Dental plaque is a mixed microbial biofilm growing on teeth and is the prime aetiological agent of the two main oral diseases, dental caries and periodontal disease. The microbial composition of plaque varies between individuals and the location on the tooth and generally reflects the complex nature of the ecology of the mouth.

In common with other biofilms, the microbial composition of dental plaque is capable of change in response to changes in the environment, notably the diet. These responses are modulated by homeostatic mechanisms inherent in the plaque in ways which are, as yet, poorly understood.

The major sites of plaque accumulation are in the fissures of molar teeth, in the area bounded by the margin of the gum and the tooth and between adjacent teeth. In addition, plaque can cause gingival inflammation which may result in loss of epithelial attachment to the tooth leading to the formation of sub-gingival pockets. These pockets may also harbour dental plaque which is significantly different from supra-gingival plaque in a number of important respects in particular a much lower redox potential which selects for a variety of anaerobic bacterial species.

Although dental plaque varies considerably in composition, it has been possible to piece together a sequence of events which lead to its establishment. The consensus view of plaque development begins with a clean tooth surface covered by a conditioning film of salivary proteins and glycoproteins, called the tooth pellicle, being colonised by so-called "pioneer species". These multiply forming, first a monolayer and, subsequently, palisades of cells perpendicular to the tooth surface.

During and after this outgrowth period secondary colonisation by a variety of Gram positive and negative species occurs leading to a large increase in the species diversity. Foremost among the events contributing to this secondary colonisation is the process known as co-aggregation whereby colonising microbes attach to cells already part of the developing biofilm. This allows species which can not attach, or can attach only poorly, to the tooth pellicle to participate in the biofilm. At 24 hours the maturing dental plaque contains a wide variety of bacteria and it is possible to detect easily identifiable inter-species associations such as the well documented "corn-cob-configurations", although a wide variety of other inter-species associations will be present.

Further colonisation and growth of established bacteria takes place as the plaque matures to form a stable, climax, community. This pattern of development leading to a climax community has been termed "bacterial succession". The resulting community consists of individual microbes and microcolonies acting in complex consortia which can convey a range of beneficial properties. These include feeding synergies, improved antibiotic resistance and a host of cooperative mechanisms which are the subject of much current research.

Probably the single most important phenomenon in the development of a biofilm is the process of microbial adhesion either to the substratum or to other adhering cells be they of the same or another species. Microbial adhesion is now recognised as a crucial step in the pathogenicity of a wide range of bacterial diseases and this has generated enormous interest into the mechanisms by which it is achieved.

In the case of some mono-specific diseases such as those caused by Salmonellae, enteropathogenic Escherichia coli or Neisseria gonorrhoeae it has been possible to piece together a highly detailed picture of the process. In situations where diseases is associated with biofims, whether mono-specific eg infective endocarditis, or multi-specific eg dental caries, the process is very much more complex and we are less certain of the details.

The adhesion of a microbe to a surface, including another microbe, appears to be governed by two major forces. The first is shear force which tends to sweep bacteria away from or off a surface. The second is electrostatic repulsion which works in combination with van de Waal's forces to provide areas of weak attraction which are thought to be important in the overall adhesion process. The current view of the adhesion process is that it takes place in two distinct phases.

In Phase 1 the microbe is held, for a brief period, by a weakly attractive force some 10nm from the surface . Shear force or Brownian Motion may disrupt this initial attraction. On the other hand, a number of specific adhesion mechanisms may hold the cell close to the surface for a significant time period. These specific interactions may be a combination of lectin-like , electrostatic and hydrophobic interactions which in some instances may involve delicate structures called fibrils or fimbriae which project from the cell surface.

In Phase 2, the adhesion is rendered essentially irreversible by the synthesis of extracellular polymers. Ultimately these polymers form a significant part of the biofilm matrix. In the case of dental plaque, these polymers comprise soluble and insoluble high molecular weight polysaccharides synthesised from sucrose by extracellular glucosyl- and fructosyltransferases. In view of the established association between sucrose and dental caries, it is not surprising that these polymers have been the subject of a considerable amount of research.

A variety of lectin-like interactions have been demonstrated in vitro and they have been implicated in a range of co-aggregation events thought to be connected to secondary colonisation. While we have detailed information regarding the possibility of such interactions contributing to plaque development there is, as yet, little evidence that such interactions are important in vivo. Similarly, there is a body of research implicating the involvement of hydrophobic and electrostatic interactions but again evidence of in vivo significance is lacking. So, although we might be able to draw diagrams of complex interactions ocurring between microbes within dental plaque we do not know for certain that this is in any way an accurate picture of what is actually happening.

The situation with extracellular polymers appears to be clearer because there is reasonable evidence that, in the case of those comprising glucose, they are closely involved with the cohesiveness of plaque and that they play a role in vivo in bacterial adhesion. Furthermore, there is considerable evidence using electron microscopy that the polymers are present in vivo.

The species most able to colonise a cleaned tooth surface are:

1. Streptococcus oralis
2. Streptococcus mitis
3. Streptococcus sanguis

These species have developed mechanisms which enable them to adhere to a tooth by exploiting the composition of dental pellicle. This thin film is so firmly attached to all tooth surfaces that it resists normal toothbrushing and requires professional prophylaxis for its removal.

After a tooth surface has been thoroughly cleaned, pioneer species quickly attach. Within 2 hours, apart from the streptococci above, various species of Actinomyces and Neisseria are also commonly isolated but the most common organisms by far are the streptococci which comprise upwards of 90% of the cultivable flora in the first few hours.

Once attached, pioneer species multiply. The forming microcolony spreads first in the plane of the surface and then, as space becomes limited, upwards creating palisades of cells.

The colonies of pioneer species now act as the substrate for further colonisation by the process of aggregation between different species known as co-aggregation. A large amount of research has implicated lectin-like interactions in these events but other types of interaction such as hydrophobic and electrostatic have also been shown to be involved. The relative importance of these interactions to real-life plaque is not yet known. However, the fact that some very specific co-aggregations between certain bacterial species are commonly found eg corn-cob-configurations, is good evidence that a high degree of specificity is a feature of secondary colonisation.

Remember also that during this process cell multiplication is taking place causing the biofilm to develop.

As the biofilm spreads outwards and upwards it will experience an increased shear force as saliva is moved over its surface by tongue movements. Some organisms will be removed and join the planktonic population in saliva which will either colonise fresh sites or be lost by swallowing.

Notwithstanding the large force exerted by saliva movements and mastication, plaque of considerable depth can form illustrating the strength of the cohesive forces within plaque binding the bacteria together.

At some point during development the rate of change in the overall composition of plaque becomes slow or even stops. The plaque has acquired a degree of stability and is described as a climax community. It is, however, not possible to define precisely when this happens but it does take several days.

Environmental pressures on the system favour the setting up of a wide variety of complex interactions between species comprising this climax community. Examples of these are feeding synergies where metabolic by-products of one species are used as a food source by another thereby setting up a web of food utilisation, sometimes called a food chain. Another example would be the cooperative breakdown of oligosaccharides with different organisms providing specialist glycosidases ensuring complete utilisation of the available carbohydrate.

The physical properties of plaque, the metabolic action of the organisms and the nature of the surrounding environment set up a number of gradients in plaque which influence the presence, location and activity of many of the species found in the biofilm. Examples include pH, nutrients, redox potential, metabolic by-products and oxygen.

Although mature plaque has a degree of stability conferred by inter-species cooperativity, physical and metabolic associations and its actual physical density it does respond to environmental change albeit slowly. The best recorded response is that due to changes in diet.

Plaque formed on the teeth of individuals with a low carbohydrate, high protein diet, contains fewer acidogenic-aciduric organisms. The pH gradient will be different and the overall pH of the plaque alkaline because of the ammonia produced as a by-product of amino acid breakdown. The higher pH of the plaque will itself inhibit acidogenesis and favour Gram negative organisms which will be present in greater numbers. The proteolytic nature of the plaque will result in the presence of particular peptides such as putrescene and cadaverine which have a characteristic offensive odour.

If this same individual, previously on a high protein, low carbohydrate diet switched to a low protein, high carbohydrate diet the formed plaque would slowly adjust its microbiological composition. The resting pH of the plaque would reduce to somewhere between pH 6.3 to 6.8 (figures are approximate) as a result of the production of organic acid by-products from the fermentation of carbohydrate. This lower, more acid, pH favours aciduric organisms such as streptococci and lactobacilli and the proportion of these would greatly increase. This would be coupled with a reduction in the numbers of Gram negative anaerobic rods which do not flourish under these conditions.

If the change in diet included an increase in sucrose consumption then the plaque matrix would contain large amounts of extracellular polysaccharides of both the fructan and glucan variety.

If the diet included frequent intake of carbohydrate eg snacking on confectionary then the plaque would contain significantly increased numbers of highly aciduric organisms such as Streptococcus mutans and lactobacilli.

These changes would occur over a time period of a few days even if the plaque had not been removed from the teeth when the diet changed. It follows that individuals on such extremes of diet produce such characteristic plaque even though they practice normal oral hygiene.

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