If you absolutely cannot pay a single dollar:
If you use a 15th-edition PDF, you will miss exam questions about Omicron, new antibiotic resistance mechanisms, and updated classification of Archaea. Brock Biology Of Microorganisms 16th Edition -FREE-
If your library doesn’t own it, request an ILL. They will scan chapters or send a physical copy from another university – often free or for a small fee ($2–$5). If you absolutely cannot pay a single dollar: