Source : Webster's Revised Unabridged Dictionary (1913)

Microbiology \Mi`cro*bi*ol"o*gy\, n. [See {Microbe}; {-logy}.]
   The study of minute organisms, or microbes, as the bacteria.
   -- {Mi`cro*bi`o*log"ic*al}, a. -- {Mi`cro*bi*ol"o*gist}, n.

Source : WordNet®

microbiology
     n : the branch of biology that studies microorganisms and their
         effects on humans