Persistence of gut microbiota may require repeated colonizations

Reports indicate that, variously, 10–50% of bacteria in the gut are coated with IgA and that bacterial diversity correlates with IgA amounts. In support of these observations, it was recently shown that there is an overall reduction in bacterial diversity in Rag1−/− mice. In extension to these findings, we report that taxa that are severely depleted in Rag1−/− mice include Proteobacteria and SFB, which are IgA-coated in wild-type (WT) mice. We reasoned that IgA might be required for these taxa to take up residence in the gut. Further, since germ-free SCID mice can be stably colonized with SFB, we explored the possibility that retention of SFBs may require continuous exposure to the organisms. We tested this by co-housing Rag1−/− mice with WT mice and found that by day 5, Rag1−/− mice had acquired both Proteobacteria and SFB. In addition, when F2 littermates obtained from WT x Rag1−/− crosses were compared for microbial diversity, both were present in the Rag1−/− pups. These data raise the possibility that mice are being colonized continuously. We also report that members of Proteobacteria separate equally into IgA-bound and IgA-unbound fraction, raising the possibility that the IgA-unbound fraction may represent new colonizers. Finally, in agreement with earlier reports, which showed that transfer of IgA-coated bacteria increases the susceptibility to DSS-colitis in germ-free mice, we find that Rag1−/− mice, which lack coated bacteria, are more resistant to DSS-colitis.