Before we consider the inhibition of caries by saliva, we should first understand how caries occur. Cariogenic mutans streptococci is the principal bacterial component responsible for the initiation and the development of dental caries (Lenander-Lumikari & Loimaranta, 2000, p.40). Usually, solid surfaces are required for both streptococcal colonization and multiplication; hard tooth surface a very suitable site for this. Fermentation of carbohydrates by cariogenic-bacteria produces organic acids which causes a pH drop in plaque. Consequently, demineralization of the tooth is promoted and the acidic medium is an advantageous environment for further growth of Streptococcus mucci.
Saliva is well equipped to prevent the development of dental caries. These shall be explored in greater detail below.
Factors affecting the development of dental caries. Saliva and gingival fluid prevents caries through the flow rate, buffer effect, inorganic components (supersaturation with respect to hydroxyapatite), antimicrobial and immunological function. External factors such as fluoride and oral hygiene may prevent dental caries. Referenced from Lenander-Lumikari & Loimaranta, 2000, p.41.
The anti-caries effects of saliva can be categorized as static or dynamic (Edgar & Higham, 1995, p.235).
Static effects are exerted continuously and include the following:
1. Effects on the bacterial composition of plaque through antibacterial or metabolic factors
2. Protective effects of pellicle formation
3. Effects of salivary ions (including fluoride) in maintaining a supersaturated environment for the tooth mineral.
Dynamic effects are mobilized over the time-course of the Stephan curve, which describes the changes in plaque pH in response to a cariogenic challenge such as an oral sucrose rinse (see "More on the Stephan Curve"). Dynamic effects are related to the level of salivary stimulation, and are thus activated during eating or drinking, i.e. whenever their action is required.
1. Metabolism of carbohydrate
2. Removal of acid products of plaque metabolism
3. Alkalinity and buffering power to restore plaque pH towards neutrality
Static and dynamic effects are summarised in the following figure:
Anti-caries effect of saliva classified as static or dynamic. Referenced from Edgar & Higham, 1995, p.235.
More on the Stephan Curve
The Stephan Curve describes the changes in plague pH in response to a cariogenic challenge (University of Newcastle Dental School. Stephan Curves: The Basics). Here, the subject rinsed with 10ml of 10% sucrose solution for 10 seconds; plaque samples were removed at intervals and the pH recorded. An initial rapid drop in plaque pH observed, and this is dependent on the speed with which plaque microbes are able to metabolise sucrose, producing acid as a by-product which lowers pH.
The lowest pH attained depends on the types of microbials in the plaque, the nature of the fermentable carbohydrate, and the rate of diffusion of substrates and metabolites in and out of the plaque.
The subsequent slower rise in pH is due to acidic by-products diffusing out of the plaque (carried away by saliva) and salivary bicarbonate diffusing into the plaque and neutralizing these by-products. The speed of pH recovery ranges from 15-40 minutes depending on the individual’s saliva neutralizing effects. Thus saliva flow rate exerts a major effect on pH recovery.
Plaque metabolism causing the initial rapid drop in pH due to acids produced. Microbial composition determines how much acid is produced and the lowest pH obtained. Rate of diffusion of materials into and out of plaque depends on saliva flow which may not reach sheltered sites such as fissures; these sites may have lower resting pH values and caries tend to develop here. Referenced from University of Newcastle Dental School. Stephan Curves: The Basics.
In the following section, we shall explore how saliva inhibits caries through a number of mechanisms that are related to its immunological function, enzymatic function, mechanical cleasing function, supersaturation with respect to hydroxyapatite and protective remineralisation (promoted by fluoride).
Immunological Function - Specific Defense Factors
Saliva also has an immunological function in the inhibition of dental caries. This is due mainly to immunoglobulins, such as IgG, IgM, IgA, and secretory IgA (slgA), which form the basis of the specific salivary defense against oral microbial flora, including mutans streptococci (Lenander-Lumikari & Loimaranta, 2000, p.44).
In saliva, dimeric slgA (an antibody) is most abundant. It is composed of at least two monomeric IgA molecules covalently linked through a J chain and a secretory component (Phalipon, Cardona, Kraehenbuhl, Edelman, Sansonetti & Corthésy, 2002), and serves to protect mucosal surfaces. There is specificity in its mode of action – the formation of specific IgAs in saliva correlates with the colonization of bacteria in the oral cavity (Lenander-Lumikari & Loimaranta, 2000, p.44). Studies show that the presence of active caries lesions may induce the formation of specific IgGs, which can then act to inhibit the caries process.
As these immunoglobulins are found in dental plague and readily bind to the pellicle, they are strategically located in close proximity to the bacteria that are the very causes of caries. Igs act by neutralizing various microbial virulence factors, limiting microbial adherence, agglutinating the bacteria and preventing the penetration of foreign antigens into the mucosa. These may inhibit the caries process.
Enzymatic Function - Innate Defense Factors (non-immune salivary factors)
Innate antimicrobial proteins in saliva include salivary peroxidase, peroxidase-generated hypothiocyanite (HOSCN/OSCN-), lysozymes, lactoferrin, cystatin, histatin or proline-rich proteins (PRP) (Lenander-Lumikari & Loimaranta, 2000, p.44).
Studies have shown that the above mentioned proteins affect cariogenic bacteria such as mutans streptococci lactobacilli, and fungi in vitro, but their clinical role in the human mouth has yet to be fully determined (Bardow et al., 2008, p.201). Presently, no definite link between the inhibition of caries and the concentration of these proteins in saliva has been established – an increase in antimicrobial protein concentrations in saliva by some oral hygiene products did not inhibit dental caries in healthy individuals with normal saliva flow. These proteins mainly function to control microbial overgrowth in the mouth only. For example, lysozyme can hydrolyse the β(1-4)bonds in the peptidogylcan layer of bacterial cell walls. See “Antibacterial, Anti-microbial, Anti-fungal Function of Salivary Proteins” for more details on how they control bacterial growth.
Having said that, if certain antimicrobial proteins have certain modes of action – those that promote adhesion or maintain inorganic component homeostasis in the oral cavity (Rudney, 1995) – the inverse relationship between caries and the various antimicrobial concentrations in saliva may still be expected. In these cases, the dynamic process of remineralisation and demineralisation of tooth minerals is affected, and would certainly contribute to inhibiting caries. For example, the action of peroxidases (Bardow et al., 2008, p.201) is likely to inhibit caries in the following way:
1. Usually, peroxidases catalyze the oxidation of thiocyanate (SCN-) by hydrogen peroxide (H2O2) to the antimicrobial component, hypothiocyanite (OSCN-) H2O2 + SCN- → OSCN- + H2O
2. The higher the concentration of hypothiocyanite, the lesser the bacteria (such as mutans streptococci) in dental plague produces acids by fermenting carbohydrates.
The inverse relationship between hypothiocyanite concentration in saliva and plague acidogenicity has a clear implication – increased concentrations of antimicrobials such as peroxidase are likely to give better protection from dental caries by interfering with bacterial glucose uptake or glucose metabolism (Lenander-Lumikari & Loimaranta, 2000, p.44), such that lesser acids are produced by fermentation of carbohydrates. Additionally, the peroxidase system helps to eliminate hydrogen peroxide (H202), which is highly toxic to mammalian cells.
Other salivary proteins also contribute to inhibiting caries by affecting the bacteria that cause them (Bardow et al., 2008, p.200). For example, mucins cause aggregation of oral bacteria, which accelerates its clearance by the mechanical effect of saliva flow. Some oligosaccharides in mucin are structurally similar to those in mucosal surfaces and so act as competitive inhibitors – they block the reactive groups in bacterial cell surfaces, thereby preventing adhesion of bacteria to soft tissues. A similar inhibition of bacterial adhesion to hard tissues, such as enamel, also occurs when mucin interacts with enamel.
Mechanical function of cleaning the tooth surface
Saliva flow inhibits dental caries mechanically as well, as studies show that low saliva flow rates in patients with dry mouth suffer more dental caries (Lenander-Lumikari & Loimaranta, 2000, p.40). The flow of saliva in the oral cavity, resulting from tongue movements, creates a shear force that prevents large amounts of bacteria from adhering to hard surfaces (i.e. the teeth) (University of Newcastle Dental School, Why Plaque Forms at Specific Sites). Usually, any microbes adhering to soft epithelial tissue of the oral cavity – such as the linings of the cheeks, gums and tongue – will not be able to form biofilms because the squamous epithelium has many layers which exfoliate continuously. In doing so, any adhering microbes are exfoliated with it, and swallowed.
Microbes carried by the saliva may adhere to hard tooth surface. However, the flow of saliva across a surface creates shear forces which easily dislodges any microbes which are weakly attached to the surface. As such, most microbes on these surfaces hardly adhere for long before saliva mechanically removes them.
When fluid flows in a tube, we observe the phenomenon of a higher velocity of flow in the centre, and reduced velocity near the surfaces of the tube. This is due to viscosity which increases drag and slows fluid flow. Similarly, when saliva flows past a surface, its velocity is reduced and this may allow microbes to become weakly attached at first. However, the accumulation of microbes on the surface significantly increases drag and shear force, thus inhibiting further development of plague.
If plague cannot develop easily, why do caries still develop? Simply put, significant build up of plague on most surfaces is inhibited, but accumulation is limted to sheltered sites such as interproximal areas, the gingival margin and fissures. Microbes that are sheltered from the flow of saliva experience less shear force and so allow for the development of plague and eventually caries.
It is important to note that the flow of saliva is concerted to the buffering capacity of saliva in caries-prevention (Lenander-Lumikari & Loimaranta, 2000, p.40). Besides faster clearance of microbes on tooth surface, higher flow rates also means faster delivery of protective salivary components (such as bicarbonate) to the site, thus limiting the rise in pH. Consequently, demineralization is less likely to occur and the caries process is halted.
Sheltered sites where caries usually develop. Fissure Caries, Approximal Caries and Advanced Gingival Margin Caries. Referenced from University of Newcastle Dental School. Bite-Sized Tutorials: Why Plaque Forms at Specific Sites.
Saliva supersaturation with respect to hydroxyapatite
Inorganic composition of saliva determines supersaturation
To understand how saliva supersaturation with respect to hydroxyapatite can affect the caries process, we must first understand the inorganic composition of saliva, and how they contribute to a certain degree of saturation. These inorganic components involved in maintaining supersaturation of saliva are calcium and phosphate ions (Bardow et al., 2008, p.196). The buffer systems in saliva work with these ions to maintain a near neutral pH in the mouth. Salivary calcium has a concentration which ranges between 1-2mM/L in both stimulated and unstimulated saliva. It is found in two main forms: bound to proteins (such as statherin and proline-rich proteins) and non bound.
Half of the non bound calcium is normally divided into ionised and non-ionised calcium, with the ratio of the two determined by saliva pH and ionic strength. When saliva pH increases at high flow rates, more calcium is found in the non-ionised form.
Salivary phosphate is found in four forms: phosphoric acid (H3PO4), dihydrogen phosphate (H2PO4-), hydrogen phosphate (HPO4 2-) and phosphate (PO4 3-). At low pH, more phosphate is found in the acidic forms, and the concentration of free ionised phosphate will thus be lower. The following figure shows this:
The phosphate buffer system as a function of pH. Physiological pH ranges from 6.0 to 7.5 (shown by the grey area). Below this, most phosphate is in the dihydrogen and phosphoric acid forms (i.e. in unstimulated saliva); saliva phosphate concentration is decreased to very low levels when pH is low. In stimulated saliva, saliva flow rate increases and so does bicarbonate concentration - this causes pH to rise. Phosphate found in the hydrogen form. Referenced from Bardow et al., 2008, p.197.
Saliva supersaturation controls demineralization and remineralisation
The processes of demineralization and remineralisation are dynamic, i.e. they are happening continuously all the time. Saliva pH and ionic strength (i.e. the concentrations of ionised calcium and phosphate) control these processes. When ionic activity product exceeds the solubility product, saliva is supersaturated and remineralisation can occur. The opposite is true for demineralization. The following video shows the mode of action of demineralisation and remineralisation.