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.