The main findings are: (1) T1D-associated gut dysbiosis is characterized by increased LPS biosynthesis and decreased butyrate production and bile acid metabolism; (2) the combination of nine bacterial species and nine fecal metabolites yields excellent discriminatory power of new-onset T1D; (3) human T1D-associated gut microbiota could induce elevated fasting glucose levels and declined insulin sensitivity in antibiotic-treated mice; and 4) butyrate and LPS exert protective and destructive effects, respectively, on glucose metabolism and islet structure and function in T1D mice. Here, INS is linked to type 1 diabetes mellitus.