bacillus

noun pl. bacilli

1. any of a genus (Bacillus) of rod-shaped bacteria that occur in chains, produce spores, and are active only in the presence of oxygen
2. any rod-shaped bacterium
3. loosely any of the bacteria, esp. those causing disease

Origin: ModL < LL, little rod < L bacillum, dim. of baculus, var. of baculum, a stick < IE base *bak-, staff > peg, Gr baktron