The application relates to methods based on modulating mammalian intestinal microbiota and related probiotic and prebiotic compositions. Specifically, the application relates to the use of novel bacterial genera Ileibacterium and Dubosiella, including their species I. valens (I. valens) and Dubosiella newyorkensis (D. newy), respectively, and closely related OTUs within the family Erysipelotrichaceae that resemble either Ileibacterium spp. or Dubosiella spp. with 90% 16S rRNA sequence identity, for modulating weight and intestinal inflammation and immunity (including modulating intestinal immune gene expression such as, e.g., modulating expression of RORγT, IL-17A, IL-17F, RegIIIγ, Relmβ, and Defβ), as well as for treatment and diagnosis of (i) obesity and related conditions such as metabolic syndrome and diabetes mellitus； (ii) allergic and autoimmune diseases, and (iii) gastrointestinal disorders (such as, e.g., inflammatory bowel disease (IBD), ulcerative colitis, Crohn's disease, and celiac disease).
