Nitrifying bacteria are chemoautotrophic

Chemotroph

Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be organic or inorganic . The chemotroph designation is in contrast to phototrophs, which utilize solar energy...

 or chemolithotrophs depending on the genera (Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrococcus) bacteria that grow by consuming inorganic nitrogen compounds. Many species of nitrifying bacteria have complex internal membrane systems that are the location for key enzymes in nitrification: ammonia monooxygenase which oxidizes ammonia to hydroxylamine

Hydroxylamine

Hydroxylamine is an inorganic compound with the formula NH2OH. The pure material is a white, unstable crystalline, hygroscopic compound. However, hydroxylamine is almost always provided and used as an aqueous solution. It is used to prepare oximes, an important functional group. It is also an...

, and nitrite oxidoreductase

Nitrite oxidoreductase

Nitrite oxidoreductase is an enzyme involved in nitrification. It is the last step in the process of aerobic ammonia oxidation, which is carried out by two groups of nitrifying bacteria: ammonia oxidizers such as Nitrosospira, Nitrosomonas and Nitrosococcus convert ammonia to nitrite, while...

, which oxidizes nitrite to nitrate.
Nitrifying bacteria are widespread in soil and water, and are found in highest numbers where considerable amounts of ammonia are present (areas with extensive protein decomposition, and sewage treatment plants).