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.

---

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.

---

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

---

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.

---

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”; bacterium

Origin of Bacteria²

First recorded in 1860–65; from New Latin; 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

These conditions can facilitate the spread of viruses, bacteria and parasites.

From Los Angeles Times • Apr. 13, 2026

Beneficial bacteria grow more slowly, which can lead to an imbalance known as dysbiosis and allow the disease to return.

From Science Daily • Apr. 13, 2026

The Los Angeles County Public Health Department is warning beachgoers to avoid all contact with the water due to the potential of bacteria from street runoff from recent rains.

From Los Angeles Times • Apr. 13, 2026

Proponents of raw milk say that pasteurization, a heating process that kills harmful bacteria, removes natural health benefits and that warnings around the consumption of raw milk are overstated.

From The Wall Street Journal • Apr. 11, 2026

If any fish had managed to survive the eruption, they didn’t last long— bacteria began to grow rampantly in the hours after the lake was changed forever.

From "Mountain of Fire" by Rebecca E. F. Barone