Search Result for "nitrobacterium": 
Wordnet 3.0

NOUN (1)

1. 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; 



WordNet (r) 3.0 (2006):

nitrobacterium
    n 1: 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