Tooth decay. Scanning electron micrograph (SEM) of the surface of a decaying molar tooth. The surface enamel (white) has begun to break down (for example, at lower right). Tooth decay, or dental caries, is caused by bacteria in plaque, a substance that covers the surface of unclean teeth. The bacteria break down sugars in the mouth, releasing an acid that progressively destroys the teeth. This causes pain, discolouration and, eventually, tooth loss. Poor dental hygiene is the main cause of tooth decay. Magnification: x19 at 6.5x7.5 inch size.