Tooth decay. Coloured scanning electron micrograph (SEM) of a decaying tooth. Holes, or cavities, can be seen developing in places (centre). Tooth decay occurs when there is a build up of plaque in the mouth. Plaque is the sticky substance made of food remains and naturally-occurring bacteria. Bacteria feed on the sugars in the food particles and produce acid as a waste product, which corrodes the enamel coating of each tooth. Eventually a cavity is produced, which exposes the pain receptors in the tooth's sensitive interior. Cavities are prevented by brushing and treated by fillings. Magnification: x650 when printed 10 centimetres wide.