How do I join the Army as a microbiologist if I'm currently one as a civilian?

Since you tagged it, I suspect you might already know that you want to discuss this with a medical recruiter, not the standard one. If there isn't one nearby, contact the Medical Recruiting Battalion* that covers your geographical area and ask to speak to one.

Yes, you'll go the "AMEDD route". Microbiologists (71As) are part of the the Army's Medical Command's (MEDCOM) Medical Services Corp* and are within the Medical Allied Sciences medical functional area (MFA 71). You're likely looking at a direct commission as a 71A if you meet the eligibility. Again, the medical recruiter can go through all the details with you, but at first glance you appear to meet the eligibility (I assume your age is between 21 and 42).

●Master's degree in microbiology or related field from an accredited program acceptable to the Surgeon General
● At least one year of clinical laboratory experience in a Department of Defense health care facility, university or other facility
● Certification by either the American Society of Clinical Pathology or the National Certification Agency for Medical Laboratory Personnel
● Must be between 21 and 42 years of age
● Must be a U.S. citizen

So, foot-stomp again - talk to a medical recruiter.
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* https://recruiting.army.mil/mrb/
* Medical Services Corp - https://api.army.mil/e2/c/downloads/2023/05/03/844ac285/20220223-medical-service-corps.pdf

As you said, You will need to go the AMEDD route. Contact them and start the process.

Where you are at in your education, I would think you need to talk to these people. https://www.usuhs.edu/