Investigating inter-kingdom interaction between oral Streptococcus mutans and Candida species in mixed-species biofilms

Biofilms are involved in the majority of microbial diseases in humans. Dental plaque is a structured, functionally organized mixed-species biofilm that is associated with two of the most common dental conditions, caries, and periodontal disease. The formation of a biofilm not only plays an important role in the pathogenesis of these diseases, but also leads to decreased susceptibility to antimicrobial agents such as antibiotics and antiseptics. The formation of this structurally organized microbial community involves microbial cell to cell interaction. Such interaction controls bacterial behaviours at the population level and shapes the functionality and structure of the biofilm, which subsequently influences a heathy or diseased outcome of the host affected by the biofilm. Streptococcus mutans has been recognized as the major causative pathogen for dental caries, whereas Candida species are among the most common fungal species present in the oral cavity. To understand the structure, the characteristics, and the function of dental biofilms, it is essential to investigate the interactions among these oral microorganisms. In this chapter, we describe protocols to examine the relationship between the cariogenic S. mutans and C. dubliniensis in mixed-species biofilms. C. albicans has been included to provide a comparison with C. dubliniensis. We compared biofilms formed on coverslip with the biofilms formed on a BioFlux plate. We report the formation of mature mixed-species biofilms of S. mutans and C. dubliniensis for the first time and relate the altered structure and functionality of the mixed-species biofilm when compared to the single species biofilm. The methodology is useful for investigating the interkingdom interaction between S. mutans and C. dubliniensis in terms of biofilm formation and its potential implications in the pathogenesis of dental caries and may be extended to other microbes as appropriate.