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

Definition of bacterium

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 Nordenholt, however, believes that researchers will eventually defeat the bacterium and discover a way to renitrify the soil. Washington Post, 26 May 2021 However, because of their extreme density (several hundred pounds per cubic inch), all of that mass would be packed into a space about the size of a bacterium. Dan Falk, Smithsonian Magazine, 13 May 2021 Meanwhile, stronger strains of the Streptococcus pyogenes bacterium that causes scarlet fever have been found in Hong Kong and the United Kingdom. Nicolas Noulin, Scientific American, 5 May 2021 She was soon diagnosed with Clostridium difficile infection, a potentially fatal condition where bacterium can cause life-threatening inflammation of the colon. Kayla Keegan, Good Housekeeping, 2 May 2021 The original version mistakenly referred to the Xylella fastidiosa bacterium as a virus. Barbie Latza Nadeau, Scientific American, 20 Oct. 2015 There is no data on how coronavirus vaccines interact with other immunizations, such as vaccines targeting the human papillomavirus, known as HPV, or the bacterium that causes meningococcal disease. Washington Post, 12 May 2021 Because gray squirrels are California’s main vector for Borrelia burgdorferi, the bacterium that causes Lyme disease, woodlands have headlined the list of the state’s places where ticks are cause for concern. Alex Fox, Smithsonian Magazine, 27 Apr. 2021 For example, Lyme disease, caused by a bacterium, resolves in most patients after antibiotic treatment. Arkansas Online, 2 May 2021

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'bacterium.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of bacterium

1835, in the meaning defined above

History and Etymology for bacterium

New Latin, from Greek baktērion staff

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Time Traveler for bacterium

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The first known use of bacterium was in 1835

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Last Updated

23 Jun 2021

Cite this Entry

“Bacterium.” Dictionary, Merriam-Webster, Accessed 23 Jun. 2021.

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More Definitions for bacterium



English Language Learners Definition of bacterium

: any one of a group of very small living things that often cause disease


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

Kids Definition of bacterium

: any of a group of single-celled microscopic organisms that are important because of their chemical activities and as causes of disease


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

Medical Definition of bacterium

: 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 Encyclopedia article about bacterium


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