a Simplified illustration of sugar clearance of two persons after sugar intake (arrows); person 1 eliminates sugar faster than person 2.
b Consequently person 2 has a lower pH for a much longer time than person 1. Under such circumstances person 2 will likely develop caries faster than person 1.
The organic components have the following functions:
• Participating in enamel pellicle formation
• Mucosal coating
• Antimicrobial defense
• Digestive actions
Clearance rate. Oral clearance can be defined as the dilution and elimination of substances in the oral cavity, which can be fast or slow.21 Figure 1.10a illustrates diagrammatically two persons with different saliva clearance rates,21 person 1 with a fast clearance and person 2 with a slower clearance. The curves in Fig. 1.10b are the corresponding pH variations in plaque (see definition below) following the elimination of the sugar lump. This figure aims to show that a slow clearance may result in a pH drop in the saliva and/or plaque that will be lower and remain so for a longer period, which may be more harmful to the teeth than a faster clearance. It is the salivary flow rate and volumes of saliva in the mouth before and after swallowing that affect the clearance rate. Thus, stimulating saliva secretion by using chewing gum will increase the clearance rate.
Electrolytes. From the caries disease angle, the most important electrolytes are calcium, inorganic phosphate, bicarbonate, and fluoride. The concentration of the various salivary electrolytes is strongly dependent on the salivary flow rate22,23 (Fig. 1.11). It appears that when the flow rate increases, the concentration of the electrolytes increases, apart from inorganic phosphate. The pH of un-stimulated and stimulated saliva is between 6 and 7. At this level the relevant ions in saliva are supersaturated, which actually should result in precipitation of the electrolytes resulting in development of mineral on the tooth surface. Why this is not a common phenomenon is explained below.
Buffers. Saliva also has systems which buffer acids from the sugar-fermenting oral microorganisms. A buffer in this context is a substance which, to a certain degree, resists changes in pH. In the development of caries disease the two following buffer systems are important:
• The phosphate system
• The bicarbonate system
The form of phosphate in saliva is influenced by its pH. At pH 7.5–6.0 most of the phosphate is present as dihydrogen (
When the pH value decreases, that is, the H+ concentration increases, hydrogen phosphate binds a hydrogen ion and changes to a dihydrogen phosphate ion. Thus, if there is sufficient monohydrogen species to react with H+, the pH will not drop further.
The bicarbonate system works at a lower pH than the phosphate system (around 6) and takes up H+ according to the following reaction:
This system works best with stimulated saliva, because the concentration of HCO3− increases with increasing flow rate (Fig. 1.11).22–24 The release of carbon dioxide gas (CO2) from saliva further boosts the buffering capacity of the system as the reaction shifts toward the right.
The organic components of saliva. Table 1.1 presents the most important proteins and enzymes in the saliva and their known functions. It appears that several of them—lysozymes, agglutinins, and antibodies—have a strong antimicrobial function. Of note among the phosphoproteins found in saliva is statherin, which is rich in the amino acid tyrosine and is indirectly very important in the caries process. As mentioned above, neutral pH saliva is supersaturated with respect to the ions of HAP, which is the main inorganic component of tooth enamel. Phosphoproteins contain sequences of phosphorin that bind calcium very strongly, thereby maintaining the supersaturated state and at the same time preventing random crystallization from occurring.25 Statherin is so far the only salivary protein currently known to inhibit both the primary and secondary precipitation of HAP in the supersaturated environment of the saliva. As statherin and other inhibitors are proteins, they are subject to microbiological degradation, in particular caused by acids in the plaque.
Fig. 1.11 Concentrations of important salivary electrolytes depend on the salivary flow rate (modified from Dawes 2004).23
Table 1.1 Organic components in the saliva and their possible roles
| Organic component | Function |
| Amylase | Degradation of starch |
| Lysozyme | Antimicrobial activity by destruction of bacterial cell membranes |
| Lactoferrin | Antimicrobial activity by high affinity for iron |
| Peroxidase | Antimicrobial activity and protection against H2O2 |
| Agglutinin | Antimicrobial activity by agglutination of bacteria to large aggregates |
| Statherin | Inhibits spontaneous precipitation |
| Antibodies | IgA/IgG, IgM inhibition of adhesion, enhancement of phagocytosis |
Pellicle
The pellicle is a thin, bacteria-free layer covering the teeth (Fig. 1.12). It is formed by the adsorption of salivary proteins, for example, glycoproteins, which have high affinity for the mineral in the surface of the tooth.26 The positively charged HAP crystals will attract negatively charged organic components from the saliva. If the pellicle is removed, for example, by the dentist during a professional cleaning, it will start forming again within seconds. The thickness of the pellicle varies in different areas of the teeth, generally ranging from 1μm to 10μm. However, it can be thicker and it can become discolored due to the staining from foods and/or tobacco.
Fig. 1.12 Transmission electron microscopic examination of dental plaque consisting of microorganisms (M) and intercellular substances (ICM)