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The mucosal surface of the intestinal tract represents the largest interface between the body and the environment. An effective local immune system is necessary to protect the organism against the invasion of noxious antigens and microbes. A tremendous amount of antibodies is secreted into the intestinal lumen o neutralise and exclude harmful antigens. Yet along with the nutritional uptake of polypeptides antigens of immunological relevance continuously penetrate the intestinal barrier. These nutritional antigens are tolerated by the immune system of the body, i. e. no antibody-production is induced against them.While it has been shown that immune stimulation can be observed after oral application of some species of lactic acid bacteria, animal experiments provided evidence that intestinal colonisation by indigenous bacteria contributes to the development and maintenance of oral tolerance. At present it is unclear which microbial species have immune suppressing properties, what mechanisms this immune modulation is based on and why oral tolerance obviously is restricted to non-pathogen antigens.In the present study association-experiments have been conducted with gnotobiotic rats to test the hypothesis that one dominant human gut bacterium is able to modulate the specific immune reaction to another indigenous microbial species. The two bacteria engaged were Bifidobacterium adolescentis (gram-positive) and Bacteroides thetaiotaomicron (gram-negative). Each species was tested on its own to investigate intestinal colonisation and antibody-production of the rats without mutual influence of the other microbial species. Furthermore the impact of B. adolescentis on the immune reaction to B. thetaiotaomicron and on the cell counts of the gram-negative bacterium was investigated by disassociating the animals with both bacteria.To monitor intestinal colonisation, cell counts were determined in rats" feces at regular intervals. At the end of the experiments gut contents were taken from different regions of the small and large intestine to obtain information about the intestinal distribution of the bacteria. To investigate the mucosal immune reaction induced by the bacteria, specific IgA levels were determined in the fecal samples and in the gut contents of the rats. Besides intestinal colonisation and local immune defence the systemic immune reaction was observed as well to levels of circulating serum IgA and IgG. With simultaneous application of both bacterial species, development of bacterial cellcounts obviously was unchanged in comparison to colonisation of monoassociated animals. Yet preceded association of B. adolescentis hampered later colonisation by B. thetaiotaomicron for a limited period of time. While the application of B. thetaiotaomicron was followed by the production of circulating antibodies, B. adolescentis obviously did not challenge any systemic immune reaction. Yet both microbial species caused a local intestinal immune reaction. Furthermore, if diassociated with both bacteria the specific immune reaction against B. thetaotaomicron was remarkably lower. The amount of fecal antibodies as well as the levels of circulating IgG and IgA were diminished in diassociated animals. This might be indicative for a down-regulation of the specific immune response against B. thetaiotaomicron.Further investigation is needed to evaluate if other bacterial species modulate the immune system to the same extant as B. adolescentis or if the observed immune suppression is a particular attribute to this microbial species. Besides this the impact of B. adolescentis on the immune reaction against pathogen microorganisms should be investigated to find out if it is possible to use B. adolescentis for probiotic treatment.