The <b><i>rnc</i></b> Gene Regulates the Microstructure of Exopolysaccharide in the Biofilm of <b><i>Streptococcus mutans</i></b> through the β-Monosaccharides

<i>Streptococcus mutans</i> is known as the crucial pathogen of human dental caries, owing to its contribution to the biofilm development via the capacity of synthesizing exopolysaccharide (EPS), which mainly compose of α-glycosidic bond and β-glycosidic bond. β-glycosidic bond is less flexible than α-glycosidic bond because of differences between their configurational properties. Previous studies have shown that the <i>rnc</i> gene is implicated in the EPS formation and the cariogenicity of <i>S. mutans</i>. However, the effects of <i>rnc</i> on the microstructure of EPS have been not well-understood yet. Here, we further investigated how the <i>rnc</i> gene worked to modulate microstructural properties of the extracellular polysaccharide of <i>S. mutans</i> using glycomics methods. The gas chromatography-mass spectrometer showed that the proportion of glucose was decreased in water-soluble EPS and galactose was absent in water-insoluble EPS from the <i>S. mutans rnc</i>-deficient strain (Smurnc), compared with the isogenic wild-type strain (UA159). The composition of functional groups and the displacement of hydrogen bond were analyzed by infrared radiation and ¹H nuclear magnetic resonance, respectively. In addition, phenotypic modulation of the biofilm matrix was assessed by microscopy. We found that the EPS of UA159 and the <i>rnc</i> overexpression strain (Smurnc⁺) mainly consisted of β-glycosidic bonds. Conversely, the EPS of Smurnc were made up of mostly α-glycosidic bonds, leading to the attenuation of biofilm biomass and bacterial adhesion. Furthermore, the existence of β-glycosidic bond was verified by enzyme digestion. Collectively, the <i>rnc</i> gene modulates the conversion of β-glycosidic bonds, which may play important roles in regulating the micromolecule structure of the EPS matrix, thus affecting the characteristics of <i>S. mutans</i> biofilm. These data illustrate that β-glycosidic bonds mediated by <i>rnc</i> may be potential targets for the prevention and treatment of dental caries.