PLAQUE

Dental plaque is a biofilm that forms naturally on the tooth surface. It consists of a diverse microbial community embedded in a polymer matrix of bacterial and salivary origin. Because environmental conditions vary from place to place within the oral cavity, each tooth site with plaque represents its own distinct ecosystem, and the dominant microbial composition at each site depends on the outcome of numerous host-microbe and microbemicrobe interactions. Initial bacterial colonizers quickly become established on a clean tooth surface, and a pattern of subsequent bacterial succession has been identified. If left undisturbed, plaque reaches a maximum bulk after about seven days. Plaque deposition begins supragingivally—on the visible part of the tooth above the gum line—and if left undisturbed can progress subgingivally—into the crevice between the gum and the tooth. Microbial interactions usually keep the bacterial composition of plaque fairly stable, but when this homeostasis breaks down, the shifts in microbial balance can trigger the initiation of dental caries (tooth decay) or gingivitis (gum inflammation). Few bacteria can be isolated from around healthy gum tissue, although with gingivitis there is a considerable increase in the number and complexity of bacteria as the lesion develops. Subgingival plaque, if left undisturbed, can become a calcified matrix (calculus, or tartar) that can harbor harmful bacteria. Not surprisingly, subgingival calculus is closely associated with periodontal diseases. Specifically, Porphyromonas gingivalis and Bacteroides forsythus in subgingival plaque had been associated with both periodontal inflammation and bone loss.

Although dental plaque is commonly depicted in commercial advertising as the cause of both caries and periodontitis, dental plaque also benefits the host by helping to prevent intra-oral colonization by exogenous species. Plaque is also a repository for fluoride and other minerals that serve both to inhibit the demineralization of dental enamel—the first step in the development of a cavity—and to promote remineralization of early lesions. The fact that caries is initiated by a drop in plaque pH, following the fermentation of simple carbohydrates by certain bacteria resident in plaque, further exemplifies plaque's delicate ecology. Controlled plaque is beneficial, uncontrolled plaque can be harmful.

Prevention of plaque-related disease is geared toward plaque control rather than eradication. The goal in preventing periodontitis (diseases of the supporting structures of the tooth) is to prevent fresh plaque from becoming established enough to permit the growth of pathogenic bacteria. This goal is best achieved by thorough toothbrushing at least once per day with a fluoridecontaining toothpaste, plus consistent professional prophylactic care. So long as plaque remains supragingival, it can be controlled by mechanical or chemotherapeutic means. Once plaque becomes established subgingivally, however, an individual cannot remove it and professional intervention is necessary. Carrying out personal oral hygiene with a fluoride-containing toothpaste helps maintain high fluoride levels in plaque and thus inhibits the development of caries.

Brian A. Burt

(see also: Caries Prevention; Gingivitis; Oral Health; Primary Prevention )

Bibliography

Zambon, J. J. (1997). "Principles of Evaluation of the Diagnostic Value of Subgingival Bacteria." Annals of Periodontology 2:138–148.

Plaque

Plaque is the diverse community of microorganisms , mainly bacteria , which develops naturally on the surface of teeth. The microbes are cocooned in a network of sugary polymers produced by the bacteria, and by host products, such as saliva, epithelial and other host cells, and inorganic compounds such as calcium. The surface-adherent, enmeshed community of plaque represents a biofilm.

Plaque is important for two reasons, one beneficial and the other detrimental. The beneficial aspect of dental plaque is that the coverage of the tooth surface by microbes that are normally resident in the host can exclude the colonization of the tooth by extraneous bacteria that might be harmful. This phenomenon is known as competitive exclusion. However, despite this benefit, the plaque can position acid-producing bacteria near the tooth and protect those bacteria from attempts to kill or remove them. Plaque can become extremely hard, as the constituent inorganic components create a crystalline barrier. Protected inside the plaque, the acid-producing bacteria can dissolve the tooth enamel, which can lead to the production of a cavity.

A plaque is a complex community, consisting of hundreds of species of bacteria. Plaque formation generally begins with the adherence of certain bacteria, such as Streptococcus sanguis, Streptococcus mutans, and Actinomyces viscosus. Then, so-called secondary colonizers become established. Examples include Fusobacterium nucleatum and Prevotella intermedia. As the plaque matures, a varied variety of other bacteria can colonize the tooth surface.

Maturation of the plaque is associated with a shift in the type of bacteria that are predominant. Gram-positive bacteria that can exist in the presence or absence of oxygen give way to gram negative bacteria that require the absence of oxygen.

Depending on how the community evolves, the plaque can become problematic in terms of a cavity. Even within the plaque, there are variations in the structure and bacterial composition. Thus, even though one region of the plaque is relatively benign is no guarantee that another region will house detrimental bacteria.

The prevalence of acid-producing bacteria is related to the diet. A diet that is elevated in the types of sugar typically found in colas and candy bars will lower the pH in the plaque. The lowered pH is harsh on all organisms except the acid-producing bacteria. Most dentists assert that a diet that contains less of these sugars, combined with good oral hygiene , will greatly minimize the threat posed by plaque and will emphasize the benefit of the plaque's presence.

See also Bacteria and bacterial infection; Biofilm formation and dynamic behavior; Microbial flora of the oral cavity, dental caries

plaque / plak/ • n. 1. an ornamental tablet, typically of metal, porcelain, or wood, that is fixed to a wall or other surface in commemoration of a person or event.2. a sticky deposit on teeth in which bacteria proliferate.3. Med. a small, distinct, typically raised patch or region resulting from local damage or deposition of material, such as a fatty deposit on an artery wall in atherosclerosis or a site of localized damage of brain tissue in Alzheimer's disease. ∎ Microbiology a clear area in a cell culture caused by the inhibition of growth or destruction of cells by an agent such as a virus.

plaque In bacteriology, a round, clear area forming on an otherwise opaque culture plate of bacteria as a result of their lysis (see LYTIC RESPONSE) by a bacteriophage (virulent virus) or other agent. Temperate phages produce turbid plaques because of the growth of lysogenized bacteria on the floor of the plaque.

plaque
1. A thin layer of organic material covering all or part of the exposed surface of a tooth. It contains dissolved food (mostly sugar) and bacteria. The bacteria in plaque metabolize the sugar and produce acid, which eats into the surface of the enamel of the tooth and eventually causes tooth decay (dental caries).

2. A clear area in a bacterial culture grown on an agar plate due to lysis of the bacteria by a bacteriophage.