Here's why:

* Aerobic bacteria require oxygen (O2) to survive and grow. They use oxygen as the final electron acceptor in their metabolic processes, which is how they generate energy.

* Anaerobic bacteria, on the other hand, can grow in the absence of oxygen. Some anaerobic bacteria are actually poisoned by oxygen.

So, when you hear about bacteria needing "air" to grow, it's generally referring to the oxygen present in the air.