Sugar Metabolism

Dietary sugars, starches, and fermentable carbohydrates (usually collectively referred to as sugars) are present in the diet, and are in direct contact with plaque during eating, and for some time afterwards. The breakdown of sugars is an important step that influences the plaque environment. Enzymes in bacteria and saliva break down sugars’ polysaccharides and disaccharides to monosaccharides. Sucrose is the specific substrate for the bacterial enzymes, glucosyltransferases, that convert the sucrose to water insoluble extracellular glucose polymers, glucans. There are five main mechanisms by which oral Streptococci hydrolyze (break down) sucrose (Figure 8).

1. Extracellular invertase cleaves the energy rich α(1-2) glycosidic bond between the glucose and fructose moieties.

2. The bacterial cell transports the sucrose across the cell membrane and cleaves the glycosidic bond using an intracellular invertase.

3. Extracellular glycosyltransferases polymerize the glucose molecule while liberating the fructose molecule so it is free to enter the bacterial cell. Streptococci are particularly proficient at this.

4. Extracellular fructosyltransferases polymerize the fructose while the glucose molecule is liberated, so it is free to enter the cell.

5. Salivary amylase cleaves the polysaccharides.

Figure 8. Mechanisms of sucrose hydrolysis by oral bacteria.

Figure 8. Mechanisms of sucrose hydrolysis by oral bacteria.

Adapted from: Marsh PD, Lewis MAO, Rogers H, et al. Oral Microbiology. 6th ed. 2016; Edinburgh: Churchill Livingstone Elsevier.

Among the full range of fermentable carbohydrates, sucrose is the most cariogenic. It is a relatively small molecule that can penetrate the biofilm, and the bacteria use sucrose for energy production through fermentation, which produces acid. During the process, the bacteria can use sucrose to make a key carbohydrate called dextrins.