(n)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
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
prop. n. A natural family of usually rod-shaped bacteria that oxidize ammonia or nitrites: nitrobacteria. Syn. -- family Nitrobacteriaceae. [ WordNet 1.5 ]