Definition of 'bacteria'

any of a division (Bacteria) of monerans, microorganisms which are typically one-celled, have no chlorophyll, multiply by simple division, and can be seen only with a microscope: they occur in three main forms, spherical (cocci), rod-shaped (bacilli), and spiral (spirilla): some bacteria cause diseases such as pneumonia and anthrax, and others are necessary for fermentation, nitrogen fixation, etc.