The mammalian gastrointestinal tract harbors a diverse and complex resident bacterial community, which interacts with the host in many beneficial processes required for optimal host health. We are studying the importance of bacterial cell-cell communication mediated by the interspecies quorum-sensing signal autoinducer-2 (AI-2) in the beneficial properties of the gut microbiota. Our recent work provided the first evidence that AI-2 produced by Escherichia coli can influence the species composition of this community in the mouse gut. We showed that, under conditions of microbiota imbalances induced by antibiotic treatments, E. coli, which increases intestinal AI-2 levels, not only had an effect on the overall structure of the microbiota community, but specifically favored the expansion of the Firmicutes phylum. Because the Firmicutes are very important for many gut functions and were the group of bacteria most severely affected by antibiotic treatment with streptomycin, we are addressing the possibility that AI-2 can influence the balance of the major bacterial groups in the gut and promote recovery of gut homeostasis. Overall, we want to understand how bacterial chemical signaling shapes the multi-species bacterial communities in the mammalian gut and how these communities affect host physiology.