The gut bacterial species is associated with a healthier clinical profile. The purpose of this study was to determine the association between and glucose homeostasis in patients undergoing bariatric surgery (BS): gastric banding (GB) or Roux-en-Y gastric bypass (RYGB). This nonrandomized prospective study included 65 women with severe obesity. Longitudinal analysis included subjects for whom data were available at follow-up [1, 3, and 12 mo; GB ( = 10) or RYGB ( = 11)]. Glucose homeostasis markers were measured under fasting conditions (glucose, insulin, and HbA1c) or during an oral glucose tolerance test. Fecal microbiota was analyzed using shotgun metagenomics, and relative abundance was assessed with 16S rRNA quantitative PCR. relative abundance was significantly lower in severe obesity [mean body mass index, 45.7 kg/m (SD 5.4)] than in moderate obesity [33.2 kg/m (SD 3.8)] but not associated with glucose homeostasis markers. A significant increase in relative abundance after RYGB was not correlated with metabolic improvement. Baseline abundance was correlated with bacterial gene richness and was highest in the high-richness Ruminococcaceae enterotype. increased in relative abundance after BS in patients with low baseline . abundance, especially those with a type 2 enterotype classification. Although decreased in severe obesity, relative abundance of was not associated with glucose homeostasis before or after BS. A certain level of abundance might be required to observe a beneficial link to health. The severity of obesity and gut dysbiosis may partly explain the discrepancy with previous findings in less obese populations.