If you absolutely cannot pay a single dollar:
If institutional access isn't an option, you can find the textbook through official channels at significantly reduced rates: Brock Biology Of Microorganisms 16th Edition -FREE-
If you prefer a physical copy, purchasing a used 16th edition can save you significant money. Alternatively, you can ask your professor if using the 15th edition is acceptable; older physical copies are highly discounted, though you will need to cross-reference chapter outlines with a classmate to ensure you do not miss newer material. Conclusion If you absolutely cannot pay a single dollar: