bacillus n.

"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."

word-forming element used to make diminutives, from French -cule or directly from Latin -culus (masc.), -cula (fem.), -culum (neuter); these appear to be variants of the diminutive suffix -ulus (see -ule) used after -i-, -e-, -u-, and consonant stems [Gildersleeve], or might be a double-diminutive involving "an ancient diminutive suffix *-qo-" [Palmer, "The Latin Language"].

There also was a Latin instrumentive suffix -culo-, -culum in baculum "walking stick," gubernaculum "rudder, helm; management, government," operculum "cover, lid," obstaculum "a hindrance, obstacle," oraculum "divine announcement."