What does bacilli mean?
Bacilli is the plural of bacillus and refers to rod-shaped bacteria, a group that includes some of the most historically significant microbes known to medicine. The name comes from the Latin bacillum, meaning 'little staff', reflecting the elongated shape these organisms take under the microscope. In scientific and medical writing, bacilli appear constantly — from descriptions of gram-negative bacilli in laboratory reports to Koch's famous identification of the tubercle bacillus as the cause of tuberculosis. While the term is precise, it is sometimes used more loosely to mean disease-producing germs generally, though 'bacteria' serves that broader purpose in ordinary speech. Because Bacillus is also a formal genus in bacteriology, careful writers distinguish between the technical taxonomic use and the general morphological sense of the word. Familiarity with bacilli rewards anyone reading health journalism, microbiology, or historical accounts of infectious disease.
noun (plural)Rod-shaped bacteria; the plural of 'bacillus'. In loose usage, any bacteria of rod-like form, especially when disease-causing.
- Rod-shaped bacteria (plural of bacillus), used especially in medical and scientific contexts.
- Loosely, disease-causing germs of any kind, in older or informal usage.
"The scientist stained the slide and examined the bacilli under the microscope."
"Robert Koch identified the tubercle bacilli that cause tuberculosis."
"The laboratory report noted the presence of gram-negative bacilli in the sample."
'Bacilli' is itself the standard plural of the singular noun 'bacillus'; there is no further plural form.
"One bacillus can multiply into countless bacilli within hours."
The word 'bacillus' comes from the Latin for 'little walking stick' — because that's exactly what these bacteria look like under a microscope.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of bacilli
Bacilli derives from the Latin bacillus, a diminutive of baculum meaning 'staff' or 'walking stick'; the term was adopted into scientific nomenclature in the nineteenth century to describe the rod-like shape of certain bacteria. The same Latin root gives English 'bacillary', an adjective describing things shaped like or relating to bacilli, and, more surprisingly, contributed through figurative extension to the word 'imbecile', which originally evoked a weakened or unsupported 'staff'. Cognate forms appear across Romance languages descended from Latin.
Related word forms
How bacilli is actually used
'Bacilli' is the standard plural of the noun 'bacillus' and is used almost exclusively in scientific, medical, and technical writing. In everyday speech, people are more likely to say 'bacteria' or 'germs'. Note that 'Bacillus' is also a formal genus name in bacteriology, written with an italicized capital B.
Easily confused with bacilli
'Bacillus' is the singular form referring to one rod-shaped bacterium, while 'bacilli' is its plural.
'Bacteria' is the general term for all such microorganisms, whereas 'bacilli' refers specifically to those with a rod shape.