I don't see too much microbiology being practiced in a dental office like it is in a research lab.
Well firstly the study of microbiology is dentistry - bacteria, plaque, etc
but I myself am doing molecular biology, and am putting orthodontic appliances on mice, something that has shown my dexterity quite clearly to dental schools, as well as subsequent knowledge of the periodontal tissues, their structure, function, and specific periodontal cells in immense detail.
It may seem that sometimes a Masters degree in science can take you even further from the sciences required for dentistry, but if you find the right area it can be very beneficial.
Then again I am sure business would be crazy helpful, and something that I know lots of medical professionals struggle with, so you could have the leg up.
Good luck with the masters whichever route you choose, no matter what you pick I think a Masters will make you stand otu from other applicants