nitrobacterium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A nitrifying bacterium.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any if the several species of bacteria in the soil that take part in the nitrogen cycle, oxidizing ammonium, and organic nitrogen compounds to the more soluble nitrite and nitrate.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of the bacteria in the soil that take part in the nitrogen cycle; they oxidize ammonium compounds into nitrites or oxidize nitrites into nitrates