Bacteria
mid 19th century: modern Latin, from Greek baktērion, diminutive of baktēria ‘staff, cane’ (because the first ones to be discovered were rod-shaped). Compare with bacillus.
wiktionary
From New Latin bacteria, plural of bacterium, from Ancient Greek βακτήριον(baktḗrion), neuter diminutive of βακτηρία(baktēría, “rod, stick”) (cognate with English peg).
From New Latin bacteria, from Ancient Greek βακτηρία(baktēría, “rod, stick”).
etymonline
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."
