- singular of bacteria.
bacterium
Americannoun
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bacteria
plural
noun
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bacteria
plural
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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.
Other Word Forms
Noun Inflected Forms
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.
See Examples For:
In a study published in Science Advances, the team used BifidoSumIL-2, an engineered strain of Bifidobacterium longum, a probiotic bacterium naturally found in the gut, to carry an immune-stimulating treatment directly into tumors.
From Science Daily ● Aug. 19, 2026
Just this month, tularemia, also known as rabbit fever and caused by a bacterium in ticks and deer flies, was detected in people living on Long Island.
From Los Angeles Times ● Aug. 16, 2026
According to the researchers, the discovery also demonstrates the remarkable adaptability of Treponema pallidum, the bacterium responsible for treponemal diseases.
From Science Daily ● Aug. 12, 2026
Some governments also use Wolbachia, a naturally occurring bacterium found in about half of all insect species, to curb the spread of mosquito-borne diseases like dengue.
From BBC ● Jul. 23, 2026
Grown on either sugar alone, the bacterium begins to divide rapidly, doubling in number every twenty minutes or so.
From "The Gene" by Siddhartha Mukherjee
The lasting effects were not limited to antibiotics, which are already well known for their ability to disrupt populations of gut bacteria.
From Science Daily ● Aug. 24, 2026
They also want to investigate more closely how oral bacteria, chronic inflammation, and pancreatic cancer progression may be biologically connected.
From Science Daily ● Aug. 22, 2026
As scientists link oral bacteria to heart disease and cognitive decline, a push is under way to treat the mouth as a source of body-wide inflammation.
From The Wall Street Journal ● Aug. 21, 2026
But it is unsafe to eat raw poultry because of the food poisoning risk from harmful bacteria.
From BBC ● Aug. 20, 2026
When bacteria switched from glucose to lactose consumption, they induced specific lactose-digesting enzymes.
From "The Gene" by Siddhartha Mukherjee
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.
