Tooth plaque (Image 1 of 4). Coloured scanning electron micrograph (SEM) of plaque on the surface of a tooth. Plaque, the main cause of tooth decay, appears as a yellow coating over the grey enamel of the tooth. Plaque 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: x70 at 6x6cm size. (See photo's M782/119, M782/120 and M782/123).