Premolar tooth extraction. Close up of the blood-filled cavity left after the extraction of a decayed premolar tooth from a 65-year-old man's lower jaw. Dental caries is the decay and crumbling of the teeth. It is caused by the bacteria in plaque, a substance which covers the surface of unclean teeth. The bacteria break down sugar in the mouth, producing an acid which progressively destroys the teeth. In severe cases such as this, root canal surgery or tooth extraction is needed. For the tooth prior to extraction, see image C029/2196.