Focal demineralization of a tooth is known as tooth decay. The main cause of tooth decay is presence of acid on the surface of teeth. The acid which decays the teeth is produced by bacteria living in plaque. Most common bacteria which can cause tooth decay are streptococcus mutans. Bacteria living in our oral cavity eat the same products as we eat. They live as parasites in our mouth and produce lactic acid as excretory material. Most commonly bacteria consume sugar and starches. If you will eat less sugar as a result less acid will be produced in the mouth.