Justin R. Kaspar, PhD

Bacteria within multispecies communities grow and persist in complex environments by competing over scare resources such as nutrients and physical space. The human oral cavity is ideally suited as a model for the dissection of interspecies interactions. In the human mouth, billions of microbes belonging to over 700 independent taxa function cooperatively and/or antagonistically to shape the composition of the microbiome. These interactions, coupled with frequent environmental perturbations, can also disrupt microbial homeostasis; which can lead to the ecological shifts observed during development of oral diseases. For example, as is the case for dental caries, increases in the proportions of strongly acidogenic and acid tolerant bacteria, such as the mutans streptococci, are observed when bacterial fermentation of dietary carbohydrates repeatedly acidifies microbial biofilms, leading to demineralization of the tooth. Concurrently, health-associated commensal streptococci, which are less constitutionally resistant to low pH, decrease in proportions. Loss of these species compounds the problem as many of these commensal streptococci provide protection from caries development by metabolizing arginine via the arginine deiminase system (ADS), which elevates the pH through release of ammonia. Further, many commensal streptococci can directly inhibit growth and/or expression of virulence traits by mutans streptococci through multiple strategies; with the generation of hydrogen peroxide being a significant deterrent to growth of Streptococcus mutans and other oral pathogens. Research within the Kaspar Lab focuses on dissecting how S. mutans responds to health-associated oral Streptococcus species that occupy the same niche and how these responses may be modulated for development of novel therapeutic approaches.