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, eukaryote

Examples of bacterium in a Sentence

Recent Examples on the Web

Walking pneumonia describes a milder subset of the condition caused by the bacterium Mycoplasma pneumoniae. —Thomas G. Moukawsher, Newsweek, 5 Nov. 2024

The bacterium was identified in the processing equipment at the manufacturing plant after as a result of routine testing conducted. —Amaris Encinas, USA TODAY, 5 Nov. 2024

The study was successfully carried out using a bacterium called Staphylococcus epidermidis on pig skin. —New Atlas, 31 Oct. 2024

Pertussis, also known as whooping cough, is a highly contagious respiratory disease caused by the bacterium Bordetella pertussis. —Aaron Valdez, The Enquirer, 30 Oct. 2024

Word History

Etymology

New Latin, from Greek baktērion staff

First Known Use

1835, in the meaning defined above

Phrases Containing bacterium

• flesh-eating bacterium
• sulfur bacterium

Kids 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