Streptococcus mutans-derived extracellular matrix in cariogenic oral biofilms

Abstract

Biofilms are highly structured microbial communities that are enmeshed in a self-produced extracellular matrix. Within the complex oral microbiome, Streptococcus mutans is a major producer of extracellular polymeric substances including exopolysaccharides (EPS), eDNA, and lipoteichoic acid (LTA). EPS produced by S. mutans-derived exoenzymes promote local accumulation of microbes on the teeth, while forming a spatially heterogeneous and diffusion-limiting matrix that protects embedded bacteria. The EPS-rich matrix provides mechanical stability/cohesiveness and facilitates the creation of highly acidic microenvironments, which are critical for the pathogenesis of dental caries. In parallel, S. mutans also releases eDNA and LTA, which can contribute with matrix development. eDNA enhances EPS (glucan) synthesis locally, increasing the adhesion of S. mutans to saliva-coated apatitic surfaces and the assembly of highly cohesive biofilms. eDNA and other extracellular substances, acting in concert with EPS, may impact the functional properties of the matrix and the virulence of cariogenic biofilms. Enhanced understanding about the assembly principles of the matrix may lead to efficacious approaches to control biofilm-related diseases.

Keywords: Streptococcus mutans; biofilms; dental caries; eDNA; exopolysaccharides; extracellular matrix; mechanical stability; spatial heterogeneities.

Figure 1

Streptococcus mutans biofilms subjected to rheometry analysis. (A) Close-up view of biofilm samples placed between the upper and bottom plates of the rheometer. Stiffness, S, indicates rigidity of the sample whereas damping, R, is related to viscosity of the samples. m_L is the initial load applied to the sample during measurement. Gap: the size was standardized at 300 μm because the 3D structure of biofilm is not homogeneous, and 300 μm was the average maximum thickness of 115 h-old biofilms grown in 1% sucrose. (B) Storage modulus of S. mutans biofilms incubated with 2 units of each enzyme (^*P < 0.05 vs. control), using small amplitude oscillatory shear experiments performed within the linear viscoelastic region at 1 Hz. All treatments with glucanohydrolases modified the rheological properties, specifically decreased the storage modulus of treated biofilms.

Figure 2

Influence of eDNA on glucans synthesis and bacterial adhesion. (A) Glucan formed by GtfB adsorbed to sHA surface. (B) This panel of images shows the sHA bead in DIC (gray), in green is the eDNA associated to surface (which presents a punctuated distribution pattern), and red are the glucans formed. Overlay and close-up image show eDNA interspersed with glucans. (C) Glucan produced by GtfB adsorbed to S. mutans and S. gordonii cells. (D) FE-SEM analysis of S. mutans biofilms on apatitic surface. Images highlight interaction of nanofibrous eDNA (white arrows) and wool-like glucans (yellow arrows). Bacterial adhesion to apatitic surfaces in presence and absence of eDNA are shown for S. mutans (E) and S. gordonii (F). sHA: saliva-coated hydroxyapatite; gsHA: glucan formed on saliva-coated hydroxyapatite. An asterisk (^*) denotes p < 0.05. Note: (D,E) were kindly provided by Dr. Zezhang (Tom) Wen (School of Dentistry, Louisiana State University Health Sciences Center, New Orleans, Louisiana, USA).