Turnbaugh PJ, Ley RE, Mahowald MA, Magrini V, Mardis ER, Gordon JI

An obesity-associated gut microbiome with increased capacity for energy harvest

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.

Bajzer M, Seeley RJ.

Physiology: obesity and gut flora

The Firmicutes and the Bacteroidetes are divisions - or phyla - within the domain Bacteria. The microbiota of the human gut is dominated by their members, most of which are benign, although a few are pathogenic. The Firmicutes is the largest bacterial phylum. It contains more than 250 genera, including Lactobacillus, Mycoplasma, Bacillus and Clostridium. There is considerable variety in the phylum. For example, the Clostridium species are obligate anaerobes (that is, they absolutely require anoxic conditions), whereas members of Bacillus form spores and many of them are obligate aerobes. Streptococcus pyogenes, the well-known cause of ‘Strep. throat’, is also a member of the Firmicutes. The Bacteroidetes include about 20 genera. In the human gut, Bacteroides is probably the most abundant single genus, and Bacteroides thetaiotaomicron is one of the most abundant organisms. Species of Bacteroides are obligate anaerobes that are benign inhabitants of the gut. However, they are opportunistic pathogens that can cause disease if they gain access to the peritoneal cavity, for example following surgery or a perforated ulcer. Members of the Bacteroidetes are found in the intestinal tracts of many warm-blooded animals, but are also abundant in soil and sea water.

Ley RE, Turnbaugh PJ, Klein S, Gordon JI.

Microbial ecology: human gut microbes associated with obesity

Two groups of beneficial bacteria are dominant in the human gut, the Bacteroidetes and the Firmicutes. Here we show that the relative proportion of Bacteroidetes is decreased in obese people by comparison with lean people, and that this proportion increases with weight loss on two types of low-calorie diet. Our findings indicate that obesity has a microbial component, which might have potential therapeutic implications.