Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Dental plaque is a biofilm (usually of a clear color) that builds up on the teeth. If not removed regularly, it can lead to dental cavities (caries) or periodontal problems (such as gingivitis).
The microorganisms that form the biofilm are almost entirely bacteria (mainly streptococcus mutans and anaerobes), with the composition varying by location in the mouth. Examples of such anaerobes include fusobacterium and Actinobacteria.
The microorganisms present in dental plaque are all naturally present in the oral cavity, and are normally harmless. However, failure to remove plaque by regular toothbrushing means that they are allowed to build up in a thick layer. Those microorganisms nearest the tooth surface convert to anaerobic respiration; it is in this state that they start to produce acids which consequently lead to demineralization of the adjacent tooth surface, and dental caries. Saliva is also unable to penetrate the build up of plaque and thus cannot act to neutralize the acid produced by the bacteria and remineralize the tooth surface.
Plaque build up can also become mineralized and form calculus.
Prevention and treatment
Frequency of brushing and flossing with good technique is important, because the nature (i.e. composition) of the microorganisms change as the plaque ages. Therefore, plaque which is 12 hours old for example is much less damaging than plaque which has not been removed in days.
Oral hygiene practices have evolved largely during the time they have been most needed, i.e. the 20th and 21st centuries. The sudden increase in tooth decay is almost certainly attributable to changes in diet, such as the introduction of refined sugar and, later, candy. Presumably, teeth used for the processing of a natural diet (raw meat, fresh vegetables) would last a full lifetime.
Mouthwash (also mouth rinse) is used for oral hygiene. Antiseptic and anti-plaque mouth rinse claims to kill the bacteria that cause plaque, gingivitis, and halitosis. Anti-cavity mouthwash contains fluoride, protecting against tooth decay.
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