A microscopic vegetable organism, belonging to the class
   Algae, 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 Bacillus.