Nitrifying bacteria aka the good or beneficial bacteria are present after successfully cycling a new tank. Nitrifying bacteria provide natural biological aquarium filtration and are responsible for breaking down organic waste within the fish tank.
Transforming ammonia to nitrite, and nitrite to nitrates. Nitrifying bacteria settle on rocks, gravel, filter media, sand, bio-wheels, etc.
Nitrifying bacteria (nitrifiers) consist of two bacteria species. Both separate, but dependent. While the first strain settles as soon as ammonia is present, the second one settles as soon as nitrites are present.
Nitrifiers rely on a biochemical reaction (oxidation) by the means of using oxygen as a way of transporting electrons, drawn from the ammonia/nitrite compounds. Just imagine oxygen as a garbage collector.
Under low oxygen levels, the bacteria use nitrite/nitrate as an acceptor, or garbage collector, reversing the process from nitrifying to denitrifying (removal of nitrates).
As an aquarium is loaded with oxygen, the bacteria need ammonia in order to stay alive. Fish respiration and decaying food provide ammonia in sufficient amounts to ensure the proper function. So in theory: Without ammonia, the bacteria will enter a resting phase without losing much of their energy, ready to get to work again once ammonia is available.