Recent evidence suggests that the microbial community in the human intestine may 
play an important role in the pathogenesis of obesity. We examined 184,094 
sequences of microbial 16S rRNA genes from PCR amplicons by using the 454 
pyrosequencing technology to compare the microbial community structures of 9 
individuals, 3 in each of the categories of normal weight, morbidly obese, and 
post-gastric-bypass surgery. Phylogenetic analysis demonstrated that although 
the Bacteria in the human intestinal community were highly diverse, they fell 
mainly into 6 bacterial divisions that had distinct differences in the 3 study 
groups. Specifically, Firmicutes were dominant in normal-weight and obese 
individuals but significantly decreased in post-gastric-bypass individuals, who 
had a proportional increase of Gammaproteobacteria. Numbers of the 
H(2)-producing Prevotellaceae were highly enriched in the obese individuals. 
Unlike the highly diverse Bacteria, the Archaea comprised mainly members of the 
order Methanobacteriales, which are H(2)-oxidizing methanogens. Using real-time 
PCR, we detected significantly higher numbers of H(2)-utilizing methanogenic 
Archaea in obese individuals than in normal-weight or post-gastric-bypass 
individuals. The coexistence of H(2)-producing bacteria with relatively high 
numbers of H(2)-utilizing methanogenic Archaea in the gastrointestinal tract of 
obese individuals leads to the hypothesis that interspecies H(2) transfer 
between bacterial and archaeal species is an important mechanism for increasing 
energy uptake by the human large intestine in obese persons. The large bacterial 
population shift seen in the post-gastric-bypass individuals may reflect the 
double impact of the gut alteration caused by the surgical procedure and the 
consequent changes in food ingestion and digestion.