Plaque on tooth. Coloured scanning electron micrograph (SEM) showing a build up of plaque (multicolored, right) on a molar tooth (grey, left). Plaque is the main cause of tooth decay. It is a sticky substance made of food remains and the bacteria that naturally occur in the mouth (such a Streptococci). Bacteria feed on the sugars in the food particles and produce acid as a waste product. This acid corrodes the enamel coating of each tooth, forming cavities and exposing the pain receptors in the tooth's sensitive interior. Magnification: x11,000, when printed 15 centimetres wide.