Characterising the invasion of Streptococcus mutans into an oral community using in vitro and in silico modelling

Abstract

The transition of a commensal dental biofilm, to one overpopulated with acidogenic species, e.g. Streptococcus mutans, is associated with caries development. Approaches to understand this dysbiosis have not balanced representing the complexity of dental biofilms, with defining the factors underpinning caries development, including interspecies interactions and pH influence. This research aimed to characterise S. mutans invasion into a synthetic oral community, to better understand the factors contributing to colonisation, using modelling approaches in a defined environment. A 4-species synthetic oral community, comprised of Streptococcus gordonii, Actinomyces oris, Neisseria subflava, and Veillonella parvula, was exposed to S. mutans. Biofilms were grown on hydroxyapatite coupons in continuous flow bioreactors, using a developed chemically defined medium, supplemented with glucose and lactic acid. Biofilm and planktonic growth were simulated with a 2-D Individual-based model (IbM) and a 0-D continuous reactor model, respectively. High glucose and lactic acid concentrations resulted in a significant pH drop and S. mutans dominating the biofilm and planktonic communities. In substrate-limited environments, the community composition, measured by qPCR and fluorescence in situ hybridization, was more balanced. The IbM simulated S. mutans dominance at high glucose concentrations, using kinetic parameters collected experimentally. When the influence of pH on the bacterial growth kinetics was considered, rather than just on chemical speciation, the simulations corroborated with in vitro and in vivo findings. I have developed in vitro and in silico models characterising S. mutans invasion of a 4-species commensal community, improving on previous attempts to represent the complexity of the dental biofilm. These models have advanced knowledge of the importance of pH in S. mutans invasion and considering pH in growth kinetics within simulations. Models will assist safe oral care product development by enabling the impact of antibacterial agents on the dental biofilm to be studied without in vivo assessment.