This study investigates the human gut microbiota in individuals with normal weight, obesity, and those who have undergone gastric bypass surgery. Using 454 pyrosequencing, the researchers analyzed 184,094 sequences of microbial 16S rRNA genes from 9 subjects, 3 in each category. The results show that the microbial community structures differ significantly among the three groups. Firmicutes were dominant in both normal-weight and obese individuals but decreased in post-gastric-bypass individuals, who had an increase in Gammaproteobacteria. The study also found that the number of H2-producing Prevotellaceae was higher in obese individuals, and real-time PCR detected significantly higher numbers of H2-utilizing methanogenic Archaea in obese individuals compared to normal-weight and post-gastric-bypass individuals. The coexistence of H2-producing bacteria and H2-utilizing methanogenic Archaea suggests that interspecies H2 transfer may be a mechanism for increasing energy uptake in the human large intestine in obese individuals. The changes in the gut microbiota after gastric bypass surgery may reflect both the surgical alterations and the subsequent changes in food ingestion and digestion.This study investigates the human gut microbiota in individuals with normal weight, obesity, and those who have undergone gastric bypass surgery. Using 454 pyrosequencing, the researchers analyzed 184,094 sequences of microbial 16S rRNA genes from 9 subjects, 3 in each category. The results show that the microbial community structures differ significantly among the three groups. Firmicutes were dominant in both normal-weight and obese individuals but decreased in post-gastric-bypass individuals, who had an increase in Gammaproteobacteria. The study also found that the number of H2-producing Prevotellaceae was higher in obese individuals, and real-time PCR detected significantly higher numbers of H2-utilizing methanogenic Archaea in obese individuals compared to normal-weight and post-gastric-bypass individuals. The coexistence of H2-producing bacteria and H2-utilizing methanogenic Archaea suggests that interspecies H2 transfer may be a mechanism for increasing energy uptake in the human large intestine in obese individuals. The changes in the gut microbiota after gastric bypass surgery may reflect both the surgical alterations and the subsequent changes in food ingestion and digestion.