Stages of tooth decay, cutaway illustration. Dental caries is the decay and crumbling of the teeth. This is caused by bacteria that form a biofilm called plaque. The bacteria break down sugar in the mouth, producing an acid which progressively destroys the teeth. From left to right (across top), the holes formed are shown progressively penetrating the layers of the tooth. The outer layer of a tooth is enamel, with an inner layer of dentin. The core of a tooth consists of pulp (orange) with 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 tooth extraction.