Stages of tooth decay, illustration. Dental caries is the decay and crumbling of the teeth. It is caused by bacteria that form a biofilm called plaque. The bacteria break down sugar in the mouth, producing an acid that progressively destroys the teeth. In the middle image, the decay is confined to the outer layer of the tooth, the enamel (white). In the image from second to right, it has reached the inner layer of dentin (orange). At upper centre it has penetrated the pulp (yellow) at the core of the tooth, which contains the nerves and blood vessels. Small areas of decay can be treated with fillings. When tooth decay reaches the sensitive inner area, it produces the intense pain of a toothache. Treatment at this stage may require root canal work or even extraction.