Etymology
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bacteria (n.)

1847, plural of Modern Latin bacterium, from Greek bakterion "small staff," diminutive of baktron "stick, rod, staff, cudgel." So called because the first ones observed were rod-shaped. Introduced as a scientific word 1838 by German naturalist Christian Gottfried Ehrenberg. A classical plural sometimes also erroneously used as a singular.

The Greek word is from a PIE *bak- "staff used for support, peg" (compare Latin baculum "rod, walking stick;" Irish bacc, Welsh bach "hook, crooked staff;" Middle Dutch pegel "peg, pin, bolt"). De Vaan writes, "Since *b was very rare in PIE, and Celtic shows an unexplained geminate, we are probably dealing with a loanword from an unidentified source."

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bacterial (adj.)
"of or pertaining to bacteria," 1869, from bacteria + -al (1).
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bacteriology (n.)
"scientific study of microbes," 1884, from German; see bacteria + -ology. Related: Bacteriological (1886); bacteriologist. Bacteriological warfare is from 1924.
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archaebacteria (n.)
1977, from archaeo- "ancient" + bacteria. Singular is archaebacterium.
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eubacteria (n.)
singular eubacterium, 1939, coined in German 1930; see eu-, here meaning "good," + bacteria. Classically, as an adverb, eu should form compounds only with verbs.
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bacteriophage (n.)

"virus that parasitizes a bacterium by infecting it and reproducing inside it," 1921, from French bactériophage (1917), from bacterio-, combining form of bacteria, + -phage.

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bacillus (n.)
1877, medical Latin, from Late Latin bacillus "wand," literally "little staff," diminutive of baculum "a stick, staff, walking stick," from PIE *bak- "staff" (also source of Greek bakterion; see bacteria) + instrumentive suffix -culo. Introduced as a term in bacteriology 1853 by German botanist Ferdinand Cohn (1828-1898).
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brucellosis (n.)
1930, Modern Latin, from Brucella, name of the bacteria that causes it, which is named for Scottish physician Sir David Bruce (1855-1931), who in 1887 discovered the bacteria, + -osis.
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phage (n.)
virus that destroys bacteria, 1917, an abbreviated form of bacteriophage.
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