Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. 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 in plants or animals.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A single celled organism with no nucleus.

from the GNU version of the Collaborative International Dictionary of English

  • n. A microscopic single-celled organism having no distinguishable nucleus, belonging to the kingdom Monera. Bacteria have varying shapes, usually taking the form of a jointed rodlike filament, or a small sphere, but also in certain cases having a branched form. Bacteria are destitute of chlorophyll, but in those members of the phylum Cyanophyta (the blue-green algae) other light-absorbing pigments are present. They are the smallest of microscopic organisms which have their own metabolic processes carried on within cell membranes, viruses being smaller but not capable of living freely. The bacteria are very widely diffused in nature, and multiply with marvelous rapidity, both by fission and by spores. Bacteria may require oxygen for their energy-producing metabolism, and these are called aerobes; or may multiply in the absence of oxygen, these forms being anaerobes. Certain species are active agents in fermentation, while others appear to be the cause of certain infectious diseases. The branch of science with studies bacteria is bacteriology, being a division of microbiology. See bacillus.

from The Century Dictionary and Cyclopedia

  • n. One of the micro-organisms which are concerned in the putrefactive processes, and are known as Schizomycetes, or fission fungi, in distinction from Saccharomycetes, or budding fungi, which produce alcoholic fermentation.
  • n. A genus of microscopic fungi, consisting of a single short cylindrical or elliptical cell, or of two such cells united end to end, and capable of spontaneous movement.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. (microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants

Etymologies

New Latin bactērium, from Greek baktērion, diminutive of baktron, rod.
(American Heritage® Dictionary of the English Language, Fourth Edition)
From New Latin, from Ancient Greek βακτήριον (bakterion, "small staff") + -ium. (Wiktionary)

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