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. )

Example Sentences

Raw, or unpasteurized, dairy has not undergone the heating process that kills harmful bacteria while leaving nutrients largely intact.

From Los Angeles Times • Apr. 2, 2026

Proponents of raw dairy have long insisted that it prevents allergies and promotes beneficial bacteria, claims that are not supported by research.

From Los Angeles Times • Apr. 2, 2026

If bacteria stop growing, the antibiotic in that tube is effective.

From Science Daily • Mar. 31, 2026

Scientists at TU Wien, working with data storage company Cerabyte, produced a QR code measuring just 1.98 square micrometers, which is smaller than most bacteria.

From Science Daily • Mar. 29, 2026

Other times, he asked for tea and then went on to examine the sugar bowl and the tin of milk, muttering, “The capabilities of bacteria are quite extraordinary.”

From "Half of a Yellow Sun" by Chimamanda Ngozi Adichie