Tooth decay. Coloured scanning electron micrograph (SEM) of a cavity (lower centre) in a human incisor. Cavities are caused by dental plaque (brown), a film of bacteria embedded in a glycoprotein matrix. The matrix is formed from bacterial secretions and saliva. The bacteria feed on sugars in food, producing acid as a waste product. This acid corrodes the teeth's enamel coating, resulting in cavities.