Distribution of gut microbiota across intestinal segments and their impact on human physiological and pathological processes

In recent years, advancements in metagenomics, metabolomics, and single-cell sequencing have enhanced our understanding of the intricate relationships between gut microbiota and their hosts. Gut microbiota colonize humans from birth, with their initial composition significantly influenced by the mode of delivery and feeding method. During the transition from infancy to early childhood, exposure to a diverse diet and the maturation of the immune system lead to the gradual stabilization of gut microbiota's composition and distribution. Numerous studies have demonstrated that gut microbiota can influence a wide range of physiological functions and pathological processes by interacting with various tissues and organs through the gut-organ axis. Different intestinal segments exhibit unique physical and chemical conditions, which leads to the formation of vertical gradients along the intestinal tract: aerobes and facultative aerobes mainly live in the small intestine and anaerobic bacteria mainly live in the large intestine, and horizontal gradients: mucosa-associated microbiota and lumen-associated microbiota. In this review, we systematically summarize the distribution characteristics of gut microbiota across six intestinal segments: duodenum, jejunum, ileum, cecum, colon, and rectum. We also draw a conclusion that gut microbiota distributed in different intestinal segments affect the progression of different diseases. We hope to elucidate the role of microbiota at specific anatomic sites within the gut in precisely regulating the processes of particular diseases, thereby providing a solid foundation for developing novel diagnostic and therapeutic strategies for related diseases.