bac·​te·​ri·​um | \ bak-ˈtir-ē-əm How to pronounce bacterium (audio) \
plural bacteria\ bak-​ˈtir-​ē-​ə How to pronounce bacteria (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 So researchers instead began looking at indirect ways of identifying Borrelia, such as searching for antibodies produced by the immune system against the bacterium. Carrie Arnold, Popular Science, "COVID-19 testing problems are all too familiar to Lyme disease patients," 28 June 2020 When the tick attaches to a human, that infection-causing bacterium, virus, or parasite can be passed along as the little bloodsucker engorges its body with your blood. Karen Pallarito,, "11 Tick-Borne Diseases You Need to Know About This Summer," 25 June 2020 After multiplying inside a bacterium, phages use lysins to break through the cell wall of their host, instantly killing it. Claudia Wallis, Scientific American, "The Virtuous Side of Viruses," 26 May 2020 One 2017 study found that honey and cinnamon appear to work together to fight against Streptococcus mutans, a bacterium known for its role in tooth decay. Miriam Fauzia, USA TODAY, "Fact check: A mix of honey and cinnamon may have some health benefits, more studies needed," 15 June 2020 Based on all this, Dr Lebeer suggests that developing a nasal probiotic therapy from this bacterium has the potential to help people with chronic sinus infections, and is something that ought to be explored. The Economist, "Nasal probiotics The nose, it seems, is protected by bacterial guards," 13 June 2020 The disease made its way here during what’s known as the Third Great Cholera pandemic, with the Vibrio cholerae bacterium spreading from Asia to Europe to North America between 1846 and 1860. Paula Allen,, "Fast-moving illness was 19th-century scourge," 5 June 2020 To understand this tiny thing, far smaller than a bacterium, is about as easy as understanding the Earth and its orbit from the vantage point of the universe. Ai Weiwei, The Atlantic, "When Time Loses All Meaning," 2 June 2020 In a Leipzig waste site, scientists found a soil bacterium that can break down components of polyurethane—and survive the toxic chemicals released in the process. Sarah Lewin Frasier, Scientific American, "In Case You Missed It," 26 May 2020

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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The first known use of bacterium was in 1835

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Last Updated

4 Jul 2020

Cite this Entry

“Bacterium.” Dictionary, Merriam-Webster, Accessed 10 Jul. 2020.

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How to pronounce bacterium (audio)

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 to humans 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 bacteria (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

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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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