Whereas much of the microbiota is dead in the feces, the cecum is an active bioreactor with high metabolic fermentative capacity. Thus we focused our study on the cecal microbiota. Cecal microbiota composition was analysed by pyrosequencing of the V1-V2 region from the bacterial 16S rRNA gene. PCR amplicons were generated from cecal DNA obtained from WT and FXR KO mice that underwent either VSG or sham operations (n= 12 WTsham, 7 WT-VSG, 9 KO-sham, 8 KO-VSG). Sequence **data** were analyzed by using the QIIME software package (version 1.5.0) 50 .