[ bak-teer-ee-uh ]
/ bækˈtɪər i ə /
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plural noun, singular bac·te·ri·um [bak-teer-ee-uhm]. /bækˈtɪər i əm/. Microbiology.
ubiquitous one-celled organisms, spherical, spiral, or rod-shaped and appearing singly or in chains, comprising numerous and variously classified phyla: among the inestimable number of species are those involved in fermentation, putrefaction, infectious diseases, and nitrogen fixation.
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Origin of bacteria

First recorded in 1860–65; from New Latin, from Greek baktḗria, plural of baktḗrion, diminutive of baktēría “staff, cane”; see origin at bacterium


bac·te·ri·al, adjectivebac·te·ri·al·ly, adverbnon·bac·te·ri·al, adjectivenon·bac·te·ri·al·ly, adverb

Other definitions for bacteria (2 of 2)

[ bak-teer-ee-uh ]
/ bækˈtɪər i ə /

noun (used with a singular or plural verb)Microbiology.
(in the three-domain system of classification) the taxonomic domain comprising the bacteria.
See also domain (def. 6).

Origin of Bacteria

First recorded in 1860–65; from New Latin; see origin at bacteria
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023


What is bacteria?

Bacteria is a collective name for a large number of single-celled, microscopic organisms that live in the soil, water, or animals, including humans. They come in several different shapes, including spheres, rods, and spirals, and may organize themselves into clusters or chains.

Bacteria consist of only a single cell. Additionally, bacteria are prokaryotes, which means their single cell does not have a nucleus like an animal or plant cell does. Instead, their genetic material (DNA) just floats around inside the cell. Bacteria often have a single loop of DNA and possibly some other genes they have picked up in the environment.

Bacteria reproduce by what is known as binary fission. A single bacteria splits itself into two identical copies (clones) of the original, meaning the two “children” have the exact same genetic material as the “parent.” Bacteria can often become immune to antibiotics quickly because they never lose any genes they have picked up that help them survive. Bacteria can reproduce extraordinarily fast under the right conditions with a single bacteria making millions of copies of itself in just a few hours.

Bacteria are incredibly common in nature and live in almost every environment on Earth. Most bacteria are harmless or help out other organisms in some way. For example, there are several kinds of bacteria that live in the human intestines that help breakdown foods or assist in warding off disease-causing organisms.

The singular of bacteria is bacterium.

Bacteria are often confused with another disease-causing microscopic organism known as a virus. Unlike bacteria, viruses cannot reproduce on their own. Instead, they infect other cells and force them to make copies of the virus. Because of their often destructive reproductive method, viruses are typically seen as harmful parasites, and there are far fewer examples of helpful viruses than helpful bacteria.

Why is bacteria important?

The first records of the word bacteria come from around 1860. It ultimately comes from the Greek word baktḗria, meaning “little canes” or “little sticks.” Some bacteria resemble sticks or rods, while others have different shapes. Bacteria are found everywhere on Earth and have been found in fossils that are hundreds of millions of years old. Scientists have argued that bacteria first appeared on Earth around 3.5 billion years ago, while humans have only been around for about 200,000 years.

Besides helping us and other animals digest food, bacteria have other beneficial uses. Several kinds of bacteria are essential to making yogurt, buttermilk, and cheese. Bacteria are also used to make different kinds of alcohol and to make vinegar.

Some bacteria, such as certain species of E. Coli, cause disease in animals and plants. This is a result of the bacteria releasing or secreting toxic chemicals that cause damage to the cells or tissues and result in diseases such as tetanus or cholera. While the immune system tries to kill most bacteria by itself, humans have developed medicines known as antibiotics that are designed to kill bacteria.

Did you know … ?

Your body is actually made up of ten times as many bacteria cells as human cells! Bacteria live both inside you, as well as all over your skin. However, their small size means they only make up about 1 percent of total body mass.

What are real-life examples of bacteria?

This image shows a zoomed-in photo of a species of bacteria known as Clostridioides difficile, which causes diarrhea in humans.

Many people are most familiar with bacteria from the “bad eggs” that cause disease in humans.

What other words are related to bacteria?

Quiz yourself!

True or False?

Bacteria only have a single cell.

How to use bacteria in a sentence

British Dictionary definitions for bacteria

/ (bækˈtɪərɪə) /

pl n singular -rium (-rɪəm)
a very large group of microorganisms comprising one of the three domains of living organisms. They are prokaryotic, unicellular, and either free-living in soil or water or parasites of plants or animalsSee also prokaryote

Derived forms of bacteria

bacterial, adjectivebacterially, adverb

Word Origin for bacteria

C19: plural of New Latin bacterium, from Greek baktērion, literally: a little stick, from baktron rod, staff
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

Scientific definitions for bacteria

[ băk-tîrē-ə ]

Plural of bacterium.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for bacteria


sing. bacterium

Microorganisms made up of a single cell that has no distinct nucleus. Bacteria reproduce by fission or by forming spores.

notes for bacteria

Some bacteria are beneficial to humans (for example, those that live in the stomach and aid digestion), and some are harmful (for example, those that cause disease).
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.