ABSTRACT The microbial etiology of dental caries remains a subject of debate with considerable differences of opinion regarding the relative contributions of specific species or classes of bacteria. The `low pH streptococci' is a designation given to atypical, highly acidogenic strains of oral streptococci originally thought to belong to the primary plaque colonizing species S. oralis, S. mitis, S. gordonii, and S. anginosus. This sub-group of streptococci is proposed to have considerable cariogenic potential, perhaps preceding, augmenting or even overshadowing the contribution of the mutans streptococci (MS). However, assessing the role of the low pH strep is not straightforward. Accurate assignment of species among the viridans streptococci is a challenge, and the low pH phenotype cannot be ascertained by the gene-based strategies routinely employed in non-culture based studies of the oral microbiome. In this application we propose a unique prospective study that combines state-of-the-art bioinformatics with phenotypic analyses of low pH phenotypes to track changes in the plaque microbiome, with a specific focus on the MS and non-MS low pH streptococci, as caries develop in children at high risk for early childhood caries. Specifically, our objectives are to determine if there is ascendancy of non- MS low pH streptococci within the plaque oral microbiome with potential shifts in composition of other organisms during the transition from a sound enamel site to a carious enamel site. In addition, we propose a novel and exciting approach of categorizing low pH streptococci based on their potential to produce alkaline products. We believe that gaining a more comprehensive assessment of the low pH streptococcal species or taxa, and our novel grouping of these organisms based on alkaline-producing phenotypes, as part of our determination of the plaque microbiome composition has the potential to make significant contributions to implementation science. To meet our objectives we propose the following aims: 1) Conduct an 18-month longitudinal study on high-caries risk children aged 1-3 to determine changes in the overall plaque microbiome preceding caries development, with a focus on the non-MS low pH streptococci; and 2) Use genomic sequencing to assign species identity or taxonomical relatedness to the non-MS low pH streptococci recovered in Aim 1. By coalescing and analyzing the data from the microbiome and low pH streptococci as caries develop we will define patterns or drivers of caries or health that may provide a basis for advances in implementation science.