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

About Viruses and Bacteria: Usage Guide

Viruses differ from bacteria in several important ways. Viruses are not living organisms; they can only replicate in the cells of a host. By contrast, bacteria are single-celled organisms that can reproduce on their own and are many times larger than viruses. While both viruses and bacteria can cause disease, most bacteria are harmless and many are beneficial.

Examples of bacterium in a Sentence

Recent Examples on the Web

Researchers at Cedars-Sinai found traces of Chlamydia pneumoniae – a bacterium that's best known for causing respiratory infections – in the eye's retinal tissue, and higher levels of this pathogen correlated with advanced degeneration in Alzheimer’s disease patients. —New Atlas, 8 Feb. 2026

Lung, stomach and cervical cancers accounted for nearly half of the cases that were linked to modifiable risks, with many linked to viruses and bacteria like the human papillomavirus (HPV), hepatitis B and C, and Helicobacter pylori (a common bacterium that infects the stomach lining). —Melissa Rudy, FOXNews.com, 5 Feb. 2026

De Witte emphasized that the bacterium poses no risk to humans or food safety and said the industry has invested heavily in vaccine research. —Moná Thomas, PEOPLE, 3 Feb. 2026

The bacterium can cause rapidly progressive and severe respiratory disease, which spreads quickly and causes a high rate of serious illness and death. —Veronica Fernandez-Alvarado january 30, Sacbee.com, 30 Jan. 2026

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

Rhymes for bacterium

• delirium
• deuterium
• imperium
• puerperium
• venereum
• agrobacterium
• corynebacterium
• mycobacterium
• cyanobacterium
• magisterium
• ministerium

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