries; 3) validate S. mutans-associated microbial virulence signatures and host salivary biomarkers for precise and comprehensive caries assessment.