What does bacillus mean?
A bacillus is any bacterium whose cells have an elongated, rod-like shape, distinguishing it from spherical cocci and spiral spirilla. The word carries two related senses: broadly, it describes any rod-shaped bacterium, while narrowly it names members of the genus Bacillus, a group of hardy, spore-forming organisms that includes Bacillus anthracis, the cause of anthrax, and Bacillus subtilis, a common soil bacterium. Coined in the nineteenth century during the rise of microbiology, the term became central to germ theory: scientists such as Robert Koch used bacilli to demonstrate that specific microbes cause specific diseases, including tuberculosis. In modern usage, 'bacillus' appears mostly in medical, laboratory, and educational contexts, and it retains a faintly clinical connotation, since many famous disease-causing bacteria are bacilli — though many others are harmless or beneficial. Its plural is 'bacilli', and its adjective form is 'bacillary'.
nounA rod-shaped bacterium; also, in strict taxonomic use, a member of the genus Bacillus of spore-forming bacteria.
- Any rod-shaped bacterium, used loosely in medicine and general science.
- Strictly, a member of the bacterial genus Bacillus, such as Bacillus anthracis or Bacillus subtilis.
"The scientist examined the slide under the microscope and identified the characteristic rod shape of a bacillus."
"Robert Koch's identification of the tubercle bacillus in 1882 transformed the study of infectious disease."
"Bacillus thuringiensis is widely used as a natural insecticide in organic farming."
The plural follows the Latin second declension and is standard in all registers; 'bacilluses' is nonstandard.
"The culture plate showed colonies of both cocci and bacilli."
When German scientist Ferdinand Cohn coined 'bacillus' in 1872, he borrowed a Latin word for a little walking stick — because these bacteria look exactly like tiny rods under the microscope.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of bacillus
The word 'bacillus' comes from Latin, where it was a diminutive meaning 'little staff' or 'small walking stick', formed from 'baculum', meaning a rod or staff. The German botanist Ferdinand Cohn adopted it in 1872 as a scientific term after observing that these bacteria resemble tiny rods under the microscope. The same Latin root survives in English 'baculum', the anatomical name for a bone found in the penis of many mammals.
Related word forms
How bacillus is actually used
In scientific writing, lowercase 'bacillus' describes any rod-shaped bacterium generically, while capitalized italicized 'Bacillus' names the formal genus. The standard plural 'bacilli' follows the Latin second-declension pattern. The word belongs chiefly to medical, microbiological, and scientific register rather than everyday speech.
Easily confused with bacillus
A bacillus is a rod-shaped bacterium — a single-celled organism — whereas a virus is a much smaller infectious agent that can only reproduce inside living cells.
'Bacteria' refers collectively to the whole group of microorganisms, while 'a bacillus' denotes one specific rod-shaped individual or type.