bacterium

noun

1. (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 to be plants

   noun.animal

   Synonyms bacteria

   Source: WordNet 3.0

2. A microscopic vegetable organism, belonging to the class Alg&aelig;, usually in the form of a jointed rodlike filament, and found in putrefying organic infusions. Bacteria are destitute of chlorophyll, and are the smallest of microscopic organisms. They are very widely diffused in nature, and multiply with marvelous rapidity, both by fission and by spores. Certain species are active agents in fermentation, while others appear to be the cause of certain infectious diseases. See <u>Bacillus</u>.

   noun

   Source: webster1913