The worldwide obesity epidemic is stimulating efforts to identify host and environmental
factors that affect energy balance. Comparisons of the distal gut microbiota of genetically
obese mice and their lean littermates, as well as those of obese and lean human volunteers
have revealed that obesity is associated with changes in the relative abundance of
the two dominant bacterial divisions, the Bacteroidetes and the Firmicutes. Here we
demonstrate through metagenomic and biochemical analyses that these changes affect
the metabolic potential of the mouse gut microbiota. Our results indicate that the
obese microbiome has an increased capacity to harvest energy from the diet. Furthermore,
this trait is transmissible: colonization of germ-free mice with an 'obese microbiota'
results in a significantly greater increase in total body fat than colonization with
a 'lean microbiota'. These results identify the gut microbiota as an additional contributing
factor to the pathophysiology of obesity.