Koning, JW; (2012) Interactions Between Streptococcus mutans and Veillonella dispar. Doctoral thesis , UCL (University College London).

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Abstract

Dental plaque, a collection of bacteria forming a biofilm, is the cause of the most common diseases of mankind: caries and periodontal diseases. This study reports on the interactions occurring between two key species in this biofilm Streptococcus mutans and Veillonella dispar. These organisms are hypothesised to form a cooperative metabolic system in which a waste product, lactic acid, produced by S. mutans is utilised by V. dispar. The objective of this thesis was to evaluate whether these species cooperate to determine whether knowledge of their interaction can be applied to prevent caries. The hypothesis was examined in single and dual species planktonic cultures and biofilms which were continued for up to 14 days, with their growth, vitality, microstructure and environment closely monitored. S. mutans gene expression was quantified during two stages of biofilm growth to determine the effects of V. dispar co-culture. A qualitative model was developed that coupled the growth and decline of S. mutans and V. dispar around lactic acid. Experiments were conducted in silico to determine whether the interaction could be modulated to improve health, and the model was expanded to include a third species to investigate the production of lactic acid as a competitive strategy. The results demonstrated the two species had a mutually beneficial relationship initially, but lactic acid rapidly accumulated and killed V. dispar. S. mutans gene expression changed considerably in co-culture, including upregulation of bacteriocins. The initial hypothesis that the species cooperate because V. dispar removes lactic acid was rejected as S. mutans produces lactic acid as a chemical warfare agent and does not want it removed. The principal conclusion is that S. mutans employs a strategy of acidogenicity and aciduricity to gain dominance in the dental plaque biofilm. This strategy overwhelms the benefit from cooperative interactions that remove lactic acid and thus sociobiological approaches to prevent caries should focus on competitive interventions.

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