In Vitro and in Vivo Characterization of Candida albicans and Streptococcus mutans Interactions

Abstract

The oral cavity is a complex environment harboring diverse microbial species that often co-exist within biofilms formed on oral surfaces. Within a biofilm, inter species interactions can be synergistic in that the presence of one organism generates a niche for another enhancing colonization. Among these species are the opportunistic fungal pathogen Candida albicans and the bacterial species Streptococcus mutans, the causative microorganisms strongly linked to the development of oral candidiasis and dental caries, respectively. Recent studies have reported the enhanced prevalence of C. albicans in children with early childhood caries indicating that this fungal-bacterial interaction may have clinical implications. In this study, we aimed to elucidate and characterize this interaction between these diverse species. Specifically, we designed in vitro and in vivo studies to validate the hypothesis that the presence of C. albicans in the oral cavity augments S. mutans colonization, potentially mediating dental caries development. Using various C. albicans mutant strains and a GFP-tagged S. mutans, metabolic viability and fluorescent biofilm assays were performed to assess S. mutans recovery from mixed biofilms and to elucidate the mechanisms of interactions. Additionally, to visualize the architecture of formed biofilms, confocal scanning laser fluorescent and electron scanning microscopy were used. Importantly, a clinically relevant mouse model of oral co-infection was developed to demonstrate C. albicans mediated enhanced S. mutans colonization in a host. The findings demonstrated significantly higher recovery of S. mutans from biofilms with C. albicans in vitro. Images revealed a high bacterial affinity to C. albicans, and secreted fungal cell wall polysaccharides were identified as the key factor mediating biofilm formation, particularly mannans. Importantly, analyses of harvested tissue demonstrated significantly higher S. mutans recovery from teeth of co-infected mice compared to mice infected only with S. mutans. Collectively, the findings strongly indicate that the presence of C. albicans in the oral environment may impact the development of dental caries and should be considered as a factor in evaluating the risk of caries. Results obtained in this thesis will support future studies using animal models of dental caries to further characterize this relationship in a closely related model in our laboratory.