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Background Over two million people worldwide suffer from ulcerative colitis (UC). Biologic therapy has significantly improved treatment, but nearly two-thirds of patients attenuate response. Fecal microbiota transplant (FMT) is an emerging therapy for the treatment of UC, but the microbial mechanism responsible for clinical response is poorly understood. Using samples from our pilot FMT study (Jacob V, et al 2017), we aim to define the core transferable microbiota (CTM) in UC patients responsive to FMT therapy and its therapeutic mechanism. Methods IBD disease activity scores were used to define clinical response. Metagenomic sequencing of donor, recipient, and 4 week post-FMT fecal samples was performed to define the CTM and strain level transferability. To define the transferable immune-reactive microbiota (TIM), IgA-seq was also performed on donor and recipient samples. Patient TIM strains were isolated and tested in gnotobiotic mouse models to evaluate their impact on mucosal immunity and colitis. Results We defined a CTM associated with clinical response to FMT. CTM strain tracking confirmed that clinical response correlated with strain transferability. We defined a core TIM by IgA-seq that correlated with clinical response. In humanized mouse models, TIM induced IgA in a T cell independent manner. Colonization of germ-free mice with a core TIM strain, Odoribacter splanchnicus, robustly induced mucosal Th17 and RORgt+/Foxp3+ iTreg cells and reduced the severity of transfer T cell colitis. Our data highlights a core TIM in UC responders to FMT and the mechanistic impact of it in shaping mucosal immunity and guiding the response to UC. This provides a framework for rational selection of TIM for microbial-therapy in IBD.