Bacteria aggregate in dental plaque on the outer surface of teeth. Bacteria convert glucose, fructose, and sucrose into acids through a process called glycolysis, which is the main energy generating pathway in all bacteria, including caries-associated Streptococcus mutans. In Figure 1 below, the monosaccharides glucose, galactose, and fructose can 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 metabolizing pyruvate (pyruvic acid) further, to generate yet more energy and more acid byproducts. When excess sugars are available, they favor the lactate dehydrogenase pathway to produce lactic acid. This causes the pH in the immediate environment of the tooth to decrease rapidly, making saliva and the interbacterial fluid in dental plaque more acidic.7-9
Figure 1. Glycolytic pathway of Streptococcus mutans, from monosaccharides to acid.
How quickly acid is produced is due in part to the microbial composition of dental plaque: In general, the more acidogenic, aciduric bacteria, such as Streptococcus mutans, are present in plaque, the faster acid is produced. The rate of acid production is also dependent on the speed with which plaque bacteria are able to metabolize the dietary carbohydrate. While sucrose is metabolized quickly, prompting a rapid decrease in pH, a large molecule such as starch diffuses into plaque more slowly because it would need to be broken down before it can be assimilated by plaque microbes.7,8 The rate of acid production is also influenced by the density of plaque. Less dense bacterial plaque, which can be penetrated by buffering saliva and oxygen, produces less acid than very dense plaque, which cannot be accessed by saliva and oxygen.7,9,10
When sugars are not available - typically between meals - bacteria use their energy reserves and form formic and acetic acids in the process. These are weaker acids that are not associated with damage to tooth structure.7