Can I do MBA after completing B.Sc in Microbiology? If yes, which field and how do I start?

You can go into Marketing field. Join a health care based company or hospital chain to apply your B.Sc skills. I am currently in Doon Business School pursuing my MBA.

Yes, you can start from sales and marketing. But your field is better than Sales and Marketing. You can go for M.Sc Microbiology that is best choice or your interest.