Hello Josh,
Different college having different time to conduct entrance exam for Msc in microbiology. Most of the universities conduct entrance exam just after 3year bachelor's degree final examination. Here I'm sharing some entrance exam details.

• IISc Entrance Exam: Held by IISc in April every year, it is a national level entrance test to take admission for post graduate program at IISc. This test asks questions from Mathematics, Chemistry, Ecological Sciences and Biological Sciences

• Osmania University Entrance Exam: Conducted every year.

• Goa University Entrance Exam: This entrance examination is held every year. This test covers questions from Mathematics, Chemistry, Ecological Sciences and Biological Sciences.

Hope it helps
All the best!