Dental plaque is a firmly adherent microbial biofilm attached to teeth. It is the prime aetiological agent of dental caries and periodontal disease. Careful observation of the distribution of plaque in the mouth reveals that it forms most readily only at certain sites, viz, molar fissures, the gingival crevice and on those surfaces between adjacent teeth (approximally).

Sticking to a surface has very real benefits for many microorganisms, they maintain their position in the environment, they can exploit the organic and inorganic molecules which adsorb to surfaces and they can interact with other bacteria in cooperative consortia to maximise their potential.

The mouth contains two main types of surface, soft and hard. The soft surfaces are the soft epithelial tissue lining the inside of the mouth such as the cheeks, gums and tongue. The hard surfaces are the teeth. Both provide a variety of quite different sites which are exploited by microbes.

The major forces acting against microbial adhesion at all sites are the same

1. shear force

2. electrostatic repulsion

Soft tissue has the added advantage in that surface layers exfoliate carrying with them adhering bacteria. This is the main reason why biofilms fail to develop on epithelial surfaces in the mouth even though cells can have 100-200 adhering bacteria .

Microbes free in saliva are carried around the mouth by saliva flow. Adhesion occurs when the cell comes into contact with a surface. The current view is that adhesion is a two-stage event. Initially a cell is held a short distance, approximately 10nm, from the surface by a combination of electrostatic and van der Waal's forces. In the second stage this initial, weak attraction, is firmed-up, by multiple adhesion mechanisms which may include synthesis of polymer.

The flow of saliva over a surface creates a shear force which tends to remove the microbe from the vicinity of the surface or detach those which are being weakly held. Saliva flows at relatively high velocities, however, near surfaces the flow is reduced by viscosity which increases drag. This phenomenon is widely recognised in fluid flowing in tubes where velocities are greatest at the centre and rapidly reduce at very small distances from the sides of the tube.

At microscopic distances the velocity of even quite rapidly flowing fluids can be reduced to very low values.

The relatively high viscosity of some saliva, notably that produced by sublingual/submaxillary glands will reduce flow velocity near to tooth surfaces which will create a greater chance for microbial adhesion than if the drag did not exist. On the other hand the high viscosity will increase drag on the microbial cell and increase the shear force.

The increased velocity and thus increased shear will inhibit accumulations of cells. As plaque builds up it experiences greater shear which inhibits development.

However, even so-called smooth surfaces of teeth contain imperfections, either areas of erosion or areas where prisms meet the surface perpendicularly. These may provide some shelter from shear force but are not usually sufficient to allow for significant plaque development.

Shear force exerts a major influence on plaque development. It effectively limits it to sites in the mouth which are sheltered from the flow of saliva.

Although bacteria can be isolated from all dental surfaces, significant amounts of plaque only accumulate at three sites which experience relatively little or no shear force.

These sites are:

Given time, gum-margin plaque can develop to the point where it begins to overgrow the smooth areas of teeth in the direction of the crown. This is often referred to as "Smooth surface plaque" but this is not strictly correct. The ability of plaque to grow over smooth areas of the tooth speaks volumes for the tenacity of its adhesive and cohesive properties.

The distribution pattern of dental caries closely follows that of plaque. Thus, the sites in the mouth which are most prone to caries are those where plaque accumulates.

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