< /bəˈsɪl aɪ/.

  1. any rod-shaped or cylindrical bacterium of the genus Bacillus, comprising spore-producing bacteria.
  2. (formerly) any bacterium.

noun plural -cilli (-ˈsɪlaɪ)

  1. any rod-shaped bacterium, such as a clostridium bacteriumCompare coccus (def. 2), spirillum (def. 1)
  2. any of various rodlike spore-producing bacteria constituting the family Bacillaceae, esp of the genus Bacillus

1877, medical Latin, from Late Latin bacillus “wand,” literally “little staff,” diminutive of baculum “a stick,” from PIE root *bak- “staff,” also source of Greek bakterion (see bacteria). Introduced as a term in bacteriology 1853 by German botanist Ferdinand Cohn (1828-1898).

n. pl. ba•cil•li (-sĭl′ī′)

  1. Any of various rod-shaped, usually gram-positive aerobic bacteria of the genus Bacillus that often occur in chains and include Bacillus anthracis, the causative agent of anthrax.
  2. Any of various bacteria, especially a rod-shaped bacterium.


  1. A genus of rod-shaped gram-positive bacteria capable of producing endospores.

Plural bacilli (bə-sĭl′ī′)

  1. Any of various pathogenic bacteria, especially one that is rod-shaped.
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