Bacteria such a S mutans in the mouth start the decay process. The bacteria along with the sugars in the food form a biofilm called plaque on the teeth. The bacteria remain protected in the biofilm. The acids produced by these bacteria remain concentrated at a particular point and dissolve the enamel. Once the enamel is dissolved the softer dentin is destroyed rapidly. If the tooth is not treated with a filling at this point, the bacteria reach the innermost layer of the tooth called the pulp. Infection of the pulp with the bacteria causes pain.