bac·​te·​ria bak-ˈtir-ē-ə How to pronounce bacteria (audio)

plural of bacterium

diseases caused by bacteria
Overprescribing antibiotics can contribute to the development of antibiotic-resistant bacteria.Maanvi Singh

Note: Microscopic single-celled organisms lacking a distinct nucleus are known as bacteria. They may be shaped like spheres, rods, or spirals. They inhabit virtually all environments, including soil, water, organic matter, and the bodies of animals. Many bacteria swim by means of long whiplike structures called flagella. The DNA of most bacteria is found in a single, circular chromosome, and is distributed throughout the cytoplasm rather than contained within a membrane-enclosed nucleus. Though some bacteria can cause food poisoning and infectious diseases in humans, most are harmless and many are beneficial. They are used in various industrial processes, especially in the food industry (for example, in the production of yogurt, cheeses, and pickles).

Is bacteria singular or plural?: Usage Guide

In its established and uncontroversial uses, bacteria is the plural of bacterium.

… many of the bacteria isolated from these deep environments are anaerobic … Stephen Jay Gould

In speech and in some, typically nontechnical, journalistic writing, it also occurs in a singular sense, synonymous with bacterium.

Lyme disease is a potentially serious arthritis-like ailment caused by a bacteria borne by certain tiny ticks. The Wall Street Journal
… this bacteria is closely associated with poor health and, in old people, frailty … Leah Hardy

Although the singular use of bacteria is often identified as an error to be avoided, it is common in published writing. The plural form bacterias is also seen but is relatively rare.

The bacteria seems to prefer living in water and is more resistant to chlorine and elevated water temperatures than other bacterias. Allan Bruckheim

Examples of bacteria in a Sentence

Recent Examples on the Web LifeStraw Personal Water Filter The LifeStraw boasts a game-changing (or life-saving, in some cases) filtration system that filters out more than 99.99 percent of bacteria and parasites. Katie Jackson, Travel + Leisure, 24 Feb. 2024 Research has found that healthy gut bacteria like Bifidobacterium and Lactobacillus feast on dark chocolate. Selene Yeager, Health, 24 Feb. 2024 So was being a cigarette-smoker; the blood of smokers showed an unusually high inflammatory response to bacteria. Celia Ford, WIRED, 14 Feb. 2024 The Health Department says the Legionella bacteria can be found at low levels in any public water system. Staff Report, Twin Cities, 13 Feb. 2024 Septicemic plague occurs when the bacteria gets into the bloodstream. Sara Moniuszko, CBS News, 12 Feb. 2024 This also happens in the world of bacteria and antibiotics — antibiotic resistance is another major public health threat — but doctors still have many more antibiotics to choose from. Kaitlin Sullivan, NBC News, 11 Feb. 2024 The main types present in dairy are: Lactic acid bacteria: These friendly microbes are the workhorses of the dairy world. Ali Francis, Bon Appétit, 12 Feb. 2024 The chemical is thought to calm skin inflammation and get rid of bacteria—sort of a cheaper, easier-to-obtain alternative to antibiotics—for people with this chronic skin condition, who are prone to infections. Amy Norton, SELF, 8 Feb. 2024 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'bacteria.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


plural of bacterium

First Known Use

1864, in the meaning defined above

Time Traveler
The first known use of bacteria was in 1864

Dictionary Entries Near bacteria

Cite this Entry

“Bacteria.” Dictionary, Merriam-Webster, Accessed 28 Feb. 2024.

Kids Definition


plural of bacterium

Medical Definition


1 of 2

plural of bacterium


2 of 2 noun
bac·​te·​ria bak-ˈtir-ē-ə How to pronounce bacteria (audio)
: bacterium
not usually used technically
caused by a bacteria borne by certain tiny ticksWall Street Journal
a single bacteria—there are roughly 200 in each cough—apparently can infect a personCheryl Clark
plural capitalized : a domain in the system of classification dividing all organisms into three major domains of life that includes the prokaryotes that are bacteria but not those that are archaebacteria or archaea compare eubacteria

More from Merriam-Webster on bacteria

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