Dental caries and plaque. An angled mirror is employed to show dental caries and the formation of plaque and tartar (brown) behind the lower incisors of a 67-year-old female patient. 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. Plaque hardens into yellow or brown tartar. A filling may be used to repair a decayed tooth, but in severe cases, root canal surgery or tooth extraction may be required.