Host factors

The two main host factors are the structure of enamel and the composition and flow of saliva. The susceptibility of different areas of enamel on the same tooth to a standard acid attack in vitro can vary markedly. This and other information strongly suggest that some areas of the same tooth are much more susceptible to carious attack than others. This fact is not often considered when the role of microorganisms in caries is studied in vivo or in vitro. Susceptibility to demineralization by acid is related to the mineral content (especially fluoride) and structure of particular areas of enamel.

Saliva performs a number of important roles in maintaining dental health, some which are related to dental caries (see Chapter 3). For example, the mechanical washing action of saliva is a very effective mechanism in removing food debris and unattached oral microorganisms from the mouth. The importance of this factor is highlighted in patients with severe Sjögren’s syndrome (a degenerative disease of salivary glands) who have a very low salivary flow rate, retain food debris in their mouth for long periods and suffer from rampant dental caries. Saliva has a high buffering capacity which tends to neutralize acids produced by plaque bacteria on tooth surfaces. It is also supersaturated with calcium and phosphorus which are important in the remineralization of white spot lesions; fluoride is also important in this process.

The roles of the other salivary antimicrobial factors in dental caries, e.g. lysozyme, the lactoperoxidase system and immunoglobulins (see Chapter 3), are not clear.

Diet

A number of epidemiological studies have demonstrated clearly a direct relationship between dental caries and the intake of carbohydrate. The most cariogenic sugar is sucrose and the evidence for its central role in the initiation of dental caries includes the following: (1) increase in the caries prevalence of isolated populations with the introduction of sucrose-rich diets; (2) clinical association studies; (3) short-term experiments in human volunteers using sucrose rinses; and (4) experimental animal studies. In addition, sucrose is highly soluble and diffuses easily into dental plaque, acting as a substrate for the production of extracellular polysaccharide and acids. Although sucrose is not required for the initial attachment of Strep. mutans cells to tooth surfaces, it is necessary for subsequent plaque development as an intercellular adhesive. However, the direct relationship between sucrose and dental caries is more complex than can be simply explained by the total amount of sugar consumed. There is good evidence that the frequency of sugar intake, rather than the total sugar consumption, is of decisive importance in caries development. Also important are the stickiness and concentration of sucrose consumed, both factors influencing the period for which sugar is retained in the mouth in close contact with the tooth surface.

Carbohydrates other than sucrose, e.g. glucose and fructose, are also cariogenic but less so than sucrose. Carbohydrates with low cariogenicity also exist, e.g. xylitol (a sugar alcohol).

Microbiology

Dental caries does not occur in vivo if microorganisms in the form of dental plaque are absent. The different types of plaque and the factors involved in their development have been described in Chapter 3 and will not be repeated here. Over the years there has been debate about whether one or more specific bacteria are principally involved in the initiation of caries or if the disease is caused by a non-specific mixture of bacteria. At present, a number of different opinions exist, for example: (1) Strep. mutans is involved in the initiation of almost all carious lesions in enamel; (2) Strep. mutans is important, but not essential, in the aetiology of the disease; and (3) the association of Strep. mutans and caries is weak and no greater than for other bacteria in supragingival plaque. The evidence advanced to support or disprove these different opinions is incomplete and it is likely that all three may be correct in specific circumstances. Given the extreme variation found in the microbial composition of supragingival plaque collected from the same site in the same mouth with respect to time, it seems unlikely that the initiation and progression of all carious lesions are associated with identical or even similar plaques. However, there is evidence that overall some bacteria (Strep. mutans, Lactobacillus spp. and Actinomyces spp.) are more important than others.