Bacterial Acid Production

Bacteria aggregate in dental plaque on the surface of teeth, where they convert glucose, fructose, and sucrose into acids through a process called glycolysis. This is the primary energy-generating pathway in all bacteria, including the caries-associated Streptococcus mutans. In Figure 4 below, the monosaccharides glucose, galactose, and fructose enter the glycolysis pathway at the points shown in the diagram. The dotted lines in the pathways indicate that there are additional intermediate steps. Streptococcus mutans is capable of further metabolizing pyruvate (pyruvic acid) to generate additional energy and more acid byproducts. When excess sugars are available, S. mutans favors the lactate dehydrogenase pathway to produce lactic acid, which significantly lowers the pH in the immediate environment of the tooth. This drop in pH makes saliva and the interbacterial fluid in dental plaque more acidic, creating an environment that promotes enamel demineralization.

Figure 4. Glycolytic pathway of Streptococcus mutans, from monosaccharides to acid.

Figure 4. Glycolytic pathway of Streptococcus mutans, from monosaccharides to acid.

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

The rate at which acid is produced is influenced by the microbial composition of dental plaque. In general, the more acidogenic and aciduric bacteria, such as Streptococcus mutans, are present in plaque, the faster acid is produced.

Sucrose is metabolized rapidly by S. mutans, leading to a rapid decrease in pH, while larger molecules like starch diffuse more slowly into plaque. These large molecules must first be broken down into simpler sugars before they can be assimilated by plaque microbes.^10,11 In addition to carbohydrate metabolism, the rate of acid production is influenced by the density of plaque. Less dense plaque, which can be penetrated by buffering saliva and oxygen, produces less acid than very dense plaque, which is more resistant to the penetration of saliva and oxygen. ^10,12,13

When sugars are not available - typically between meals - bacteria use their energy reserves to produce formic and acetic acids, which are weaker acids compared to lactic acid. These acids are less likely to damage tooth structure, as they do not significantly lower the pH to the critical threshold needed for enamel demineralization.