bac·te·ri·um | \bak-ˈtir-ē-əm \
plural bacteria\bak-ˈtir-ē-ə \

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

The group A streptococcus bacterium, the microbe responsible for strep, causes 616 million cases of sore throat each year worldwide. Kate Sheridan, Newsweek, "Strep Vaccine Could Save Lives and Reduce Health Care Costs—So Why Don't We Have One?," 26 Feb. 2018 This is the most serious E. coli outbreak since 2006, when the bacterium spread from spinach. Helena Oliviero, ajc, "Kennesaw man sues restaurant, says daughters got E. coli from lettuce," 19 May 2018 These have stymied efforts to make the reconstructed bacterium viable. Elie Dolgin, Scientific American, "Scientists Downsize Bold Plan to Make Human Genome from Scratch," 1 May 2018 The institute also helped develop a bacterium with a minimal genome, eliminating all elements not necessary for life. Bradley J. Fikes,, "Genome pioneer Craig Venter retiring from Human Longevity; returning to JCVI," 25 May 2018 Lyme disease is an infection caused by a bacterium, Borellia burgdorferi, that is carried by deer ticks. Jennifer Walter, Milwaukee Journal Sentinel, "The Tick App offers resources to identify, remove ticks as part of Lyme disease study," 3 July 2018 Legionnaires’ disease is caused by a type of bacterium found in the environment and transmitted by breathing in a contaminated mist or vapor, occurring more often in hot and humid weather, according to health officials. Angie Leventis Lourgos,, "9 sickened by Legionnaires' disease across McHenry County; public health officials investigating," 11 July 2018 The bacterium is found in the faeces of those infected, as well as a small proportion of people who have recovered but remain asymptomatic carriers. The Economist, "A strain of typhoid could become virtually untreatable," 26 Apr. 2018 Suddenly, one bacterium shoots out a long appendage, latches onto a DNA fragment and reels in its catch. New York Times, "Using Harpoon-Like Appendages, Bacteria ‘Fish’ for New DNA," 14 June 2018

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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Last Updated

15 Oct 2018

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Time Traveler for bacterium

The first known use of bacterium was in 1835

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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 \
plural bacteria\-ē-ə \

Kids Definition of bacterium

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


bac·te·ri·um | \bak-ˈtir-ē-əm \
plural -ria\-ē-ə \

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

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evasion of direct action or statement

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