At the heart of gut health is the gut microbiome — the large and complex community of bacteria living mostly in the large intestine. A healthy microbiome is not about having one “good” bacterium in large amounts. It is about balance, variety, and cooperation between many species. Scientists often compare it to a forest. A forest made up of many different plants is stable and self-supporting, while a forest built around a single species is fragile and easily disturbed.