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Dental plaque is a biofilm, usually a pale yellow, that develops naturally on the teeth. Like any biofilm, dental plaque is formed by colonizing bacteria trying to attach themselves to the tooth's smooth surface. It has been speculated that plaque forms part of the defense systems of the host by helping to prevent colonization of microorganisms that may be pathogenic. The oral cavity contains the only known anatomical aspect of the human body that does not have a regulated system of shedding surfaces: the teeth. This allows microorganisms to adhere to the surface of teeth for long periods of time. These multiple species of bacteria become dental biofilm.

Dental biofilm, more commonly referred to as dental plaque, is composed of about a thousand species of bacteria that take part in the complex ecosystems of the mouth. The natural, non-frequent regulation of tooth shedding plays a large role in making dental biofilm the most diverse biofilm in the human body despite the relatively small size of the teeth.

The human oral cavity is also called the human oral microbiota. This is because the human oral cavity can contain several environments at a given moment that could vary from tooth to tooth. Additionally it has been estimated that 25,000 species of bacteria reside in the mouth. This is in contrast to the previously estimated 700+ species.

Studies have found that out of the 25,000 species that exist in the oral cavity, about 1,000 species can exist as part of the dental biofilm ecosystem. This is also in contrast to the previous estimate of more than 500 species as part of the dental biofilm. These 1,000 species have the ability to change their environment through a series of biotic relationships. At first, the biofilm is soft enough to come off by using the fingernail. However, it starts to harden within 48 hours, and in about 10 days the plaque becomes dental calculus (tartar), which is hard and difficult to remove. Dental plaque can give rise to dental caries (tooth decay)—the localised destruction of the tissues of the tooth by acid produced from the bacterial degradation of fermentable sugars—and periodontal problems such as gingivitis and chronic periodontitis.