Dental Plaque: Structure and Formation

 

Dental Plaque: Structure and Formation

The oral cavity comprises a diverse variety of microbial flora, of which bacteria are predominant and are housed as distinct microbial communities on mucosal surfaces as well as on teeth. The host and the microbial community maintain a harmonious relationship and there is a delicate balance maintained as far as the growth and proliferation of these micro-organisms are concerned. Salivary proteins help form a lubricating layer called the salivary pellicle on all surfaces in the oral cavity.

While this helps in lubrication of the mucosal surfaces, it also provides for binding sites that help in adherence of bacteria. Luckily, epithelial cells of the mucosal surfaces are continuously shed and this prevents an excessive accumulation or build-up of bacterial deposits.

The teeth, however, provide for non-shedding surfaces and promote accumulation of bacterial communities if oral hygiene is not well maintained. Micro-organisms, primarily bacteria, thrive and proliferate in soft deposits or biofilms formed on teeth. These soft deposits are called dental plaque. Dental plaque is a yellowish-white deposit or biofilm that comprises predominantly of bacterial communities in a matrix of exfoliated epithelial cells and salivary proteins. It attaches tenaciously to the surfaces of teeth as well as any prostheses or appliances in the oral cavity.

Dental plaque usually accumulates in the gingival thirds of the tooth surface. This is because formation of initial plaque in the coronal 2/3rds can be disrupted by movement of mucosal tissues over the teeth during mastication. Pits and fissures in teeth, areas around malaligned teeth and overhanging restorations may also favour its accumulation. Dental plaque according to its site of deposition can be classified as supra-gingival and sub-gingival plaque.

Sub-gingival plaque apart from being tooth associated can also be associated with the sulcular tissue.

While supra-gingival and tooth associated sub-gingival plaque could contribute to calculus formation and dental caries, tissue associated sub-gingival plaque could be detrimental to the periodontal tissues contributing to periodontitis. The mature plaque is a biofilm with diverse microbial colonies having at least 500 distinct microbial species.

Though predominantly bacterial, the microflora also harbours yeasts and viruses. These micro-organisms are housed in a matrix consisting of organic and inorganic materials derived from the saliva, bacteria and the gingival crevicular fluid. In fact, the plaque behaves like an organism by also having primitive circulatory channels within the mass of the plaque matrix.

These channels help in movement of nutrition and waste products in and out of the plaque mass. This provides for a micro-environment for the bacteria to thrive and proliferate. Organic and inorganic portion of the plaque matrix comprise around 20-30% of the plaque mass. The organic portion of the matrix comprises salivary glycoproteins, extracellular polysaccharides like glucans and fructans produced by bacteria as well as a little lipid material. Calcium and phosphorus from the bulk of the inorganic portion together with small amounts of sodium, potassium and fluoride.

 

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Plaque formation starts with the formation of a thin coating called the acquired or salivary pellicle on the surface of the tooth. Acquired pellicle is a coating of salivary proteins, predominantly glycoproteins and is formed within seconds to minutes after the tooth surface is cleansed. Apart from being derived from salivary proteins, it may also have components derived from the crevicular fluid. After a few hours of pellicle formation, bacteria start to colonize. The initial colonizers are Gram positive facultative anaerobic cocci and rods, usually Streptococcus species and Actinomyces species respectively.

The bacterial colonization is, initially, purely physical and is a reversible attachment. With time, the attachment becomes stronger and bacteria firmly adhere to the pellicle. This attachment is facilitated by binding of bacterial proteins called adhesins to proline rich proteins in the pellicle. The initial colonizers primarily derive nutrition from salivary glycoproteins and simple sugars and continue to divide and proliferate forming microbial colonies. Streptococcus species produce extra-cellular polysaccharides like glucans and fructans that help in both adhesion of more bacteria and providing an energy source for the bacterial colonies.

The bacteria continue to grow laterally to cover the tooth surface and then begin to grow away from the tooth as columnar microbial colonies. The initial colonizers being facultative anaerobes begin to deplete the oxygen in the micro-environment with their growth and pave the way for the growth of anaerobic bacteria. As the environment becomes more anaerobic, gram -ve rods and filaments begin to colonize at the outer surface of the plaque mass. These include Prevotella intermedia, Capnocytophaga species, Fusobacterium nucleatum and Porphyromonas gingivalis. These organisms adhere to the bacteria already on the plaque mass via receptors and interact with the primary colonizers, a process called co-aggregation.

A classic example of co-aggregation is the presence of “corn-cob” like structures. These corn-cob structured interactions comprise a central gram negative organism like F.nucleatum and outer bacterial cocci attached along the central rod bacteria. As the plaque mass accumulates supra-gingivally, it begins to cover the tooth surface and grow beyond the gingival margin.

This may lead to the gingival tissues getting inflamed causing a deepening of the gingival sulcus.

The environment in the gingival sulcus becomes anaerobic and nutrition for the bacteria is derived primarily from the periodontal tissues and blood. This leads to tissue breakdown, further deepening pockets and increasing plaque microflora. The sub-gingival plaque structurally resembles the supra-gingival plaque and may also harbour more spirochetes in addition to the same gram negative bacterial community present in the supra-gingival plaque…

 

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