## TL;DR

The gut microbiome is the ecosystem of ~39 trillion bacteria in the human digestive tract — roughly equal to human cells. It influences digestion, immunity, mental health (gut-brain axis), metabolism, and disease risk. Shaped by diet (fiber feeds beneficial bacteria), antibiotics (disrupt), birth method (vaginal vs. C-section).

## Core Explanation

Diversity: more diverse microbiome = generally healthier. Prebiotics: fiber that feeds beneficial bacteria (garlic, onions, bananas). Probiotics: live bacteria (yogurt, kimchi, supplements). Fecal microbiota transplant (FMT): transferring stool from healthy donor — 90%+ effective for C. difficile infections. Gut-brain axis: gut produces 95% of body's serotonin. 'You are what you eat' — your microbiome reflects your diet.

## Further Reading

- [I Contain Multitudes (Ed Yong)](https://www.harpercollins.com/products/i-contain-multitudes-ed-yong)