bacteria, bacterium - (microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered plants

nitrate bacterium, nitric bacterium - any of the nitrobacteria that oxidize nitrites into nitrates

nitrite bacterium, nitrous bacterium - any of the nitrobacteria that oxidize ammonia into nitrites