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
Mitochondria descend from an ancient bacterium that was swallowed, millions of years ago, by an ancestral cell from which all life descends.
—
Martin Picard, Scientific American, 3 Jan. 2025
Back in April, scientists at Berkeley Lab found that an algae and a bacterium have merged to do something neither of them can alone: grab nitrogen, a key nutrient, from the air around them.
—New Atlas, 2 Jan. 2025
For instance, several strains of the bacterium Pseudomonas aeruginosa infect humans and have evolved to resist many of our antibacterial treatments.
—Carlyn Kranking, Smithsonian Magazine, 31 Dec. 2024
One of the most serious such disturbances is caused by the campylobacter bacterium, which can cause severe diarrhea in children and even lead to stunted growth.
—Jeffrey Kluger, TIME, 20 Dec. 2024
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Martin Picard, Scientific American, 3 Jan. 2025
Back in April, scientists at Berkeley Lab found that an algae and a bacterium have merged to do something neither of them can alone: grab nitrogen, a key nutrient, from the air around them.
—New Atlas, 2 Jan. 2025
For instance, several strains of the bacterium Pseudomonas aeruginosa infect humans and have evolved to resist many of our antibacterial treatments.
—Carlyn Kranking, Smithsonian Magazine, 31 Dec. 2024
One of the most serious such disturbances is caused by the campylobacter bacterium, which can cause severe diarrhea in children and even lead to stunted growth.
—Jeffrey Kluger, TIME, 20 Dec. 2024
See all Example Sentences for bacterium
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Cite this Entry
“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 22 Jan. 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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