E1 Chemical-biological interface

Topic

The chemical-biological interface of the oral environment is defined by the interaction between two elements: dental biofilm (also known as bacterial plaque) and saliva. While dental biofilm tends to acidify this chemical-biological interface, saliva tends to neutralize it and restore its balance; thus, the state of the interface at any given moment depends on the equilibrium between these two opposing elements.

Dental biofilm is an organized microbial community embedded in an extracellular matrix that adheres to the tooth surface; within this matrix reside the bacteria that acidify the oral environment's chemical-biological interface.

Bacteria within the dental biofilm metabolize dietary fermentable sugars, producing organic acids—such as lactic, formic, and acetic acids—in the process. These organic acids lower the local pH of the biofilm below the critical pH for enamel demineralization (approximately 5.5 for hydroxyapatite and 4.5 for fluorapatite), thereby triggering the enamel dissolution characteristic of dental caries.

The rate at which dental biofilm bacteria metabolize fermentable sugars does not increase indefinitely with the amount of available sugar; initially, the metabolic rate rises rapidly as more fermentable sugar becomes available, but once the bacteria are metabolizing near their maximum capacity, adding further sugar results in only a negligible increase in the rate of organic acid production.

In response to this acidification, saliva acts as a buffer: it neutralizes the organic acids produced by the dental biofilm through two buffering systems—one based on bicarbonate and carbonic acid, and the other on phosphate—thereby restoring the local pH that the biofilm bacteria had lowered. In addition to neutralizing organic acids, saliva supplies tooth enamel with the calcium and phosphate needed for remineralization—a process that competes with the demineralization caused by organic acids from dental biofilm to determine whether the enamel gains or loses minerals.

The organic acids produced by dental biofilm do not reach the enamel instantly; instead, they must diffuse through the biofilm itself and through the salivary pellicle—a thin layer of salivary components covering the tooth surface. The denser the biofilm or pellicle the acids must traverse, the slower that diffusion proceeds, and the longer it takes for the local pH at the enamel surface to reflect the presence of the newly produced organic acids.

When a person consumes sugar, this interaction between dental biofilm and saliva can be tracked over time by observing changes in the local pH within the plaque: the local pH drops rapidly during the first few minutes as biofilm bacteria metabolize the ingested sugar and produce organic acids, and then recovers much more slowly as saliva neutralizes those acids and restores the local pH toward its resting value. This pattern of rapid decline followed by slow recovery encapsulates, in a single dynamic, the balance between acidification and neutralization that defines the chemical-biological interface of the oral environment described earlier.

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