Abstract
Streptococcus mutans is a cariogenic pathogen that produces an extracellular polysaccharide (glucan) from dietary sugars, which allows it to establish a reproductive niche and secrete acids that degrade tooth enamel. While two enzymes (GlmS and NagB) are known to be key factors affecting the entrance of amino sugars into glycolysis and cell wall synthesis in several other bacteria, their roles in S. mutans remain unclear. Therefore, we investigated the roles of GlmS and NagB in S. mutans sugar metabolism and determined whether they have an effect on virulence. NagB expression increased in the presence of GlcNAc while GlmS expression decreased, suggesting that the regulation of these enzymes, which functionally oppose one another, is dependent on the concentration of environmental GlcNAc. A glmS-inactivated mutant could not grow in the absence of GlcNAc, while nagB-inactivated mutant growth was decreased in the presence of GlcNAc. Also, nagB inactivation was found to decrease the expression of virulence factors, including cell-surface protein antigen and glucosyltransferase, and to decrease biofilm formation and saliva-induced S. mutans aggregation, while glmS inactivation had the opposite effects on virulence factor expression and bacterial aggregation. Our results suggest that GlmS and NagB function in sugar metabolism in opposing directions, increasing and decreasing S. mutans virulence, respectively.
Highlights
Streptococcus mutans is a commensal bacterium present in the oral cavity and one of the first bacteria to colonize the tooth surface
Since GlcNAc significantly affected the growth of the glmSknockout mutants in S. aureus [8], we investigated the growth of these S. mutans mutants in the presence or absence of 10 mM GlcNAc (Fig. 1)
Effects of glmS and nagB on virulence factor expression Since numerous studies have explored the effects of sugars on virulence factors, including glucan synthesis, biofilm formation, and acid production, in S. mutans [5,6,23], we investigated the expression of virulence factors in our glmS and nagB mutants in the exponential growth phase (OD = 0.8) in chemically defined medium (CDM)-G50 with or without 10 mM GlcNAc
Summary
Streptococcus mutans is a commensal bacterium present in the oral cavity and one of the first bacteria to colonize the tooth surface This bacterium can be isolated from humans with or without dental cavities, suggesting that the oral environment of the host plays an important role in the virulence of S. mutans. Growth of this species changes local environmental conditions, allowing it to attach to the tooth surface by producing an extracellular polysaccharide called glucan, which is involved in the formation of dental plaque [1,2,3]. The enzymatic conversion of sugars by S. mutans, accompanied by the extracellular production of cell-surface protein antigen (PAc; known as SpaP), glucan binding protein, dextranase, and acid tolerance factor (H+ pump), facilitates acid-catalyzed tooth decay and leads to erosion of the hydroxyapatite of the teeth [5,6,9,10,11]
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