Biology BS from Excelsior

[Yenisei]

For medical microbiology, look at Eastern Oregon University, and University of California Berkeley extension will also have some courses from that list.

If you do go with Excelsior, for their Genetics requirement, check and see if Empire State College's course (which is 4 credit hours) has a lab component so you can use it as your upper level lab and also meet the 20 credit requirement for the exam.

Thomas Edison in particular and Charter Oak offer flexibility when it comes to biology electives, which given your intention to take the Microbiology exam, is something you should consider unless you don't mind taking several more courses beyond Excelsior's requirements.

EDIT: Berkeley extension offers Immunology for 3 credits. Add 4 for Genetics from either ESC or U Colorado Denver and the 6 from the Molecular Biology certificate the OP mentioned above and probably 4 from Microbiology and 3 from Cell Biology and it looks like the OP is in the clear even without needing Medical Microbiology.