Fig. 5

Slave tissue hypothesis. Microbial tissue is the additional fundamental tissue of the human body, a slave tissue alongside nervous, epithelial, connective and muscular tissues.^701,702 The maternal microbiota exerts a regulatory influence on fetal growth and development and can partially transfer seed microbiota to the newborn through microbial exposure. Microbes that colonize in body site (including but not limited to the gastrointestinal tract, respiratory tract, reproductive tract, skin and urinary tract) play a vital role in digestion, immunity, neural regulation and metabolic crosstalk throughout human growth and ageing, and ultimately participate in the degradation of the body upon death.^{37,415,703,704} The human microbiota has undergone co-speciation, co-evolution, co-adaptation, and co-diversification with humans over a long period of time.⁵³ Throughout the life cycle, factors such as mode of delivery, genetics, gender, diet, medication, environment and behavior (e.g. exercise) can potentially contribute to differential microbial tissue formation^705,706,707