Saliva role in protection against caries

The role of saliva in caries protection

Saliva
The term saliva refer to the mixture of secretions in the oral cavity, this mixture consists of fluids derived from the major salivary glands, minor glands of the oral mucosa and traces from the gingival exudates. Saliva is a complex secretion. 93% by volume is secreted from the major salivary glands and the remaining 7% by the minor glands. These glands are located in every region of the mouth except the gum and anterior part of the hard palate. Saliva is sterile when it leaves the salivary glands but ceases to be so as soon as it mixes with the crevicular fluid, remains of food, microorganisms, desquamated oral mucous cells, etc.
Daily secretion rates range between 500 and 700 ml and the average volume in the mouth is 1.1 ml. saliva production is controlled by autonomous nervous system. At rest, secretion ranged from 0.25-0.35 ml/min and is mostly produced by the submandibular and the sublingual glands. Sensory, electrical or mechanical stimuli can raise the secretion to 1.5 ml/min. the greatest volume of saliva is produced before, during or after meals, reaching its maximum peak at around 12 a.m. and fall considerably at night, while sleeping.
Saliva composition is 99% of water and the other 1% of organic and inorganic molecules. Saliva is a good indicator of the plasma levels of various substances such as hormones and drugs and can therefore be used as a non-invasive method for monitoring plasma concentration of medicines or other substances.
The normal quantity of saliva can be reduced. This is known as hyposalivation or hypoptyalism and significantly affects the individual’s quality of life as well as his or her oral health. The main symptoms and signs associated with hyposalivation are a “dry mouth” feeling or xerostomia, frequent thirst, difficulty in swallowing, difficulty in speaking, difficulty in eating dry foods, frequent need to drink water, difficulty in wearing dentures, pain and irritation of the mucosa, a burning feeling in the tongue. The signs most frequently encountered are loss of glossiness of the oral mucosa dryness of the mucosa, which become thin and cracked, fissures in the dorsum of the tongue, angular cheilitis, thick saliva, increased frequency of oral infection especially by Candida species, presence of caries in atypical locations and increased size of major salivary glands.
Less frequently, salivary secretion can increase, this is called hypersalivation, ptyalism or sialorrhea, and it may be physiological or pathological. It is diagnosed through the symptoms reported by the patient, who suffer the inconvenience of constantly having to swallow saliva or, in patient with cerebral palsy or other severe neurological disorders, constant drooling that causes frequent chapping of the lips and of the skin of the face and neck, with the risk of secondary infection. Sialometry will demonstrate an increase in the unstimulated salivary flow rated.
The part that saliva plays in protection against caries can be summarized under four aspects: diluting and eliminating sugars and other substances, buffer capacity, balancing demineralization/ remineralization and antimicrobial action.

Diluting and eliminating sugars and other substances

One of the most important functions of saliva is to remove microorganisms and dietary components from the mouth. Studies has established that following the ingestion of carbohydrates, the concentration of sugars in saliva rises exponentially, very quickly at first and then more slowly.
After ingesting sugars, the mouth contain a small volume of saliva, around 0.8 ml. The sugar is diluted in this small quantity of saliva, where it reaches a high concentration. This stimulates the secretory response of the salivary glands, causing an increase in flow, which can reach 1.1 ml. when the food is swallowed, some sugar remains in the mouth. It is gradually diluted and the volume of saliva in the mouth then return to its normal level. Consequently, a high volume of saliva at rest increases the speed of sugar removal, explaining the increased risk of caries in patients with a low unstimulated salivary flow rate.
Elimination does not occur equally in all areas of the mouth. It is faster in the areas that are closest to the places where the salivary gland ducts drain into the mouth, as saliva circulates faster there than in areas where it forms a pool. Again, the speed of clearance from the mucosa and teeth varies considerably (0.8-8 ml/min); even on the teeth, clearance will be slower from the surfaces that are more retentive and more difficult for saliva to reach.
The sugar in saliva easily spread to the bacterial plaque. A few minutes after sugar ingestion, the plaque is already supersaturated with greater concentrations than are found in saliva; there is a correlation between pH changes in the plaque and sugar clearance from the saliva. These changes in pH and the ability of pH to recover are expressed by Stephan’s curve.

Buffer capacity

Although saliva as such plays a part in reducing the acids in the plaque, it also contains specific buffer mechanisms such as bicarbonate, phosphate and some protein systems which not only have a buffer effect but also provide ideal conditions for automatically eliminating certain bacterial components that require a very low pH to survive.
The carbonic acid-bicarbonate buffer acts above all when the stimulated salivary flow rate rises. The phosphate buffer plays an essential role when salivary flow is low. At a pH greater than 6 the saliva is supersaturated with phosphate with regard to hydroxyapatite (HA). When pH falls below the critical level (5.5), the HA begin to dissolve, freeing phosphates that attempt to restore the pH balance. In the final analysis, this depends on the phosphate and calcium ions content of the surrounding medium. Certain proteins, such as histatins or sialin, as well as certain alkaline products generated by the metabolic activity of bacteria on amino acids, peptides, proteins and urea, are also important for controlling pH of saliva.
As in the case of sugar removal, the buffer mechanisms do not act equally on all the tooth surfaces. Their effect is greater on the free surfaces, which are covered by a thin layer of bacterial plaque, than on interproximal surfaces. The mouth is often exposed to foods that have a far lower pH than that of saliva and can start to dissolve the enamel (chemical erosion). Under these conditions also, the buffer mechanisms come into action to normalize pH as fast as possible.


Balance between demineralization and remineralization
Caries lesions are characterized by sub-surface demineralization of the enamel that is covered by a fairly well mineralized layer, unlike chemical erosion, where the outer surface of the enamel is demineralized but there is no sub-surface lesion. The factors that regulate the hydroxyl apatite (HA) balance are the pH and the concentration of free calcium, phosphate and fluoride ions. both saliva and plaque (particularly the extracellular plaque that is in close contact with the tooth) are supersaturated with calcium, phosphate and hydroxyl ions with regard to HA.
Some proteins are able to bind to the HA and inhibit the spontaneous precipitation of calcium and phosphate, thus maintaining the integrity of the enamel crystals. Proline-rich proteins statherins, histatins and cystatins act in this way, while the action of some proteases and of salivary callicrein affect this regulatory process.
Caries process begins when bacteria ferment carbohydrates, resulting in the production of organic acids that lower the pH of saliva and the plaque. In the dynamic balance of the caries process, supersaturation of the saliva provides a barrier to demineralization and tips the balance towards remineralization. The presence of fluoride assists this balance.
Calcium is found in greater quantities in unstimulated than in stimulated saliva as its main source is the saliva secreted by the submaxillary and the sublingual glands, whereas when stimulation occurs, it is the parotid gland which produces the greater volume of secretion. The phosphate concentration in saliva from submaxillary glands is approximately 1/3 of that in parotid saliva but is 6 times higher than that of the saliva produced by the minor salivary glands.

Antimicrobial action

Saliva plays an important role in maintaining the equilibrium of the oral ecosystem. This is essential for dental caries control. Saliva is able to perform its function in maintaining the oral micro biota balance because it contains certain proteins. These are essential constituents of the acquired pellicle, encourage bacterial aggregation, are a source of food for certain bacteria and possess an antimicrobial effect because some of them are capable of modifying the bacteria’s metabolism and ability to adhere to the tooth surface.
The most important proteins involved in oral ecosystem maintenance are proline-rich protein, lysozyme, lactoferrin, peroxidases, agglutinins and histidine, as well as secretory immunoglobulins A ,G and M.

The role of saliva in bacterial plaque formation

Bacterial plaque is a biofilm that cover all the oral structures. It is partly cellular, fundamentally bacterial and partly a cellular from bacterial, salivary and dietary sources. It appears as a yellowish-white deposit which adhere strongly to the tooth and is not dislodged by chewing or jets of air or water, unlike the material alba, which is composed of food debris, desquamated cells, leucocytes and unadhered bacteria and can be flushed away by jet of water.
The first stage in bacterial plaque formation is the formation of acquired pellicle, which takes place only a few minutes after the teeth have been well brushed. The acquired pellicle is an acellular coating, between 2 and 10 µm thick, made up of salivary proteins and other macromolecules. It provides the basis for initial colonization by microorganisms which, under certain conditions, form the dental plaque. However, the acquired pellicle also provides an important protection against attrition and abrasion and acts as a diffusion barrier, as it carries a negative electric charge.
Primary bacterial colonization takes place through specific, irreversible adhesion between the acquired pellicle receptors and the bacterial molecules known as adhesion. Proline-rich proteins are an important part of this process, as their amino-terminal segment adheres to the tooth, leaving the carboxy-terminal region free to bind to the bacteria. This stage lasts between 4 and 24 hours, with a predominance of aerobic bacteria.
Secondary colonization can last from 1 to 14 days, after which the bacteria multiply actively by aggregation and coaggrigation, although some bacteria may also employ adhesion. The plaque thickness increases and unaerobic microorganisms begin to predominate in the deeper layers.
Bacterial competition is established and nutrients are obtained from the breakdown of the acellular matrix and the excretion of certain bacterial metabolites that can be used as nutrients by other species. Approximately two weeks later, the mature plaque forms.
Oxygen and nutrients are scarce in its deeper areas and the accumulation of waste products increases. Although this places the number of viable cells at risk, the composition of the plaque maintains a certain stability.
The mature plaque may mineralize and form calculus, which has similar microbial composition but may have a lower number of viable cells. A prerequisite for calculus formation is that the plaque must have a more alkaline pH than the surrounding saliva or crevicular fluid. This may be the result of high proteolytic activity. Protease activity in the saliva is closely linked to calculus indices and high concentrations of urea in the plaque encourage deposition of calcium and phosphorus on the plaque. Further processes such as those described may be repeated on the calcified plaque, increasing its thickness.

Dental pellicle

The acquired enamel pellicle is a thin film consisting mainly of salivary proteins selectively absorbed to the surface of the enamel. The pellicle protects the enamel from dissolution. Diffusion fluxes are reduced by 50% in the presence of pellicle, leading to a decreased demineralization potential of the acids secreted by bacteria. The pellicle is also a base to which the bacteria can adhere when they enter the oral cavity. The binding of bacteria is mediated by non-specific electrostatic and Van der Waals forces, but also by specific interaction between bacteria and the proteins on the salivary pellicle.
Thus, colonization of microbial flora on the tooth surface is strongly modified by salivary proteins. Several proteins-like parotid saliva agglutinins, a-amylase, statherins, mucins, and salivary immunoglobulin are reported to bind with oral streptococci. These proteins are also found in the salivary pellicle, and therefore, they are likely to mediate the specific adhesion of bacteria to tooth surfaces. It has been suggested that high molecular-weight parotid saliva agglutinins, and similar proteins found in submandibular-sublingual saliva, are the most important salivary proteins in promoting the adhesion of Streptococcus mutans.
On the other hand, when these same proteins exist in the liquid phase, they may promote bacterial aggregation and, hence, the clearance of bacteria from the oral cavity. The two most abundant agglutinins in saliva are high molecular-weight agglutinin from parotid saliva and mucins.


Dental plaque
Dental plaque is a soft deposit that accumulates on the teeth, it can be defined as complex microbial community, with greater than 1010 bacteria/milligram. It has been estimated that as many as 400 distinct bacterial species may be found in plaque, in addition to bacterial cells, plaque contains a small number of epithelial cells, leucocytes, and macrophages.
Plaque can be classified into supra gingival and sub gingival based on its relationship with the gingival margin. Streptococci and Gram positive aerobic bacteria mostly found in supra gingival plaque while Gram negative anaerobes dominate sub gingival plaque.
Plaque can be classified by its relationship with the tooth surface, as either attached or unattached plaque.
Plaque can also be classified in association with disease state into “health associated” and “disease associated”, this is related to differences in microbial composition of dental plaque in health versus disease.
When a clean enamel surface is exposed to the oral environment, it becomes covered by an amorphous organic film called the pellicle, this consists mainly of a glycoprotein precipitated from the saliva, this can attract and anchor specific types of bacteria to the tooth surface.
The earliest colonizers tend to be aerobic (oxygen tolerant) bacteria, as the oxygen level in plaque falls, the Gram negative rods such as Fusobacteria, and Gram negative cocci such as Veillonella tend to increase. In the early colonizers Streptococcus sanguis first appear followed by Streptococcus mutans, the Gram negative spesies are thought to predominate in the plaque during later inflammatory periodontal disease development.

Dental plaque and dental caries

The organisms which initially colonize the pellicle are predominantly Streptococci, only 2% of these are Streptococcus mutans. When a plaque becomes thicker and a mixture of different types of microorganisms compromises the bacterial community, consequently, the flora of the plaque changes from its initial predominantly coccal form to a mixed flora consisting of coccal, rods and filaments.
Carbohydrate intake leads to acid production by plaque bacteria may lead to a fall in the pH of the fluid medium to even less than 5.5, which is the critical pH of the saliva, resulting in undersaturation of the fluid medium with respect to hydroxyapatite causing dissolution of the hydroxyapatite crystals from their ionic component, in response to the increased pressure gradient the calcium and phosphate ions diffuse from the tooth toward the surrounding environment.
The acid attack can be terminated by depletion of fermentable carbohydrate. By buffer capacity of both plaque and saliva, the pH rise again and remineralization episodes occur by reprecipitation of hydroxyapatite crystals, if the sum of demineralizing episodes is greater than that of remineralizing because there will be a continuous loss of calcium and phosphateions and increase in the porosity of enamel, which may progress to cavitation.