Bacteria: Single-celled microorganisms that can exist either as independent (free-living) organisms or as parasites (dependent on another organism for life). The plural of bacterium. Examples of bacteria include Acidophilus, a normal inhabitant of yogurt; Gonococcus which causes gonorrhea; Clostridium welchii, the most common cause of gangrene; E. coli, which lives in the colon and can cause disease elsewhere; and Streptococcus, the bacterium that causes the common throat infection called strep throat.
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Last Editorial Review: 5/13/2016