Two concepts are very important for microorganisms in the intestinal microbiota: 1. species diversity, 2. number. Preserving diversity of the species and numerical balance is in favor of health. Preserving the balance (homeostasis) in both is of critical importance for mental and brain health. The metabolic products of bacteria, which are tens to hundreds of times more than the number of trillions of bacteria in the intestine, can create general systemic effects for our entire body. In summary, there is a two-way interaction between the intestinal microbiota and the brain. Microbiota balance disorders are the key to our mental health.