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ubiquitous one-celled organisms, spherical, spiral, or rod-shaped and appearing singly or in chains, comprising the Schizomycota, a phylum of the kingdom Monera (in some classification systems the plant class Schizomycetes), various species of which are involved in fermentation, putrefaction, infectious diseases, or nitrogen fixation.
Contemporary Examples

bacterial pathogens include gonorrhea (yes) which can cause blindness in a matter of hours, and chlamydia.
Everything You Wanted To Know About Bob Costas’s Olympic Pinkeye Kent Sepkowitz February 11, 2014

During production, this washing procedure is repeated to keep the surface of the cheese moist and hospitable to bacterial growth.
Why We Love Stinky Cheese Stacey Slate January 13, 2010

The bacterial infection that cost Aimee Copeland her leg and hands is rare, but when it strikes, it can be deadly.
Necrotizing Fasciitis: Georgia Woman Fights Flesh-Eating Bacteria Casey Schwartz May 11, 2012

In your body, bacterial cells outnumber human cells by 10 to 1.
An Epidemic of Absence: Destroying the Bugs in Our Bodies Can Be Dangerous to Our Health Moises Velasquez-Manoff September 8, 2012

So far, at least 60 homeless people living in shelters in the area have contracted the bacterial respiratory infection since 2007.
L.A. Hunts for 300 Missing Tuberculosis Cases Christine Pelisek February 28, 2013

Historical Examples

A nucleus as such is not present in bacterial cells, except in a few large rare forms and in very young cells.
The Fundamentals of Bacteriology Charles Bradfield Morrey

What effect does cooking have upon the bacterial flora of foods?
Human Foods and Their Nutritive Value Harry Snyder

Take another similarly prepared pipette and aspirate into it equal volumes of washed cells, bacterial emulsion and pooled serum.
The Elements of Bacteriological Technique John William Henry Eyre

How far chemical and bacterial agencies have also been effective is not clear.
The Economic Aspect of Geology C. K. Leith

A bacterial disease that has made its appearance in California has not been seen in this state.
Walnut Growing in Oregon Various

plural noun (sing) -rium (-rɪəm)
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
adj.

1869, from bacteria + -al (1).
n.

1847, plural of Modern Latin bacterium, from Greek bakterion “small staff,” diminutive of baktron “stick, rod,” from PIE *bak- “staff used for support.” So called because the first ones observed were rod-shaped. Introduced as a scientific word 1838 by German naturalist Christian Gottfried Ehrenberg (1795-1876).

bacteria bac·te·ri·a (bāk-tǐr’ē-ə)
n.
Plural of bacterium.
bac·te’ri·al adj.
bacterium
(bāk-tîr’ē-əm)
Plural bacteria
Any of a large group of one-celled organisms that lack a cell nucleus, reproduce by fission or by forming spores, and in some cases cause disease. They are the most abundant lifeforms on Earth, and are found in all living things and in all of the Earth’s environments. Bacteria usually live off other organisms. Bacteria make up most of the kingdom of prokaryotes (Monera or Prokaryota), with one group (the archaea or archaebacteria) often classified as a separate kingdom. See also archaeon, prokaryote.

bacterial adjective

Our Living Language : It is important to remember that bacteria is the plural of bacterium, and that saying a bacteria is incorrect. It is correct to say The soil sample contains millions of bacteria, and Tetanus is caused by a bacterium.

bacteria
(bāk-tîr’ē-ə)
Plural of bacterium.

sing. bacterium

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

Note: 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).