The Collaborative International Dictionary of English v.0.48:

Microbiology \Mi`cro*bi*ol"o*gy\, n. [See Microbe; -logy.]
   The branch of biology studying minute organisms, or microbes,
   such as the bacteria, fungi, viruses, and protozoa. --
   Mi`cro*bi`o*log"ic*al, a. -- Mi`cro*bi*ol"o*gist, n.
   [Webster 1913 Suppl. +PJC] Microcephalic

WordNet (r) 3.0 (2006):

microbiology
    n 1: the branch of biology that studies microorganisms and their
         effects on humans

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "microbiology":
   aerobiology, agrobiology, anatomy, astrobiology, bacteriology,
   biochemics, biochemistry, biochemy, bioecology, biological science,
   biology, biometrics, biometry, bionics, bionomics, biophysics,
   botany, cell physiology, cryobiology, cybernetics, cytology,
   ecology, electrobiology, embryology, enzymology, ethnobiology,
   exobiology, genetics, gnotobiotics, life science,
   molecular biology, pharmacology, physiology, radiobiology,
   taxonomy, virology, xenobiology, zoology