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 Dyson’s desert bacterium is one species that could help, but there are plenty of others adapted to extreme environments that could also offer a way out. Wired, 22 July 2022 However, even though the bacterium has been around for quite some time, little is known about its history. Jane Recker, Smithsonian Magazine, 23 June 2022 The bacterium typically doesn’t pose a risk to healthy people. Ginger Adams Otis, WSJ, 21 Mar. 2022 Less certain still is just how the deadly bacterium was introduced to the area. Barbie Latza Nadeau, Scientific American, 24 June 2015 The presence of this bacteria led scientists to hypothesize that this bacterium, and maybe other bioluminescent bacterial species as well, are the cause of milky seas. Grrlscientist, Forbes, 11 July 2022 If the culture shows that a particular bacterium is present in your urine, your doctor can prescribe the correct antibiotic to wipe it out and if the culture doesn’t show any bacteria, your doctor can explore other causes for your discomfort. Kaitlyn Pirie, Good Housekeeping, 13 May 2022 The disease was caused by a bacterium that would later become known as Yersinia pestis. Evan Bush, NBC News, 15 June 2022 Some ticks are harmless, but others can become infected with a virus or bacterium, typically after feeding on animals, and then transmit an illness-causing pathogen to humans through a blood-sucking bite. Serena Coady, SELF, 23 Apr. 2022 See More

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.

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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The first known use of bacterium was in 1835

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Last Updated

29 Jul 2022

Cite this Entry

“Bacterium.” Dictionary, Merriam-Webster, Accessed 17 Aug. 2022.

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More Definitions for bacterium


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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