Answer: The bacteria uses the oxygen present in the mouth.
Step-by-step explanation:
The plaque that is formed in the teeth is a highly anaerobic bacteria but it can sustain in the environment rich in oxygen. The oxygen is used by the bacteria and there is a absence of oxygen in the teeth.
It is formed in between the teeth which needs oxygen to survive and if remains for tooth for a longer period of time then it began to grow without oxygen.
It basically uses the oxygen available in the mouth and makes the condition anaerobic.