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]