bacterium
Americannoun
noun
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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.
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.
Researchers say the findings indicate the bacterium may help remove nanoplastics from the body by binding to the particles in the intestine and promoting their excretion.
From Science Daily • May 18, 2026
Scientists in South Korea say a probiotic bacterium found in kimchi may help the body get rid of nanoplastics by binding to the particles inside the intestine and helping remove them through waste.
From Science Daily • May 18, 2026
Their findings offer a clearer explanation of how this bacterium may influence depression.
From Science Daily • Apr. 25, 2026
The key difference turned out to be a virus living inside the bacterium.
From Science Daily • Apr. 22, 2026
Pneumococcal pneumonia after influenza infection—the epidemic within the epidemic—was of such concern that the ministry had deployed teams of scientists to study the bacterium and develop a vaccine against it.
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.
