Colonisation of model oral biofilms by Streptococcus mutans

Abstract

Dental plaque is a mixed-species microbial biofilm that forms on tooth surfaces. Incorporation of acidogenic bacteria such as Streptococcus mutans within the biofilm results in development of carious lesions. This research project aims to establish a reproducible model of early colonising oral bacteria and use it to assess S. mutans integration into biofilms. The Modified Robbins Device (MRD) was used to develop mixed species biofilms from Streptococcus gordonii DL1, Actinomyces oris MG1, and Veillonella parvula PK1910, with or without 0.1% sucrose. These biofilms were challenged with S. mutans UA159 or GS5. Quantitative PCR and a variety of microscopic imaging approaches were used to assess the structure and stability of early coloniser biofilms and to study S. mutans incorporation. Reproducible and stable early coloniser biofilms were established. The presence of sucrose had no effect on biofilm formation. Challenging preformed biofilms with S. mutans had no effect on the early coloniser species, but resulted in differences in the appearance of biofilm matrix. Significant differences were observed between S. mutans strains: after 24 h S. mutans UA159 was about 20-fold more abundant in the biofilm than S. mutans GS5. The impact of extracellular DNA on S. mutans GS5 colonisation was studied by replacing the wild-type S. gordonii with a mutant disrupted in the ssnA gene encoding an extracellular deoxyribonuclease. There was no significant difference in S. mutans between preformed biofilms with wild-type and mutant S. gordonii, indicating that SsnA does not play a role in determining S. mutans colonisation in this system. A reproducible system for culturing early coloniser oral biofilms has been established here that will be useful for further investigations of biofilm colonisation by oral bacteria. Differences in the ability of different S. mutans strains to colonise may be further explored through targeted mutagenesis to find key factors responsible for colonisation.