The physiological intestinal flora consists of numerous bacterial species that live together with the host in a complex symbiotic ecosystem. Shortly after birth and the suckling phase, the gastrointestinal flora is established and remains largely stable for the rest of the life.
However, within the intestinal tract, there are considerable differences in distribution. While pathogen counts in the duodenum and the jejunum are rather low due to the influence of gastric acid, bile and pancreatic enzymes as well as the present mucosal defence systems, they massively increase in the ileocecal area and reach their highest concentration in the large intestine. The number of anaerobes and facultative anaerobic pathogens is 1,000 to 10,000 times higher than the number of aerobic microflora. The highest concentrations are reached by Bacteroides spp., lactobacilli and bifidobacteria as well as Enterobacteriaceae.