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On the basis of association-experiments with gnotobiotic rats, we described the immunogenicity of two selected bacterial species (Bifidobacterium adolescentis and Bacteroides thetaiotaomicron). B. adolescentis is a gram-positive lactic acid producing bacterium, strains of which are claimed to have probiotic properties. B. thetaiotaomicron is a gram-negative rod, autochthonous to the human as well as to the rats' intestinal tract. Colonization of the gut was monitored by determination of bacterial cell counts in the animals' feces. In order to investigate the systemic immune reaction, the amounts of specific serum-IgG and -IgA against both bacterial species were measured in the serum. The intestinal immune reaction was examined by measuring the specific IgA in the rats' feces. Knowing about the antibody levels in gnotobiotic rats induced by monoassociation we subsequently disassociated two groups of rats in order to investigate the impact of B. adolescentis on the immune reaction against B. thetaiotaomicron. One group was disassociated simultaneously with B. adolescentis and B. thetaiotaomicron, the second group was disassociated with these bacteria in sequence. B. adolescentis was merely able to induce a mucosal immune reaction, while B. thetaiotaomicron challenged the mucosal as well as the systemic immune system. Furthermore B. adolescentis obviously suppressed the systemic and mucosal immune reaction against the autochthonous B. thetaiotaomicron.