Microbiota and the human nature: know thyself

Summary Biology has been driven by the human desire for self‐knowledge. The discovery of our intimate symbiosis with microbes raises the question about our identity. A central issue is whether the microbiome associated with humans changes our phenotype in an observable way. As we deal with a great multitude of colonizing microbes and as even monozygotic twins differ substantially for their microbiome, we might deal with a dynamic system that is highly sensitive to initial conditions for which long‐term prediction are impossible according to chaos theory. The overall colonization of the human alimentary tract can be teleological rationalized by a strong antimicrobial activity in the proximal and a mutualistic but controlled relationship with the microbiome in the distal gut segments. Yet the association of a specific microbiome with physiological traits turned out to be complicated and became frequently only clear after microbiota transfer experiments into gnotobiotic mice as a reductionist approach. As pathogenic bacteria create human phenotypes by their presence, mutualistic bacteria create symptoms (phenotypes) by their absence as exemplified by lactobacilli in bacterial vaginosis.