How could you explain the mechanism responsible for the formation of dental caries by resident microorganisms?

Dynamic Process. When sugars or carbohydrates are consumed, bacteria have the ability to produce acid which leads to DE-mineralization and tooth decay.
Demineralization. Acids produced by resident bacteria demineralize the enamel matrix, causing decay of the tooth. http://en.wikipedia.org/wiki/Dental_caries.
Bacteria. Literally have thousands of different types of bacteria living in our mouths, most of which help us digest our food. The bacteria will layer in specific order on and around the teeth. Food debris, in particular sugars/starches/acids, will gradually be converted into hydrochloric acid which will etch calcium out of the enamel, allowing bacteria to penetrate the tooth, destroying tooth material.