The Role of Exopolysaccharides in Oral Biofilms

Abstract

The oral cavity contains a rich consortium of exopolysaccharide-producing microbes. These extracellular polysaccharides comprise a major component of the oral biofilm. Together with extracellular proteins, DNA, and lipids, they form the biofilm matrix, which contributes to bacterial colonization, biofilm formation and maintenance, and pathogenesis. While a number of oral microbes have been studied in detail with regard to biofilm formation and pathogenesis, the exopolysaccharides have been well characterized for only select organisms, namely Streptococcus mutans and Aggregatibacter actinomycetemcomitans. Studies on the exopolysaccharides of other oral organisms, however, are in their infancy. In this review, we present the current research on exopolysaccharides of oral microbes regarding their biosynthesis, regulation, contributions to biofilm formation and stability of the matrix, and immune evasion. In addition, insight into the role of exopolysaccharides in biofilms is highlighted through the evaluation of emerging techniques such as pH probing of biofilm colonies, solid-state nuclear magnetic resonance for macromolecular interactions within biofilms, and super-resolution microscopy analysis of biofilm development. Finally, exopolysaccharide as a potential nutrient source for species within a biofilm is discussed.

Keywords: bacteria; caries; extracellular matrix; microbiology; periodontal disease; plaque/plaque biofilms.

Figure 1.

Various aspects of exopolysaccharides in oral microbes. A major component of the oral biofilm matrix is composed of exopolysaccharides. This self-synthesized polymer is essential to the structure of the biofilm and maintenance of the bacteria in the oral cavity. Presented here is the current state of extracellular polysaccharide (EPS) research: its synthesis, contribution to matrix development, regulation, and promotion of immune evasion. Future and novel directions are covered in detail to highlight emerging technologies by which to study EPS.

Figure 2.

Exopolysaccharide export in Aggregatibacter actinomycetemcomitans. Schematic representation of the export mechanisms of extracellular polysaccharide (EPS) in A. actinomycetemcomitans. The PgaABCD proteins make a gap (a) or a bridge (b) for the EPS to traverse the periplasmic space. In the gap model, both N- and C-terminal domains of PgaB can interact, whereas in the bridge model, only 1 domain of PgaB can interact with the PgaATPR domain. IM, inner membrane; OM, outer membrane; PG, peptidoglycan. Open circles represent glucosamine and dark circles represent N-acetylglucosamine.