Passage Summary: Because most microbes rely on each other and cannot be grown alone in a lab, scientists haven't been able to learn everything about them.

Conclusion: Microbiologists do not possess a thorough understanding of most species of microbes.

Reasoning: Most microbes live in complex, interdependent communities and currently cannot be grown individually in a laboratory setting.

Analysis: There is a logical gap between the physical inability to grow a microbe in isolation and the intellectual state of 'lacking complete knowledge.' The argument assumes that isolation in a lab is a necessary requirement for complete understanding. To make this conclusion follow logically, we need an assumption that explicitly links these two concepts—essentially stating that if you can't cultivate it alone, you can't fully know it.