- 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:
Plague is a disease caused by the Yersinia pestis bacterium, the same pathogen that killed millions of people in Europe during the Middle Ages.
From Barron's ● Oct. 7, 2026
The bacterium that causes the plague, Yersinia pestis, found among rodents such as marmots in the area.
From BBC ● Oct. 7, 2026
The bacterium Yersinia pestis, which can cause pneumonic and bubonic plagues, has been identified from time to time in Siberia, linked to exposure to vermin.
From The Wall Street Journal ● Oct. 5, 2026
It uses a harmless bacterium to insert DNA into the rootstock exactly where it is needed to give the tree resistance.
From The Wall Street Journal ● Sep. 19, 2026
All of us carry the bacterium in our bodies, probably since birth or immediately thereafter.
From "The Tipping Point" by Malcolm Gladwell
The research work included testing stem cell production, creating and testing IV fluid, and studying how the bacteria that causes pneumonia can lead to long‑term heart damage.
From Los Angeles Times ● Oct. 8, 2026
"You're altering the microbiome of your gut, and perhaps creating antibiotic resistant bacteria in your body that will fight off the drug therapy the next time you really need it," he said.
From Barron's ● Oct. 8, 2026
It’s caused by a bacteria called Yersinia pestis that infects fleas and the small mammals that host fleas, and there are actually three types of plague depending on which part of the body becomes infected.
From Slate ● Oct. 8, 2026
The researchers say dietary sugar may magnify the microbiome disruption caused by antibiotics because potentially harmful bacteria that survive treatment may be able to use those sugars to expand.
From Science Daily ● Oct. 7, 2026
When the sugar source was switched to lactose, the bacteria turned on an entire module of genes—several lactose-metabolizing genes—to digest lactose.
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.
