The Collaborative International Dictionary of English v.0.48:

Microbe \Mi"crobe\, Microbion \Mi*cro"bi*on\, n. [NL. microbion,
   fr. Gr. ? little + ? life.] (Biol.)
   A microscopic organism; a microorganism; -- particularly
   applied to bacteria and especially to pathogenic forms; as,
   the microbe of fowl cholera.
   [1913 Webster] microbial

WordNet (r) 3.0 (2006):

microbe
    n 1: a minute life form (especially a disease-causing
         bacterium); the term is not in technical use [syn:
         microbe, bug, germ]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "microbe":
   adenovirus, aerobe, aerobic bacteria, aerobic organism, amoeba,
   anaerobe, anaerobic bacteria, anaerobic organism,
   autotrophic organism, bacillus, bacteria, bacterium, being, bug,
   coccus, creature, crumb, disease-producing microorganism, dot,
   drop, droplet, echovirus, enterovirus, filterable virus, fleck,
   flyspeck, fungus, genetic individual, germ, gnat, grain,
   gram-negative bacteria, gram-positive bacteria,
   heterotrophic organism, individual, iota, jot, living being,
   living thing, microorganism, midge, minim, minutia, minutiae, mite,
   mold, morphological individual, mote, nonfilterable virus, ont,
   organic being, organism, organization, particle, pathogen,
   physiological individual, picornavirus, pinhead, pinpoint, point,
   protozoa, protozoon, reovirus, rhinovirus, rickettsia, scrap, snip,
   snippet, speck, spirillum, spirochete, spore, staphylococcus,
   streptococcus, tittle, trypanosome, vanishing point, vibrio, virus,
   zooid, zoon