competitive ones. In the long term (evolutionary length), competition for survival has not been the best strategy, as 99% of all the species that have ever lived are now extinct. However, there are some remarkable examples of cooperation which have lasted 3 billion years. The relationship between mitochondria and all nucleated animal cells started off as a host–parasite relationship. The cell gave shelter to the mitochondria which were able to use oxygen to produce high-energy molecules and thus could be a great benefit to an active cell. The partnership was so successful that it led to the evolution of the eukaryote cell, with nucleus, mitochondria, and cilia. The eukaryote cells enabled an unprecedented surge in evolution of multicellular forms of life. The mitochondria have retained their genetic identity over the millions of years of this partnership and their separate process of reproduction through the female lines of organisms. The relationship is still recognizable as a form of symbiosis, the living together of different organisms. There are many other examples of symbiotic relationships, which, as they are better understood, indicate that they are of significant importance. A close and highly interdependent relationship between bacteria is found in a biofilm. This is a dense aggregate of many species which have abandoned a solitary planktonic life for the benefits of a cooperative community existence.
4.2 The Oral Environment
The following section will describe the features of the oral environment which support and influence the ecology of the mouth. The oral cavity provides a range of fairly stable habitats for microorganisms. There is a plentiful supply of both oxygen and nutrients, and there are physical surfaces for attachment, although some surfaces are more liable to disruption than others. The oral mucosa, including the tongue and exposed tooth surfaces, is exposed to saliva flow and the disruption caused by shear forces which occur during swallowing and mastication. The surface epithelial cells of the oral mucosa are shed (desquamated) when mature and carry off with them any organisms which have colonized the cell’s surface (▶ Fig. 4.1). This process is particularly important for the health of the gingival sulcus, one of the most highly populated habitats in the oral cavity. This site provides shelter, and a rich supply of nutrients provided by the gingival crevicular fluid flowing out of the gingival sulcus. The cells of the junctional epithelium and gingival sulcus epithelium desquamate regularly, clearing away colonizing organisms. It has been noted in Chapter 3 Oral Mucosa and Periodontium that the turnover period of the junctional epithelium may be only a matter of days.
For those organisms which are able to adhere to the exposed tooth surface, there are large areas to colonize. The sheltered tooth surfaces, such as the approximal areas and occlusal fissures, are more densely colonized than more exposed tooth surfaces, and it is these sites which have the greatest risk of developing dental caries (▶ Fig. 4.2). Oral organisms are unable to attach directly to tooth enamel, but they may adhere via an intermediary layer of proteins, the salivary pellicle.
The presence of a fixed restoration, which has defective margins, and a removable restoration, which covers the oral mucosa, provides a protected environment for organisms which allows the total mass to increase (▶ Fig. 4.3). When the teeth are lost, these habitats disappear and the oral flora is dramatically altered; in general, it is less diverse.
Fig. 4.1 The desquamation of the surface epithelial cells of oral mucosa. (a) A scanning electron microscope (SEM) image of a surface cell which has partly desquamated (magnification × 1,000). (b) A diagrammatic representation of the effect of desquamating epithelial cells on the control of colonization of epithelial surfaces by oral organisms.
4.2.1 Salivary Pellicle
Salivary pellicle is a thin layer (10 μm) of various salivary proteins which heap up on top of each other, on the surface of recently cleaned enamel, within a few hours. The smaller-molecular-weight phosphoproteins and sulpho-glycopeptides are the first to adhere to freshly cleaned enamel. Some of the phosphoproteins and calcium-binding proteins form ionic bonds with the apatite crystals of enamel. Other proteins adhere because bacteria have caused them to clump together; they are less strongly bound to the enamel surface. Most of the salivary proteins are rich in the amino acid proline and are collectively described as proline-rich proteins (PRPs). The coverage and composition of the pellicle change during its early formation. After the smaller-molecular-weight proteins, the larger glycoproteins adhere, and this stage is rapidly followed by the adhesion of the first oral organisms. Pellicle has the following influence on the oral environment:
• It protects enamel from demineralization by providing a layer of proteins, which isolates the surface from changes in the acidity of fluids surrounding the tooth.
• It influences the types of microorganisms which will adhere to the tooth surface.
• It lubricates the enamel surface and may therefore reduce the rate of tooth wear (▶ Fig. 4.4).
Given a surface onto which oral organism are able to adhere, a further defining feature of the oral environment is the fluid surrounding the pellicle and bathing all the surfaces in the oral cavity. Oral fluid is a term which best describes the mixture of substances which might be found in a sample of fluid from the mouth. It would mainly consist of saliva from the major and minor glands, each with its characteristic feature. The oral fluid would also contain gingival sulcus fluid, desquamated epithelial cells, bacteria, and some blood cells, mostly neutrophils. It is difficult to detail the exact composition of saliva as the secretions from each gland are not identical, and they all vary with the rate of secretion of saliva.
4.2.2 How Saliva Defines the Oral Environment
The essential functions of saliva are both enabling and protective. Saliva enables mastication and speech as it lubricates the oral structures during function. It also protects enamel from demineralizing and enables a stable ecology of oral organisms to live as commensals while excluding more overtly pathogenic bacteria.
Lubrication: Lubrication is provided by mucins (glycoproteins) which are selectively adsorbed to mucosa and enamel. On mucosa, they protect the tissues against drying, irritants, and bacterial enzymes. Together with the water content of saliva, glycoproteins provide lubrication for speech and mastication and preparation of a food bolus which can be readily swallowed. It appears to be the carbohydrate part of the glycoproteins, which is important for lubrication. (see Chapter 6.2 Glycoproteins)
Mechanical cleansing: The flow of saliva and subsequent intermittent swallowing provide a flushing system which removes food particles, bacteria, and desquamated cells from the mouth. The rate at which this removal occurs is referred to as the clearance rate. The more complete the fluid eliminated at each swallow, the more rapid will be the clearance of a substance from the mouth. Clearance will also depend on the flow rate of saliva; it is thus dramatically reduced during sleep (see Chapter 4.2.3 Rate of Flow of Saliva). The clinical importance of clearance lies in the rate of elimination of sucrose and fluorides from the oral cavity. A high rate of salivary clearance in individuals has been associated with a reduction in caries prevalence.
