bacterium
Americannoun
noun
plural
bacteria-
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.
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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
Explanation
Use the word bacterium when you're talking about a single-celled organism. Your uncle's bad case of pneumonia might be caused by a bacterium. You're more likely to be familiar with the plural form of bacterium, bacteria. A bacterium is frequently the cause of diseases, specifically those known as bacterial illnesses. In biology, a bacterium is simply a microorganism, or a very tiny living thing, with cell walls but no distinct nucleus. The word itself comes from the Greek word bakterion, "small stick or rod," which describes a bacterium's shape when seen under a microscope.
Vocabulary lists containing bacterium
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.
The key difference turned out to be a virus living inside the bacterium.
From Science Daily • Apr. 22, 2026
A common gut bacterium found in most healthy individuals.
From Science Daily • Apr. 22, 2026
Now, researchers from the University of Southern Denmark and Odense University Hospital have identified a previously unknown virus inside a common gut bacterium.
From Science Daily • Apr. 22, 2026
One lowly bacterium possesses a flagellum—a tiny tail or rotor.
From The Wall Street Journal • Apr. 16, 2026
It took nature hundreds of millions of years to evolve a bacterium, and billions to make a grasshopper.
From "Cosmos" by Carl Sagan
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.
