What kind of micro flora and fauna you have in your mouth can mean the difference between a mouth full of cavities and a healthy, brilliant smile. The human mouth is full of bacteria, all the time, even after brushing, as is the human intestinal tract and the surface of the skin, which produces its very own bacteria to fight off invaders. Getting the right kind of bacteria is very important though, as some bacteria, like streptococcus mutans, is only good for dissolving enamel and has absolutely no other function. Other ones, like bifidus essensis may be good for your intestines and your mouth, while still others are good if they are present in small numbers, but must not proliferate too much, because they can start to do damage.