microscopic life form: a single-celled, often parasitic microorganism without distinct nuclei or organized cell structures. Various species are responsible for decay, fermentation, nitrogen fixation, and many plant and animal diseases. Kingdom Eubacteria.
[Mid-19th century. < Greek baktērion "little rod" (because the first ones discovered were rod-shaped) < baktron "rod"]
Bacterium is singular and bacteria is plural. The word is more commonly found in the plural, which can lead to its being wrongly treated as a singular noun taking a singular verb.
