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• bacteria
  bacteria

  plural noun

  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.
• Bacteria
  Bacteria

  noun

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

bacteria

¹ American  

[bak-teer-ee-uh] / bækˈtɪər i ə /

plural noun

Microbiology.

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

Derived Forms

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

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

An analysis of his microbiome revealed a particular kind of a gut bacteria that is almost non-existent among modern humans.

From Barron's • Jun. 3, 2026

Though gone from the stomachs of people in the industrialised world, the bacteria has been detected among tribes in Africa and South America, Sarhan said.

From Barron's • Jun. 3, 2026

For 40 hours, Barnica waited in the hospital for the heartbeat to stop, with her cervix exposed to bacteria.

From Salon • May 27, 2026

A simple beetroot juice routine may help explain one of the more surprising links in healthy aging: the connection between bacteria in the mouth and blood pressure.

From Science Daily • May 25, 2026

The sting of the hand sanitizer was gone now, which meant the bacteria were back to breeding, spreading through my finger into the bloodstream.

From "Turtles All the Way Down" by John Green