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singular of bacteria.
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

Pertussis, or “whooping cough,” is caused by the bacterium Bordetella pertussis.
Thanks to Anti-Vaxxers, Mumps Are Back. What’s Next? Russell Saunders March 19, 2014

Botulism is caused by the bacterium Clostridium botulinum, a hardy anaerobe.
The Deadliest Botox Has Arrived Kent Sepkowitz October 17, 2013

One type of bacterium is likely very different from its neighbors, and may have equally different effects on the body.
Your Probiotic Is Probably B.S. Carrie Arnold June 24, 2014

The bacterium is technically a pathogen, so the USDA looks at it.
Plants That Glow in the Dark Spark Heated Debate Josh Dzieza August 17, 2013

Caused by the bacterium Borrelia burgdorferi, the illness is transmitted by the bite of an infected tick.
Predator Doctors Take Advantage of Patients With ‘Chronic Lyme’ Scam Russell Saunders September 18, 2014

Historical Examples

The bacterium becomes constricted, divides, and finally there are two cells instead of one.
Canned Fruit, Preserves, and Jellies: Household Methods of Preparation Maria Parloa

Virus or bacterium, amoeba or fungus—whatever it was, it struck.
Despoilers of the Golden Empire Gordon Randall Garrett

It was in May of 1884 that he was able to announce the discovery of the coma bacillus, that is, the bacterium of cholera.
Notable Events of the Nineteenth Century Various

At the end of three days there is not a bacterium to be found in it.
Fragments of science, V. 1-2 John Tyndall

The infectious agent is a bacterium first described by the Danish investigators, Bang and Stribolt.
Contagious Abortion of Cows Ward J. MacNeal

noun
the singular of bacteria
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
n.

c.1848, singular of bacteria (q.v.).
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).

bacterium bac·te·ri·um (bāk-tēr’ē-əm)
n. pl. bac·te·ri·a (-tēr’ē-ə)
Any of the unicellular, prokaryotic microorganisms of the class Schizomycetes, which vary in terms of morphology, oxygen and nutritional requirements, and motility, and may be free-living, saprophytic, or pathogenic, the latter causing disease in plants or animals.

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).