ba·cil·lus 
(bə-sĭləs)
(bə-sĭləs)n. pl. ba·cil·li (-sĭlī′)
1. Any of various bacteria, especially a rod-shaped bacterium.
2. Any of various rod-shaped, spore-forming, aerobic bacteria of the genus Bacillus that often occur in chains and include B. anthracis, the causative agent of anthrax.
[Late Latin, diminutive of Latin baculum, rod; see bak- in the Appendix of Indo-European roots.]