Medical Definition of bacterium
plural bacteria \-ē-ə\play
: any of a domain (Bacteria) of prokaryotic round, spiral, or rod-shaped single-celled microorganisms that may lack cell walls or are gram-positive or gram-negative if they have cell walls, that are often aggregated into colonies or motile by means of flagella, that typically live in soil, water, organic matter, or the bodies of plants and animals, that are usually autotrophic, saprophytic, or parasitic in nutrition, and that are noted for their biochemical effects and pathogenicity; broadly : prokaryote
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