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Synonyms

bacterium

American  
[bak-teer-ee-uhm] / bækˈtɪər i əm /

noun

  1. singular of bacteria.

bacterium British  
/ bækˈtɪərɪəm /

noun

  1. the singular of bacteria

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
bacterium Scientific  
/ băk-tîr′ē-əm /

plural

bacteria

  1. Any of a large group of one-celled organisms that lack a cell nucleus, reproduce by fission or by forming spores, and in some cases cause disease. They are the most abundant lifeforms on Earth, and are found in all living things and in all of the Earth's environments. Bacteria usually live off other organisms. Bacteria make up most of the kingdom of prokaryotes (Monera or Prokaryota), with one group (the archaea) sometimes classified as a separate kingdom.

  2. See also archaeon prokaryote

Usage

It is important to remember that bacteria is the plural of bacterium, and that saying a bacteria is incorrect. It is correct to say The soil sample contains millions of bacteria, and Tetanus is caused by a bacterium.

Etymology

Origin of bacterium

1840–50; < New Latin < Greek baktḗrion, diminutive of baktēría staff; akin to báktron stick, Latin baculum, bacillum

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

Flea-borne typhus is caused by the bacterium Rickettsia typhi.

From Los Angeles Times • Apr. 3, 2026

So a lot of the focus is on changes to the bacterium and whether people have less immunity.

From BBC • Mar. 27, 2026

Does the explanation for its scale and speed lie solely in the bacterium itself or did other factors play a role?

From BBC • Mar. 20, 2026

When the bacterium entered the bloodstream, it significantly boosted the growth and spread of existing tumors.

From Science Daily • Mar. 19, 2026

At various places inside the monkey house, they set out patches of paper saturated with spores of a harmless bacterium known as Bacillus subtilis niger.

From "The Hot Zone" by Richard Preston

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.