L. bacterium is the latinisation of the Greek bakterion = staff, cane.
The first bacteria observed by Dutch microscopist Antonie van Leeuwenhoek in 1676, were rod-shaped. The term was introduced as a scientific word in 1838 by German naturalist Christian Gottfried Ehrenberg. A classical plural sometimes also erroneously used as a singular.
Ehrenberg also coined the term “Bacillus”, a genus of spore-forming, rod-shaped bacteria.