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 Global issue The disease caused by the bacterium Bordetella pertussis is transmitted through respiratory droplets. —Bloomberg News, TIME, 11 Apr. 2024 The bacterium binds to cancer cells, activating growth factors that promote cancer proliferation and metastasis. —Claire Bugos, Verywell Health, 8 Apr. 2024 This oral bacterium is one of the major causes of tooth decay. —Laura Baisas, Popular Science, 27 Mar. 2024 Indeed, the bacterium—and in some cases, just portions of it—were detectable in the material, boosting hopes that the same techniques could be applied to real otherworldly material. —Isaac Schultz / Gizmodo, Quartz, 24 Mar. 2024 Contracted by ingesting food or water contaminated with the bacterium Vibrio cholerae, cholera thrives in places that lack access to safe water and basic sanitation. —Maryanne Murray Buechner, Forbes, 19 Feb. 2024 There’s also the potential for the bacterium itself to fight cancer cells. —Lauryn Higgins, Health, 9 Apr. 2024 Key Takeaways Fusobacterium nucleatum, an oral bacterium, is linked to the development of colorectal cancer. —Claire Bugos, Verywell Health, 8 Apr. 2024 Diagnosis of the fungi or bacteria responsible is difficult because sometimes the fungal pathogen or bacterium found in the wounded cane is not the pathogen that caused the canker, but a secondary invader. —Rita Perwich, San Diego Union-Tribune, 6 Apr. 2024

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'bacterium.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

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

Phrases Containing bacterium

Dictionary Entries Near bacterium

bacteritic

bacterium

bacteriuria

See More Nearby Entries 

Cite this Entry

“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 28 Apr. 2024.

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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

More from Merriam-Webster on bacterium

Nglish: Translation of bacterium for Spanish Speakers

Britannica English: Translation of bacterium for Arabic Speakers

Britannica.com: Encyclopedia article about bacterium

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