Dental caries. Close up of dental caries in a lower molar tooth. The cavities are filled with remnants of food and discoloured, decayed tooth material, and parts of the tooth have broken off. 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.