bacteria

¹ American  

[bak-teer-ee-uh] / bækˈtɪər i ə /

plural noun

Microbiology.

singular

bacterium

1. 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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Bacteria ² American  

[bak-teer-ee-uh] / bækˈtɪər i ə /

noun

(used with a singular or plural verb)

1. (in the three-domain system of classification) the taxonomic domain comprising the bacteria.

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bacteria British  

/ bækˈtɪərɪə /

plural noun

1. 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 animals See also prokaryote

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bacteria Cultural  

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

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Usage

Plural word for bacteria Bacteria is the plural form of the singular bacterium. Because microscopic bacterium is most often found in multiples, the plural form bacteria is more commonly used. The plurals of several other singular words ending in -um are also formed this way, such as memorandum/memoranda and curriculum/curricula. The irregular noun bacterium’s plural derives directly from its original pluralization in Latin and Greek. Bacteria is sometimes treated as a singular collective noun. However, this is not standard in English, and bacteria should be treated as a plural form.

Discover More

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).

Other Word Forms

• bacterial adjective
• bacterially adverb
• nonbacterial adjective
• nonbacterially adverb

Etymology

Origin of bacteria¹

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

Origin of Bacteria²

First recorded in 1860–65; from New Latin; see origin at bacteria ( def. )

Explanation

Bacteria are microscopic living organisms, usually one-celled, that can be found everywhere. They can be dangerous, such as when they cause infection, or beneficial, as in the process of fermentation (such as in wine) and that of decomposition. In 1676, Anton Van Leeuwenhoek first observed bacteria through a microscope and called them “animalcules.” In 1838, the German Naturalist Christian Gottfried Ehrenberg called them bacteria, from the Greek baktḗria, meaning "little stick." An apt word, as the first observed bacteria were shaped like rods, although bacteria can also be spiral or spherical in shape. A grammar note: The word bacteria is the plural form of "bacterium" and so should be written as plural, as in "Many bacteria are harmless."

Example Sentences

The products on our bathroom shelves, marketed as clean, stripped away essential bacteria.

From The Wall Street Journal • Apr. 17, 2026

This work, carried out in collaboration with the University of York and the Rowland Institute at Harvard, highlights how bacteria can reuse existing biological systems in unexpected ways.

From Science Daily • Apr. 17, 2026

So I got a small grant, bought a swab kit and spent a semester knocking on doors to ask classmates if I could sample the bacteria on their faces.

From The Wall Street Journal • Apr. 17, 2026

Antibodies – similar to those the body makes to attack viruses or bacteria – have been engineered to spot the amyloid and clear it from the brain.

From BBC • Apr. 16, 2026

The membranes lining the inner compartment of mitochondria are unlike other animal cell membranes, and resemble most closely the membranes of bacteria.

From "The Lives of a Cell" by Lewis Thomas