I don't know if there ever was a study that separated out different bacteria species and examined their roles. Many of the bacteria in the mouth are helpful bacteria. They help us digest our food and prevent bad bacteria from getting a foothold in the mouth. However, it's a delicate balance. If you change the balance (e.g., by changing the chemistry in your mouth) and allow one particular group to multiply quickly, you'll get into trouble. Also, their waste would form plaque on your teeth and it's really bad for the teeth. It takes only a few hours for plaque to form. Gingivitis is often caused by plaque.