I want to pursue Ph.D degree in Microbiology. Is it a compulsion to have our own research topic? What will happen if I do not have a topic?

Yes, you will need to have your own research topic for a PhD in Microbiology: 

Research is central: A PhD is a research degree, and you'll get to choose a topic you're interested in. You'll be able to dive deep into the topic and contribute to the body of research in your field. 

Research training:

For a Ph.D. in Microbiology, having a research topic is important but not always mandatory when you apply. Many universities allow students to join the program first and then choose or refine their research topic with the help of their supervisor.

If you don't have a specific topic when applying, you