bacterium

noun

bac·​te·​ri·​um bak-ˈtir-ē-əm 

plural bacteria bak-ˈtir-ē-ə 

biology : any of a domain (Bacteria) (see domain sense 8) of chiefly round, spiral, or rod-shaped single-celled prokaryotic microorganisms that typically live in soil, water, organic matter, or the bodies of plants and animals, that make their own food especially from sunlight or are saprophytic or parasitic, are often motile by means of flagella, reproduce especially by binary fission, and include many important pathogens

broadly : prokaryote

Note: Bacteria lack a nuclear membrane or membrane-bound organelles and are categorized as gram-positive or gram-negative when a cell wall is present. While many bacteria are aerobic requiring the presence of oxygen to survive, others are anaerobic and are able to survive only in the absence of oxygen.

compare archaea, eukaryote

About Viruses and Bacteria: Usage Guide

Viruses differ from bacteria in several important ways. Viruses are not living organisms; they can only replicate in the cells of a host. By contrast, bacteria are single-celled organisms that can reproduce on their own and are many times larger than viruses. While both viruses and bacteria can cause disease, most bacteria are harmless and many are beneficial.

Examples of bacterium in a Sentence

Recent Examples on the Web

Four of the ticks were found to carry Anaplasma phagocytophilum, and four others carried Borrelia miyamotoi, a bacterium that causes hard tick relapsing fever and is almost always thought to be concentrated in the Northeast and Midwest. — Erika Edwards, NBC news, 13 Aug. 2026

Investigators have traced the outbreak to a bacterium known as Salmonella Javiana in jalapeno peppers grown in Sinaloa, Mexico and distributed by Coast Citrus Distributors. — Omer Awan, Forbes.com, 9 Aug. 2026

Louisiana health officials warned residents to take precautions to prevent infections because the bacterium's presence increases as coastal waters get warmer. — Mike Snider, USA Today, 8 Aug. 2026

Of these, 16 of them proved to be viable and able to kill certain strains of Escherichia coli—including strains of the bacterium that had mutated to be resistant to the original reference genome of ΦX174. — Adam Kovac, Scientific American, 7 Aug. 2026

Word History

Etymology

New Latin, from Greek baktērion staff

First Known Use

1835, in the meaning defined above

Phrases Containing bacterium

• flesh-eating bacterium
• sulfur bacterium

Rhymes for bacterium

• agrobacterium
• corynebacterium
• mycobacterium
• cyanobacterium
• delirium
• deuterium
• imperium
• puerperium
• venereum
• magisterium
• ministerium

Kids Definition

bacterium

noun

bac·​te·​ri·​um bak-ˈtir-ē-əm 

plural bacteria

-ē-ə

: any of a group of single-celled microorganisms that live in soil, water, the bodies of plants and animals, or matter obtained from living things and are important because of their chemical effects and disease-causing abilities

Medical Definition

bacterium

noun

bac·​te·​ri·​um bak-ˈtir-ē-əm 

plural bacteria -ē-ə 

: 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