< /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

plural of bacillus (q.v.).


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.

sing. bacillus

One of three forms of bacteria, usually rod-shaped.

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