The intestinal tract is not only the mother river that supplies nutrients and energy, but also the Great Wall indispensable in guarding the health of the whole body. The commensal gut microbes are like guardians and soldiers on the Great Wall for host defense. They participate in numerous aspects of host biology such as nutrient processing, immune regulation, and gut-brain interaction, and are implicated in various diseases with potential in clinical translation. (Adapted from The Mother River of Life: Gut Science and Art Illustration Collection)