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, eukaryoteExamples of bacterium in a Sentence
Recent Examples on the Web
In years past, many believed that the bacterium known as Mycobacterium leprae caused leprosy and that it was only spread in America by early European explorers and settlers.
—
Nick Butler, FOXNews.com, 5 June 2025
Yersinia pestis itself is a bacterium with some deep historical roots and has caused three major pandemics in recorded human history.
—Laura Baisas, Popular Science, 29 May 2025
Salmonella Montevideo is a type of Salmonella enterica bacterium.
—Mirna Alsharif, NBC news, 20 May 2025
Salmonella is a bacterium that settles in human and animal intestinal tracts.
—Hollie Silverman, MSNBC Newsweek, 6 May 2025
See All Example Sentences for bacterium
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Nick Butler, FOXNews.com, 5 June 2025
Yersinia pestis itself is a bacterium with some deep historical roots and has caused three major pandemics in recorded human history.
—Laura Baisas, Popular Science, 29 May 2025
Salmonella Montevideo is a type of Salmonella enterica bacterium.
—Mirna Alsharif, NBC news, 20 May 2025
Salmonella is a bacterium that settles in human and animal intestinal tracts.
—Hollie Silverman, MSNBC Newsweek, 6 May 2025
See All Example Sentences for bacterium
Cite this Entry
“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 10 Jun. 2025.
Copy CitationKids 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
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