The oral environment
This chapter discusses the oral environment. The oral cavity shows many of the biological characteristics of the gut as a whole. However, because of its exposed position at the beginning of the alimentary canal, its intrinsic biological properties are continually modified by environmental factors, particularly by contamination with microorganisms and by the intake of food. The bacteria that inhabit the mouth constitute a community of many different species, and a study of their interactions with the tissues of the oral cavity is necessary for an understanding of oral biology in health and disease. Organisms and their surroundings together constitute an ecosystem, and each ecosystem has physical, chemical, and biological properties that determine the composition of the community and dictate which organisms dominate the system and which will fail to survive. In the mouth, those populations for which it has favorable nutritional and physiological characteristics survive and unsuited species fail to become established, so that the community is maintained in balance by the operation of natural selection. Each organism has a particular biological role or niche within the community so that species with identical biological properties compete for the same niche; a mixed community becomes stable only when each organism has a different role to play thus avoiding competition. The mouth, together with the rest of the gut, constitutes a natural open system, and the oral cavity is regarded as a fermentation chamber providing a suitable environment for the continuous culture of microbial populations.
The oral cavity shows many of the biological characteristics of the gut as a whole. However, because of its exposed position at the beginning of the alimentary canal, its intrinsic biological properties are continually modified by environmental factors, particularly by contamination with microorganisms and by the intake of food. The bacteria which inhabit the mouth constitute a community of many different species and the study of their interactions with the tissues of the oral cavity is necessary for an understanding of oral biology in health and disease.
Organisms and their surroundings together constitute an ecosystem and each ecosystem has physical, chemical and biological properties that determine the composition of the community and dictate which organisms will dominate the system and which will fail to survive. In the mouth, those populations for which it has favourable nutritional and physiological characteristics survive and unsuited species fail to become established, so that the community is maintained in balance by the operation of natural selection. Each organism has a particular biological role or niche within the community so that species with identical biological properties will compete for the same niche; a mixed community becomes stable only when each organism has a different role to play thus avoiding competition.
The mouth, together with the rest of the gut, constitutes a natural open system and the oral cavity can be regarded as a fermentation chamber providing a suitable environment for the continuous culture of microbial populations. Relatively constant conditions are ensured by the flow of saliva which, in association with a frequent intake of food, provides a regular source of fresh substrates. At the same time, soluble products of microbial activity are continually removed and swallowed, so that their concentrations do not rise to any significant extent. The mouth is at a constant temperature, and the flow of saliva not only keeps it moist but also serves to remove many of the bacteria as well as helping to maintain the pH and ionic composition within a limited range. In other words, the mouth can be considered to contain a mixed bacterial culture preserved in a steady state. The bacteria, in turn, modify the physical and chemical composition of their surroundings and establish a commensal relationship with their host and a stable, interspecies relationship among themselves.
So far the mouth has been considered as a single entity; it can, however, be subdivided into a number of small independent ecosystems, each with a unique set of organisms selected on the principles already described. The main problems confronting the dentist involve the relatively restricted regions of the tooth surface and its supporting tissues, since the bacterial populations in these areas modify their environment in a manner that may lead to disease. The relationship between the tooth surface and the rest of the mouth can be summarized as follows:
Dynamic interactions between the saliva, diet and oral bacteria lead to the accumulation of material on the tooth surface. This is known as dental plaque which can be defined as the deposit which forms on the tooth in its natural environment and, unlike food debris, cannot be removed by a water spray. It is composed of localized concentrations of bacteria specific to this surface together with some degraded mammalian cells. These are surrounded by a matrix containing both protein and polysaccharide and the whole is bathed in a fluid derived from the saliva. On this basis, plaque can be said to have the characteristics of a simple tissue.
Plaque is almost always found on the non-occluding surfaces of the teeth which are only subject to abrasion if the teeth are brushed. Plaque may be present in healthy mouths and does not necessarily lead to pathological changes. However, it is recognized as the common factor in the two main dental diseases, namely dental caries and periodontal disease.
Dental caries is characterized by the loss of calcified material from the tooth while periodontal disease is a chronic inflammatory process that leads to the gradual destruction of the supporting tissues of the tooth and is often associated with the deposition of calculus.
Saliva is a mixed secretion more than 90% of which is produced by the parotid, submandibular and sublingual glands. The remainder is contributed by accessory glands present on the soft palate and on the internal surfaces of the lips and cheeks. The type of secretion varies according to the gland and, whereas parotid saliva is serous or watery in consistency, that from the submandibular and sublingual glands contains different glycoproteins and is much more viscous. Their viscosities relative to water at (1·0) 37°C are parotid 1·3, submandibular 2·6 and sublingual 11·0.
The volume of saliva secreted each day is difficult to determine. An average value of between 1·0 and 1·5 litres is often cited but this may well be an overestimate. Under resting conditions, the rate of flow is about 0·3 ml min−1 (range 0·05–1·8 ml min−1) and this increases to about 2· 5–5·0 ml min−1 on stimulation. On this basis, since during sleep the flow is negligible, it may be calculated that between 700 and 800 ml are produced each day so that over a period of weeks or months the surfaces of the teeth are exposed to large volumes of saliva.
Estimates suggest that about 1·0 ml of saliva is present in the mouth at any one time. It has been found that the average volume required to initiate the swallowing reflex is 1·1 ml and that 0·8 ml remains after swallowing, thus removing about 0·3 ml each time. Because the estimated surface area of the teeth and soft tissues is very large, about 200 cm2, it follows that these surfaces are covered by a film of saliva, no more than 100 μm thick. This allows rapid exchange between the saliva and the underlying surface and favours the deposition of salivary macromolecules providing a more dynamic relationship between the enamel and saliva than is generally realized. Assuming a daily salivary volume of 700–800 ml it follows that the 1·0 ml pool of saliva is replaced 700–800 times each day, requiring about 2500 swallows in the process! This high dilution rate is essential for the effective removal of food debris and prevents the accumulation of excessive numbers of bacteria. The average adult swallows between 1·0 and 2·5 g of bacteria each day and defective salivary flow quickly results in microbial overgrowth and oral stagnation.
The epithelial surface of the whole of the gastrointestinal tract is covered by a continuously flowing layer of mucus so that the environment of these surfaces is largely independent of the chemical composition of the contents. The glycoproteins in saliva provide both mechanical and chemical protection. Also, because of its large volume and continuous flow, saliva flushes away large numbers of bacteria and accumulating toxins. If the flow of saliva is suppressed, pathological changes may supervene and excessive dryness leads to stomatitis, i.e. inflammation of the mouth and an increase in dental caries.
The properties of saliva which make it a good lubricant also help to protect the soft tissue surfaces from physical damage which might otherwise be caused by hard textured foods or excessive temperatures. Similarly, buffers present in saliva help to protect both hard and soft tissues from chemical damage resulting from bacterial acid production. Protection against the damaging effects of bacteria may also be provided by such specific components as immunoglobulins and lysozyme (page 409), which break down the coat polysaccharides of certain types of bacteria.
Lubrication with saliva helps to soften the food and aids the formation and swallowing of a food bolus. The rate of removal of substances from the mouth depends on factors such as the rate of saliva flow, the adhesive properties of the food and the activity of salivary enzymes, notably amylase. This enzyme initiates the digestion of starch (page 224) and the breakdown and consequent removal of residual material commonly found around the teeth.
By lubricating the lips and tongue saliva facilitates speech and, since it acts as a solvent, it also aids gustation. Saliva may perhaps be considered to play a role in excretion since heavy metals such as mercury and lead are secreted into it; this does not, however, provide a final solution to the problem of their disposal since they are subsequently swallowed and may be reabsorbed. Lastly, to the great convenience of the dentist, the rheological* properties of saliva help to retain dentures in position although this can hardly be part of Nature’s grand design!
Saliva, for experimental work, can be collected with special cannulae directly from the ducts of the glands. This material is pure and sterile and truly representative of the secretion of the gland in question. However, mixed saliva, which is usually collected from the mouth during paraffin wax stimulation, is of very variable composition. In addition to fractions from each gland, it contains food debris, plaque, bacteria, degraded mammalian cells and possibly some gingival sulcus fluid. In spite of its heterogeneity and variability, the composition of mixed saliva is of great importance since it is this fluid which gives rise to the acquired enamel integuments (page 490). Some idea of the composition of a normal average sample of mixed saliva can be obtained from Table 33.1.
|Solids||0·5 (stimulated)–6·0% (unstimulated)|
|Saliva (mM)||Plasma (mM)|
|Amino acids (free)||1–2||2|
|Fatty acids (mg l−1)||10||3000|
|Macromolecules (mg l−1)|
The secretion of saliva appears to be an active process as its composition is very different from that of the plasma and also, when the ducts are ligated, secretion continues even when the pressure in the ducts is about twice that of the normal blood pressure. Stimulated mixed saliva contains 99·0–99·5% water, and the solid matter is composed of approximately equal amounts of organic and inorganic matter. Saliva is hypotonic and has a specific gravity of 1·002–1·008. The pH varies from 5·6 to 8·0 but the average value is about 6·7.