Abstract
Streptococcus mutans (S. mutans) has a superior ability to rapidly metabolize sucrose into exopolysaccharides (EPS, mainly glucans), which serve as a critical virulence factor related to dental caries. Despite extensive research on sucrose-dependent EPS at the molecular and macroscale levels, however, the mechanisms underlying EPS effects at the microscale level remain poorly understood. Here, by employing bacteria tracking and fluorescence staining techniques, we investigated the role of sucrose-dependent EPS during biofilm development of S. mutans at the microscale level for both WT and ΔgtfB strains. The results showed that at the early stages of biofilm development, the sucrose-derived glucans enhanced the surface attachment of S. mutans through bamboo joint-like glucan patterns displayed on cell surfaces and altered their microcolony structures from loose 2D chains in ΔgtfB to dense-packed cell clusters in WT; then, after microcolonies formed, sucrose-dependent EPS promoted their development by speeding up the 2D-3D transition of WT microcolonies and affected final biofilm morphologies at the stage of biofilm maturation. Moreover, by tracking the long-time dynamic process of WT biofilm development at the microscale, the results demonstrated clearly the origin of liquid regions and their correlations with the structural and pH heterogeneity of biofilms. These findings establish sucrose-dependent EPS as dual-functional scaffolds-mechanically accelerating biofilm assembly, meanwhile, facilitating the formation of structural and pH heterogeneity inside biofilms that are critical for enamel demineralization, and thus provide insights for developing new anti-caries strategies. IMPORTANCE: Streptococcus mutans is a major pathogen in caries development due to its ability to rapidly metabolize sucrose into EPS. EPS serves as a major component of the S. mutans biofilm matrix, and previous studies mostly explored the effects of EPS on the macroscale. However, how EPS shapes S. mutans biofilm formation at the microscale is not well understood. By combining single-cell tracking with fluorescence staining techniques, we demonstrate that sucrose-dependent EPS governs the transition from 2D growth to 3D biofilm architecture and facilitates the formation of a liquid region at the bottom of the biofilm. These findings bridge a fundamental knowledge gap between the microscale organization and macroscale attributes of biofilms, offering novel perspectives for developing targeted anti-caries strategies.