The Role of Candida albicans Secreted Polysaccharides in Augmenting Streptococcus mutans Adherence and Mixed Biofilm Formation: In vitro and in vivo Studies

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 <i>Candida albicans</i> and the bacterial species <i>Streptococcus mutans</i>, the etiologic agents of oral candidiasis and dental caries, respectively. Recent studies have reported enhanced prevalence of <i>C. albicans</i> in children with caries indicating potential clinical implications for this fungal-bacterial interaction. In this study, we aimed to specifically elucidate the role of <i>C. albicans</i>-derived polysaccharide biofilm matrix components in augmenting <i>S. mutans</i> colonization and mixed biofilm formation. Comparative evaluations of single and mixed species biofilms demonstrated significantly enhanced <i>S. mutans</i> retention in mixed biofilms with <i>C. albicans.</i> Further, <i>S. mutans</i> single species biofilms were enhanced upon exogenous supplementation with purified matrix material derived from <i>C. albicans</i> biofilms. Similarly, growth in <i>C. albicans</i> cell-free spent biofilm culture media enhanced <i>S. mutans</i> single species biofilm formation, however, the observed increase in <i>S. mutans</i> biofilms was significantly affected upon enzymatic digestion of polysaccharides in spent media, identifying <i>C. albicans</i> secreted polysaccharides as a key factor in mediating mixed biofilm formation. The enhanced <i>S. mutans</i> biofilms mediated by the various <i>C. albicans</i> effectors was also demonstrated using confocal laser scanning microscopy. Importantly, a clinically relevant mouse model of oral co-infection was adapted to demonstrate the <i>C. albicans</i>-mediated enhanced <i>S. mutans</i> colonization in a host. Analyses of harvested tissue and scanning electron microscopy demonstrated significantly higher <i>S. mutans</i> retention on teeth and tongues of co-infected mice compared to mice infected only with <i>S. mutans</i>. Collectively, the findings from this study strongly indicate that the secretion of polysacharides from <i>C. albicans</i> in the oral environment may impact the development of S. mutans biofilms, ultimately increasing dental caries and, therefore, <i>Candida</i> oral colonization should be considered as a factor in evaluating the risk of caries.