hello there!

If you are from India you need to complete your Post graduation in relevant subject with at least 55% marks, or B+ grade or equivalent in 7 or 10 point scale CGPA. The relevant subject definition is changes as per interpretation by the authorities in which university you are taking admission.

Next step is to qualify the entrance test that is Ph.D. Entrance test or eligibility test. If you clear any othe NET or ICMR or JRF exams, then you will be exempted form the PET exam.

Then go to the university site and find out institutes of that university where you want to work and know about the guides. Approach the institute and enquire if they conduct their own entrance and or interview for admission. Apply there if they have vacancy. and then based on your performance in admission exam and or interview you will be enrolled.

The steps are

Post graduation in Microbiology or related subject

PET or NET or any other Ph.D qualifying examination

i hope this helps

goodluck!