The first step in the formation of a cariogenic plaque, meaning a plaque that causes tooth decay, is the adherence of oral streptococci to specific receptors on the tooth pellicle. The pellicle is a thin film of proteinaceous material adsorbed on the tooth from the saliva. Other species of bacteria attach specifically to earlier arrivals. Two species that might not attach to each other may attach to a third. If dietary sucrose is present, S. mutans attaches to the bacterial mass and produces glucans from sucrose through the action of extracellular enzymes. Sucrose is split by the enzymes to the monosaccharides glucose and fructose. The glucose is polymerized, yielding glucan, and the fructose is metabolized, producing lactic acid. The glucans bind the organisms together and to the tooth, and make the plaque impenetrable to saliva.
When sugar enters the mouth, the pH of cariogenic plaques drops from its normal value of about 7 to below 5 within minutes (figure 24.4); with more than a 100-fold increase in the acidity of the plaque, the calcium phosphate of teeth dissolves. The duration of this acidic state depends on how long the teeth are exposed to sugars and on the concentration of the sugars. After food leaves the mouth, the pH of the plaque rises slowly to neutrality. The delay in return of the pH to neutrality is due to the ability of S. mutans to store a portion of its food as an intracellular, starchlike polysaccharide that is later metabolized with the production of acid. Cariogenic plaque thus acts as a tiny, acid-soaked sponge closely applied to the tooth. Both S. mutans and a suitable sucrose-rich diet are required to produce dental caries on smooth surfaces of the teeth. In deep fissures or pits in the teeth, plaque can accumulate in the absence of S. mutans but
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