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4 The Ecology of the Oral Cavity
The Biofilms of the Oral Environment
Oral Ecology and Dental Caries
The Influence of Fluorides on the Oral Ecosystem
Mucosal Immunity and Oral Ecology
Abstract
The oral cavity provides a variety of niche habitats for hundreds of species of oral bacteria. There are potential habitats on the teeth, tongue, or gingival sulcus and a constant supply of nutrients from food residues, saliva, and the products of other bacteria. The human host and its bacterial partners in the oral cavity and gut have a cooperative relationship which goes back millions of years. Cooperation between different bacterial species is just as important as competition in conserving this relationship; their interdependence holds together several different species in a consortium, a sort of mutual benefit community. If dental caries or periodontal disease occurs, it is always due to a disturbance in the dominance or hierarchy of bacteria within the consortium, often caused by something as simple as an increase of sugar in the diet, or at the other extreme, malnutrition. All the members of the consortium are well-established residents, so dental caries and periodontal disease are not the result of foreign pathogens but may be classified as noncommunicable diseases. The balance between the efforts of organisms to maintain growth on the oral surfaces, by both competitive and cooperative tactics, and the host’s factors which tend to support and tolerate certain commensals, but inhibit potential pathogens, is the theme of this chapter. Oral health strategies, which work best, are those based on an understanding of the principal factors which regulate and maintain a stable oral microbiome.
Keywords: oral environment, oral ecology, salivary pellicle, oral fluids, gingival fluid, dental plaque, calculus, dental caries, diet and caries, nutrition and oral health, fluorides, mucosal immunity, oral tolerance
4.1 Introduction
Ecology is the study of the relationship between living organisms and their environment. An ecosystem is a specific environment in which plant and animal species live in an interconnected web of cooperation and competition. For example, a forest is an ecosystem in which trees, bushes, and smaller plants interact with insects, soil bacteria, birds, small mammals, and reptiles, in fact, a large spectrum of most major classes of living organisms. Within the forest, each tree could be seen as a small ecosystem on its own, as it supports a characteristic collection of interactive bacteria and insects. The complexity and stability of an ecosystem take a long time to develop. If a temperate forest were to be cleared to the ground, it is estimated that it would take 200 years to regain its original rich and varied number of species.
The oral cavity is an ecosystem on a smaller scale, which also requires time to acquire a mature ecological balance of organisms. The time is much shorter than that needed for a forest to mature, as the life cycle of bacteria is faster than trees, and there are only 500 or so species or bacteria in the mouth. However, it still takes 2 weeks before a film of plaque bacteria has reached maturity. The organisms which have established themselves in an ecosystem are those which have been able to adapt to the physical environment and to the other species in the ecosystem.
Not all the members on an ecosystem are of equal importance to its survival. Some, called keystone species, have a decisive influence even when they may represent a small proportion of the total numbers of organisms in the ecosystem. Keystone species determine the composition of the ecosystem and control the relative numbers of each species. An example of a keystone species is the sea otter. On the Western coast of North America, the otter keeps down the population of sea urchins which graze on the base of kelp. Without the sea otter the urchins would devour the kelp and destroy the entire ecosystem of the kelp beds.
There are keystone species in communities of bacteria, occurring in the gut and oral cavity, which communicate with their microbial community, via chemical messages, a communication which microbiologists have called cross-talk or quorum sensing. The chemical messages induce control of gene expression in the target organism. Communication between bacterial species is also observed in the spread of antibiotic resistance. Genes from bacteria which confer resistance to antibiotics are able to be shared with other species of bacteria.
Stability is a feature of successful ecosystems, and it is dependent on maintaining a balance in the hierarchy of member species of the system. This balance is maintained partly by the stability of the physical environment such as the availability of light, oxygen, and nutrients, but it is also dependent on the population control of each species. This control is achieved through processes of competition for resources and cooperation between species in maintaining a balanced and stable population.
The relationship between different species is not always competitive. In fact, cooperative partnerships are more significant
