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 tooth brushing 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 (tartar).
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.
