For several years, scientists worldwide have been investigating the extent to which microorganisms living in and on the human body influence central life processes and thus health and disease. Today they assume that there is a connection between the totality of the microbial colonization in the human body, called the microbiome, and the development of diseases. Chronic inflammatory bowel disease (CIBD) in particular is suspected to be closely linked to the composition and (im-)balance of the intestinal microbiome. However, the causal relationship between the microbiome and the development of disease and determining factors of the composition of the microbiome in the individual are still largely unexplained.