What Is Dental Plaque? Dental plaque is an accumulation of bacteria in the mouth. It starts off as a colourless sticky film on the surface of the tooth, as it thickens, it changes its colour to yellow or brown. The mouth contains has over 500 different species of organisms that are present. In health, they all are in a delicate balance, but, when the environment changes, some organisms take this opportunity and flourish. Some of these organisms may cause diseases which may not only affect the mouth but the rest of the body.