We have bacteria in our mouths, and well everywhere else. Researchers estimate that we have just as many, if not more bacterial cells in and on our bodies as human cells. One of the types of bacteria that are in our mouth is not so good for us because they cause tooth decay. The main culprit bacterium for cavities is S. Mutans, though there are others that contribute. This bad bacteria digests the foods that are leftover in your mouth, they particularly like carbohydrates. Then the produce acid as a byproduct. The acid dissolves the minerals in your teeth, leading to weakened teeth and eventually tooth decay.