Abstract Background and Aims We previously reported that 20% of unaffected first-degree relatives of pediatric IBD patients share the same microbial community structure (OTU-type) as IBD patients. This suggests that a preexisting dysbiosis can predispose to the development of IBD. The aims of this study were to establish that microbial community types identified in the family cohort could be reconstituted in gnotobiotic mice for further translational studies and to assess for any direct effects of this microbiota on intestinal inflammation. Methods Over 100 germ-free wild-type C57/BL6 mice were colonized for 4 weeks in the UCLA gnotobiotic facility with fecal microbiota from 30 human donors belonging to each of four groups from the pediatric family cohort: unaffected first-degree relatives with OTU-type 1, unaffected first-degree relatives with the IBD-associated OTU-type 2, Crohn’s disease (CD) with OTU-type 1, CD with OTU-type 2. Each group was represented by 8 human donors (n=3–4 mice/donor) with the exception of CD OTU-type 1 (only 5 donors available in the cohort). In addition, 35 eight week old germ-free IL-10-/- C57/BL6 mice were colonized for 12 weeks with fecal microbiota from 4 CD OTU-type 2 donors and 4 unaffected OTU-type 1 donors (n=4–5/donor). Luminal and mucosal microbial composition in the colon and small intestine was evaluated by 16S rRNA sequencing. Intestinal inflammation was assessed in the small intestine and colon by semiquantitative scoring of H&E stained sections. Results Donor-specific microbial composition was observed in the humanized gnotobiotic mice with clear separation between recipients of donor OTU-type 1 vs. OTU-type 2 stool. Parallel differences in differentially abundant microbes and microbial diversity were seen in these gnotobiotic mice as in the original human donors. No histologic evidence was found for colitis, enteritis, or fibrosis in wild-type colonized mice. However, fecal lipocalin was increased two-fold increase in recipients of OTU-type 2 microbiota from CD patients relative to the other three groups. CD humanized IL-10-/- mice exhibited lower microbial diversity and distinct microbial composition compared to non-IBD humanized IL-10-/- mice, mirroring differences in the human donors. Mice colonized with CD microbiota had increased histological disease severity in the colon and cecum (p=0.05) compared to mice colonized with non-IBD microbiota. Conclusion Our results demonstrate that human microbial community states identified in the pediatric IBD cohort could be reconstituted in gnotobiotic mice for translational studies. Moreover, CD-associated dysbiosis exacerbated colitis severity. These data support the concept that dysbiosis plays a direct role in promoting intestinal inflammation and provide an experimental framework for further mechanistic studies of IBD-associated microbial communities.