The oral environment

Publisher Summary

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:

< ?xml:namespace prefix = "mml" />Enamel+environment{mixed salivadietbacteria→Dental plaque{specific bacteriamatrixfluid

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

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.

Table 33.1

The average composition of mixed human saliva and normal values for plasma

Water	94·0–99·5%
Solids	0·5 (stimulated)–6·0% (unstimulated)
Specific gravity	1·002–1·008
pH (average)	6·7
pH (range)	6·2–7·6
	Saliva (mM)	Plasma (mM)
Inorganic
Ca²⁺	1–2	2·5
Mg²⁺	0·2–0·5	1·0
Na⁺	6–26	140
K⁺	14–32	4
NH₄⁺	1–7	0·03
	2–23	2
Cl⁻	17–29	103
	2–30	27
F⁻	0·001–0·005	0·01
SN⁻	0·1–2·0	—
Organic
Urea (adults)	2–6	5
Urea (children)	1–2	—
Uric acid	0·2	3
Amino acids (free)	1–2	2
Glucose (free)	0·05	5
Lactate	0·1	1
Fatty acids (mg l⁻¹)	10	3000
Macromolecules (mg l⁻¹)
Proteins	1400–6400	70000
Glycoprotein sugars	110–300	1400
Amylase	380	—
Lysozyme	109	—
Peroxidase	3	—
IgA	194	1300
IgG	14	13000
IgM	2	1000
Lipid	20–30	5500