bacterium

noun

bac·​te·​ri·​um bak-ˈtir-ē-əm How to pronounce bacterium (audio)
plural bacteria bak-ˈtir-ē-ə How to pronounce bacterium (audio)
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

Examples of bacterium in a Sentence

Recent Examples on the Web
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Rice contains a bacterium called Bacillus cereus, which can cause food poisoning. Thomas G. Moukawsher, Newsweek, 30 Jan. 2025 Tuberculosis symptoms explained Tuberculosis, caused by the bacterium called Mycobacterium tuberculosis, spreads through germs from people with active infections, CDC says. Eduardo Cuevas, USA TODAY, 29 Jan. 2025 In the winter of 1925, Balto was part of a team that braved subzero temperatures to bring a shipment of antitoxin to the isolated city of Nome, Alaska, where children were dying of diphtheria, a serious infection caused by a highly contagious bacterium. David Kindy, Smithsonian Magazine, 28 Jan. 2025 These days, the disease, which is caused by the bacterium Mycobacterium tuberculosis, is treated with specific antibiotics — sometimes for as long as nine months, the Cleveland Clinic says. Cara Lynn Shultz, People.com, 27 Jan. 2025 See all Example Sentences for bacterium 

Word History

Etymology

New Latin, from Greek baktērion staff

First Known Use

1835, in the meaning defined above

Time Traveler
The first known use of bacterium was in 1835

Dictionary Entries Near bacterium

Cite this Entry

“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 17 Feb. 2025.

Kids Definition

bacterium

noun
bac·​te·​ri·​um bak-ˈtir-ē-əm How to pronounce bacterium (audio)
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 How to pronounce bacterium (audio)
plural bacteria -ē-ə How to pronounce bacterium (audio)
: 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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