UPSC conducts CDS exam twice every year. Candidate have to be at least a graduate in order to apply. Final year students can also apply for this but have to provide proofs. Other eligibility criteria such as nationality, educational qualification, age, marital status, and physical conditions should be checked before applying.
A microbiology graduate can apply for IMA and OTA. The educational qualification to enter in these two jobs is degree or equivalent in any subject. For INA you need a degree in engineering and for Air Force Academy you need either an engineering degree or a degree in science subjects with physics and mathmatics(core or subsidiary). Remember the selection is based on different criteria as mentioned above. Educational qualification is only one of them. Check on the official website for further details. Good luck!