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

Yersinia pestis, the subject of her research, is the bacterium responsible for three bubonic plague pandemics over human history. Diana Gitig, Ars Technica, "Medieval European plague genomes hint at Black Death’s travels," 5 Dec. 2018 Also known as Hansen’s disease, leprosy is an infectious illness caused by the bacterium Mycobacterium leprae. Brigit Katz, Smithsonian, "Did Leprosy Originate in Europe?," 14 May 2018 In the last decade, a determined effort to bring TB under control has come under increasing threat from the rise of antibiotic-resistant strains of the bacterium. Melissa Healy,, "Scientists have a promising new approach for treating drug-resistant tuberculosis," 25 Apr. 2018 One 2012 study found that leaving the lid up versus down led to 12 times as much diarrhea-inducing bacterium Clostridium difficile in the air. Caroline Picard, Good Housekeeping, "Please, for the Love of God, Close the Toilet Lid When You Flush," 20 Aug. 2018 But after a severe, prolonged famine in the North in the mid-1990s, the TB bacterium spread rapidly among malnourished survivors. Richard Stone, Science | AAAS, "Major donor nixes effort to combat tuberculosis crisis in North Korea," 29 June 2018 Plague is a disease caused by the bacterium Yersinia pestis. Amanda Macmillan,, "An Idaho Child Was Diagnosed With the Plague. How Worried Should You Be?," 13 June 2018 Dillos Scientists have suspected that armadillos could harbor and transmit the bacterium responsible for leprosy, known as Mycobacterium leprae, since the 1970s. National Geographic, "Humans Gave Leprosy to Armadillos. Now, They’re Giving It Back," 28 June 2018 The bacterium that carries the various types of plague, Yersinia pestis, has been responsible for millions of deaths over the course of human history. Anna Gunther, CBS News, "Idaho boy recovering from rare case of plague," 13 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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10 Dec 2018

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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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Rhyming Dictionary: Words that rhyme with bacterium

Spanish Central: Translation of bacterium

Nglish: Translation of bacterium for Spanish Speakers

Britannica English: Translation of bacterium for Arabic Speakers Encyclopedia article about bacterium

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What made you want to look up bacterium? Please tell us where you read or heard it (including the quote, if possible).


a nest or breeding place

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