The Collaborative International Dictionary of English v.0.48:

Bacillus \Ba*cil"lus\, n.; pl. Bacilli. [NL., for L. bacillum.
   See Bacillari[ae].] (Biol.)
   A variety of bacterium; a microscopic, rod-shaped vegetable
   organism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bacillus
    n 1: aerobic rod-shaped spore-producing bacterium; often
         occurring in chainlike formations; found primarily in soil
         [syn: bacillus, B]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "bacillus":
   adenovirus, aerobe, aerobic bacteria, amoeba, anaerobe,
   anaerobic bacteria, bacteria, bacterium, bug, coccus,
   disease-producing microorganism, echovirus, enterovirus,
   filterable virus, fungus, germ, gram-negative bacteria,
   gram-positive bacteria, microbe, microorganism, mold,
   nonfilterable virus, pathogen, picornavirus, protozoa, protozoon,
   reovirus, rhinovirus, rickettsia, spirillum, spirochete, spore,
   staphylococcus, streptococcus, trypanosome, vibrio, virus