Decayed tooth structure. An extracted molar tooth cut in half to show the extent of decay (brown). Tooth decay, or dental caries, 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 this case the decay has destroyed part of the hard enamel and dentine of the crown. It has reached the pulp cavity, which extends down into roots and normally contains blood vessels and nerves.