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

Examples of bacterium in a Sentence

Recent Examples on the Web Here is one of our first experiments: Somebody had discovered that when the bacterium Escherichia coli enters the human intestinal tract, the environment contains oxygen. —

Veronique Greenwood, Quanta Magazine, 15 Aug. 2023 But despite its age-old connotation, the bacterium Mycobacterium leprae remains a real and very present danger, with some 200,000 people in 120 countries reporting the tell-tale signs of the disease—chiefly skin lesions, lumps, ulcers, and loss of sensation. —

Darren Orf, Popular Mechanics, 10 Aug. 2023 In lab experiments, the honey protected against the bacterium Staphylococcus aureus, which causes a wide range of diseases in humans, but did not protect against other bacteria. —

Will Sullivan, Smithsonian Magazine, 28 July 2023 In June, its frozen Organic Tropical Fruit Blend was flagged for possible contamination with the bacterium Listeria monocytogenes, just months after it had been recalled for possible hepatitis A contamination in March. —

Emma Glassman-Hughes, BostonGlobe.com, 25 July 2023 The bacterium, Vibrio vulnificus, is typically contracted by consuming raw or undercooked oysters and other shellfish. —

Mirna Alsharif, NBC News, 12 June 2023 These neutralize a key protein on the outside of Borrelia burgdorferi, the bacterium that causes most cases of Lyme disease in the U.S. —

Claire Bugos, Verywell Health, 25 May 2023 The bacterium is often found in the nose and throat of people without causing the disease. —

Monique Calello, USA TODAY, 2 Sep. 2023 Some scientists hypothesize that it may be linked to a misguided immune response to the bacterium that causes Lyme, in which the immune system attacks the patient’s own cells. —

Kay Lazar, BostonGlobe.com, 29 Aug. 2023 See More

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

See more words from the same year

Cite this Entry

Style

“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 25 Sep. 2023.

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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