14.1 Dental Plaque

The oral environment

Our teeth are moistened with saliva produced by various salivary glands in the mouth. Healthy saliva has a neutral pH and and electrolyte composition that resembles that of plasma with an increased abundance of phosphate and a few other ions. It also contains larger molecules that form mucus, enzymes, antimicrobials and other substances. Among those, some glycoproteins can bind to the exposed surface of enamel. They form a protective film, called dental pellicle, attached to the crown of each tooth. The pellicle is removed with tooth brushing but it forms again moments after the clean tooth makes contact with saliva.

Human saliva is also populated by microorganisms, including dozens of species of bacteria, fungi and archea. Most of them ferment dietary sugars and produce lactic acid. They can bind to the glycoproteins of the pellicle and remain attached to the tooth.

Plaque formation

When microorganisms grow attached to a surface, they may also secrete a sticky extracellular matrix that further stabilizes them in place. This forms a biofilm, a thin layer of microorganisms + extracellular matrix. Dental plaque is biofilm that forms on the surfaces of the crown, along the gumline, or within the gingival sulci.

The bulk of the microorganisms that form the biofilm are Strepcoccus mutans, Fusobacterium and Actinomyces. Streptococcus and other anaerobes are the initial colonisers of the pellicle and play a major role in the establishment of the early biofilm community. As more microbes attach and secrete matrix, the plaque becomes thicker. The substances secreted by them gradually alter the chemical environment within the plaque and this favors the establishment of new types of microorganisms, called the later colonizers.

The bacterial equilibrium position varies at different stages of formation. Below is a summary of the bacteria that may be present during the phases of plaque maturation:

• Early biofilm: primarily Gram-positive cocci

• Older biofilm (3–4 days): increased numbers of filaments and fusiforms

• 4–9 days undisturbed: more complex flora with rods, filamentous forms

• 7–14 days: Vibrio species, spirochetes, more Gram-negative organisms
  
  

Early colonizers

Most genera listed below include multiple species that form plaque.

• Streptococcus (60-90% of bacteria in plaque)

• Eikenella

• Haemophilus

• Prevotella

• Priopionibacterium

• Capnocytophaga

Late colonizers

• Actinomyces

• Prevotella

• Eubacterium

• Treponema

• Porphyromonas

Fusobacterium is another very common group of colonizers that appears between the early and late colonisers, linking them together.

Environment

The warm and moist environment of the mouth and the presence of teeth, makes a good environment for growth and development of dental plaque. Teeth make great settlement surfaces for the biofilm. Unlike other parts of the body, tooth surfaces are firm and non shedding. The main ecological factors that contribute to plaque formation are pH, saliva, temperature and availability of oxygen. The normal pH of saliva is neutral and plaque flourishes at pH 6.7 to 8.3. Variation in pH within the mouth favors the development of specific types of microorganisms in plaque. Saliva acts as a buffer maintaining the pH in the mouth between 6 and 7. The normal temperature of the mouth ranges between 35 °C and 36 °C, and a two-degree change has been shown to drastically shift the dominant species in the plaque. Oxidizing reactions are carried out by aerobic bacteria. Their metabolism influences the oxygen levels in plaque and affects the composition of its microbial community.

The microorganisms in plaque are normally unable to harm the teeth. They attach to the pellicle and grow at a moderate speed forming a thin plaque. The lactic acid that they secrete diffuses easily into the saliva where it is buffered and it does not affect the enamel which is protected by the pellicle. Most of the cover of pellicle and plaque is removed daily at each event of tooth brushing and flossing. Plaque is therefore present most of the time, but is it prevented from growing thick.

Enamel demineralization and remineralization

The hydroxyapatite crystals Ca5(PO4)3(OH) in enamel are influenced by the pH at the pellicle and the concentrations of of Ca++ and PO4--- in saliva. Low pH stimulates dissolution of the hydroxyapatite crystals which leads to demineralization of the enamel. At high pH, calcium, phosphate and hydroxyl groups from saliva crystalize into hydroxyapatite, remineralizing the enamel.

The teeth can be protected further by exposure to fluoride in saliva. Fluoride forms fluorapatite which is incorporated into the dental enamel. Fluoride reduces the rate of tooth enamel demineralization and increases the rate of remineralization of teeth.

This dynamic balance between demineralization and remineralization of enamel allow teeth to recover from minor acidic attacks by plaque microorganisms. Failure to remove plaque by regular tooth-brushing, however, allows plaque to proliferate unchecked. With the build up in a thick layer, plaque can become difficult to remove and it can harm the tooth and the gingiva, causing various dental diseases.

Plaque build-up and consequences

The first kind of plaque to form after the teeth are brush is supragingival biofilm, which is a dental plaque that forms above the gingiva. It is most common between the teeth, in the pits and grooves of the teeth and along the gums. It is rich in aerobic bacteria but if allowed to grow thick, anaerobic bacteria may dominate its composition.

Plaque located under the gums is called a subgingival biofilm. It follows the formation of the supragingival biofilm which grows along the dentogingival attachment line until it manages to bypass it. The subgingival biofilm is mostly made of anaerobic bacteria. As this plaque attaches in a pocket under the gums, it is less exposed to oxygen.

Gingivitis is the most common result of plaque build-up around the gingival tissues (see chapter 14 Gingiva). The bacteria found in the biofilm secrete acids and proteolytic enzymes that cause a localized inflammation of the tissue. This is characterized by a red, puffy appearance of the gums and bleeding during brushing or flossing. Gingivitis due to plaque can be resolved by removal of the plaque. If left for an extended period of time, however, the inflammation may begin to affect the periodontal ligament, in a progression referred to as periodontitis.

In periodontitis, bacteria in plaque release enzymes that attack not only the collagen but also the bone. At the same time, osteoclasts in the bone erode it to prevent further infection. This can be treated with brushing and cleaning in between the teeth as well as with surgical debridement (removal of decayed tissue).

Caries is an infectious disease caused primarily by Streptococcus mutans, characterized by acid demineralization of the enamel and deeper dental tissue. When the plaque becomes very thick, saliva is unable to penetrate and cannot neutralize the acid produced by the bacteria. These acids then lead to demineralization of the tooth surface.

Detection of plaque build-up

There are two main methods of detecting dental plaque in the oral cavity: visually or through the application of a disclosing gel or tablet.

Dental plaque may present as a yellow, tan or brown stain on the tooth. It can be detected thorough contact as a rough surface and it collects in the instrument if the tooth is scraped. The most common areas where plaque is found is between the teeth and along the cervical margins.

Disclosing agents are applied to the teeth and stain the tooth with one or multiple colors to indicate the amount, location and age of plaque build-up. Clean surfaces of the teeth do not retain the disclosant after the mouth is rinsed, but surfaces covered in plaque do. Plaque disclosing agents are available for use at home or in the dental clinic. These gels provide a visual aid in the management of dental plaque.

Calculus (tartar)

Calculus is a form of hardened dental plaque. It is caused by precipitation of minerals from saliva in plaque on the teeth. This process of precipitation kills most bacteria within dental plaque, but the rough and hardened surface that is formed provides an ideal surface for further plaque formation. Calculus buildup compromises the health of the gingiva (gums) and teeth.

Heavy staining and calculus deposits are commonly exhibited on the lingual surface of the mandibular anterior teeth, along the gumline.

Like plaque, calculus can be supragingival or subgingival. It should be removed mechanically but brushing and flossing are not effective because it is more firmly attached to the tooth than plaque. Calculus buildup has to be removed at the clinic with dental hand instruments or with an ultrasonic scaler.

Calculus is composed of about 50% inorganic (minerals) and 50% organic (cellular and extracellular matrix) components. The mineral part consists of various forms of calcium phosphate crystals. The organic component is approximately 85% microorganisms and 15% extracellular matrix. The cells are primarily bacterial, but also include at least one species of archaea (Methanobrevibacter oralis) and several species of yeast (e.g., Candida albicans). The organic extracellular matrix in calculus consists primarily of proteins and lipids (fatty acids, triglycerides, glycolipids, and phospholipids).

Plaque and calculus in animals

Calculus formation in animals is less well studied than in humans, but it is known to form in a wide range of species. Domestic pets, such as dogs and cats, frequently accumulate large calculus deposits. Grazing animals rarely form thick deposits and instead tend to form thin calculus deposits that often have a metallic or opalescent sheen. In some animals, calculus can be confused with crown cementum. The entire tooth is covered by cementum which is gradually worn away through abrasion, but remnants of it may be visible in the cervical region of the tooth.

Summary

Saliva contains glycoproteins that form a protective pellicle on the crown of each tooth. Microorganisms in saliva bind the the pellicle and form a biofilm called plaque. Most of them ferment carbohydrates in the mouth and form lactic acid. Which can demineralize the enamel. They also secrete proteolytic enzymes that cause periodontal disease. The pellicle with plaque is mechanically removed during brushing and flossing. If proper cleaning is not done, multiple layers of plaque accumulate and make it more difficult to remove. It later mineralizes forming calculus which has to be removed with scalers at the clinic.

Key terms

Pellicle, dental plaque, biofilm, Streptococcus, Fusobacterium, Actinomyces, Porphyromonas, early colonizers, late colonizers, demineralization, remineralization, supragingival, subgingival, caries, debridement, saliva, calculus , tartar, scaler, ultrasonic scaler, calcium phosphate, hydroxyapatite, Methanobrevibacter, Candida.

Figure credits

Figure 1 by Alsheik4 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=53394913

Figure 2 by Onetimeuseaccount - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=27756041

Figure 3 by Jost Jahn - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=56033606

Figure 4 by Themolarbear - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39718328

Figure 5 by Michael Ottenbruch - Dentist Thornhill, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=520959

Figure 6 by ANUG - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=38142083