Patient-friendly definition from the DentistsDirectory.org dental glossary.
Dental plaque is the soft, sticky film of bacteria — technically called a biofilm — that forms continuously on all tooth surfaces and the gum line. It is not simply a collection of bacteria floating loosely in the mouth; it is an organised community of hundreds of bacterial species embedded in a self-produced matrix of proteins and polysaccharides that helps the bacteria adhere to surfaces, communicate with each other, and resist removal.
Plaque bacteria cause the two most common dental diseases through different mechanisms. Cavity-causing bacteria (primarily Streptococcus mutans) ferment dietary sugars and produce organic acids as a byproduct — these acids dissolve tooth enamel in the demineralisation process that leads to cavities. Periodontal disease is caused by a different set of bacteria, predominantly gram-negative anaerobes that colonise the gum margin and subgingival spaces. Their metabolic byproducts trigger chronic inflammation in the gum tissue, eventually destroying the bone and ligaments that support the teeth.
Plaque is soft and can be removed by thorough brushing and flossing — but only if the mechanical action reaches all tooth surfaces. The areas where plaque consistently accumulates and causes disease are the same areas where brushing and flossing are most often inadequate: the gum line, the spaces between teeth, and the grooves on chewing surfaces. Brushing alone cleans only about 60% of tooth surfaces; interdental cleaning with floss, picks, or brushes is necessary to address the spaces between teeth. If plaque is not removed within 24–72 hours, minerals in saliva begin to calcify it into tartar (calculus), which requires professional removal.