The microbiome is the total of all microorganisms that colonise the body surfaces and the inside of an animal. Most of them are localised in the colon (1011 – 1012 bacteria/g of faeces). 99.9% of the bacteria that grow under these conditions are anaerobes. The main function of these organisms is the nutrition and protection of the mucous membrane. If there are bacterial imbalances, these tasks can only be fulfilled insufficiently. As a result, colonisation resistance is reduced and increased colonisation with obligatory or facultative pathogens takes place. Due to the reduced barrier function of the mucosa, antigens, endotoxins and histamine, amongst others, can pass from the intestinal lumen into the bloodstream and thus initiate or aggravate pathomechanisms.