Anaerobic activity, you won't like it...

As much as the makers of Listerine would like you to think otherwise, mouth bacteria are useful. In fact killing them will land you in Thrush related mouth bother. You don't want that. HOWEVER, that is just an aside, if you starve mouth bacteria of Oxygen they are forced into anaerobic mode (anaerobic=without Oxygen). This is when the misery starts... Anaerobic activity in some mouth bacteria produces lactic acid. This is the cause of tooth decay.  Plaque is very efficient at keep out Oxygen, so the problems being in areas where plaque builds up easily. Not on the brushable surfaces, but between the teeth. Does this mean that you don't have to brush? Well, no, but it highlights the importance of flossing.