Dental caries and plaque. Close-up showing dental caries and plaque formation (left) destroying the lower incisors of a female patient aged 67. 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. A filling may be used to repair a decayed tooth, but in severe cases, root canal surgery or tooth extraction may be required.