Saliva is a complex fluid produced by major and minor salivary glands. Predominantly water, it contains a rich mix of electrolytes, proteins, and organic molecules. Dental caries, or tooth decay, is a disease where the tooth structure is destroyed by acid, a byproduct of bacteria metabolizing sugars. Saliva acts as the body’s primary defense system against this acid challenge.
Mechanical Cleansing and Lubrication
The continuous flow of saliva provides a constant physical washing action inside the mouth. This fluid flushes away loose food particles, residual sugars, and acid-producing bacteria from the teeth and soft tissues. A normal, stimulated flow rate ensures constant dilution and removal of potentially harmful substances, reducing the cariogenic potential of the oral environment.
Saliva also contains mucins, which are sulfated glycoproteins. These mucins provide lubrication, protecting the oral mucosa and aiding in swallowing. They also possess an anti-caries function by aggregating bacteria, including those responsible for decay. Once clumped, these bacterial clusters are more easily cleared from the mouth and swallowed, reducing the microbial load on the tooth surface.
Chemical Buffering and Remineralization
The most direct chemical protection saliva offers is buffering, its ability to neutralize acid. When bacteria metabolize sugars, they excrete organic acids, causing the pH level on the tooth surface to drop rapidly. Saliva contains potent buffering systems, primarily bicarbonate and phosphate, that counteract this drop. These buffers quickly raise the pH back toward a neutral state, preventing the loss of minerals from the enamel.
Acid neutralization is a prerequisite for remineralization, the natural repair mechanism for early tooth damage. Saliva is naturally supersaturated with the necessary building blocks for enamel repair: calcium and phosphate ions. When the environment is acidic, these ions are pulled out of the enamel in a process called demineralization.
As salivary buffers restore the pH, the concentration gradient shifts, driving calcium and phosphate ions back into microscopic lesions on the tooth surface. This repairs the early stages of decay before a visible cavity forms. Specific proteins, such as statherins and proline-rich proteins, maintain the stability of these repair minerals in the saliva. They prevent the calcium and phosphate from precipitating prematurely, ensuring they remain available to repair the enamel structure.
Specific Antimicrobial Components
Saliva contains biological molecules that actively inhibit or destroy oral bacteria. Secretory immunoglobulin A (IgA) is a key immune component that prevents the initial attachment of bacteria to the tooth surface. By binding to bacteria like Streptococcus mutans, IgA forms complexes that block the bacteria’s ability to adhere to the pellicle, interfering with biofilm formation.
Other salivary proteins possess non-immune antimicrobial actions that directly affect bacterial viability. Lysozyme is an enzyme that damages the cell walls of specific bacteria, leading to inactivation. Lactoferrin binds to and sequesters iron, a nutrient necessary for bacterial growth. By depriving bacteria of this element, lactoferrin starves the microorganisms.
The salivary peroxidase system, composed of peroxidase and thiocyanate ions, generates powerful free radical compounds. These compounds penetrate bacterial cells and disrupt their metabolic processes, leading to cellular self-destruction. The combined actions of these antimicrobial components manage the overall bacterial population and prevent pathogenic organisms from dominating the oral environment.
The Impact of Reduced Salivary Flow
The entire protective network collapses when saliva production is significantly reduced, a condition known as xerostomia or “dry mouth.” This reduction compromises all mechanisms protecting the teeth from decay simultaneously. With less fluid movement, physical cleansing is diminished, allowing food debris and acid to linger on tooth surfaces.
The lack of flow also reduces available buffering agents, resulting in an environment that remains acidic for longer periods. This prolonged acidity accelerates the demineralization process. Furthermore, fewer antimicrobial factors allow cariogenic bacteria to proliferate unchecked. Reduced salivary flow is a common side effect of many medications, a symptom of systemic diseases like Sjögren’s syndrome, or a result of head and neck radiation therapy. This loss of the natural defense mechanism leads to an increased risk of rapid and aggressive dental caries.