"unicellular microorganisms which lack an organized nucleus," and sometimes cause disease, 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."
"rod-shaped bacterium," 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 (see -cule). It was introduced as a term in bacteriology in 1853 by German botanist Ferdinand Cohn (1828-1898).