When a grocery shopper glances at the nutritional information on a box of Cheerios, the number of calories is 110 calories for a serving size of one cup. But depending on the type of bacteria living in the shopper’s gut, the full 110 calories may not be fully absorbed. The number of calories denotes “the absolute amount of energy in that serving, and shoppers with different [types] of bacteria in their gut may harvest and store different amounts of energy from that same serving,” said Jeffrey Gordon, the director of the Center for Genome Sciences at the School of Medicine, and the Dr. Robert J. Glaser Distinguished University Professor.

Over 800 species of microbes live in the human gut. Consequently, research on bacteria may be able to offer insights about how energy and weight are stored in the human body. Gut bacteria also may be the key to drug therapies that could treat a variety of digestive or weight loss processes.

Gordon, along with Ruth Ley, an instructor in molecular biology and pharmacology, recently received approval from the National Institutes of Health to sequence 100 microbial genomes.

“We think of this as the next logical step in the human genome project because the micro-biome is an integral part of our genetic landscape,” said Gordon. “We can begin to understand the different types of properties these organisms bring to us.”

“Most people think of bacteria as being adversaries, pathogens, [but] we think that most of our encounters with microbes are friendly and mutually beneficial,” he continued.

Although the microbes that reside in human guts are foreign entities, often introduced at the time of birth, from mothers and from the environment, they are essential to digestion and fat storage.

“We’ve done experiments in [bacteria] free mice that show that when you add bacteria in a very short period of time, the mice acquire a market increase of 60 percent in the amount of fat cells,” Gordon said.

He added that, “The way that works is that the microbes help break down these indigestible components of the diet and liberate the calories, but they also help the host by allowing the energy that is liberated to be stored in fat cells. They do so by manipulating a series of genes in the host.”

“You can think of this symbiosis in a very elegant way, in that microbes not only help break down components of the diet and liberate calories that would otherwise be lost, but also to ensure that those calories are stored,” said Gordon.

Gut bacteria, for instance, can break down polysaccharides, a carbohydrate found in plants that are integrated into everything from bread to pasta.

Gut bacteria, also believed to influence metabolism, may eventually allow scientists to form drugs specific to people with certain species of bacteria living in their gut.

“Armed with that knowledge of what the metabolic potential of these microbes are, I think that we will begin to see clinical trials within the next five years. For example, the nature of nutritional advice is predicated on the knowledge of what the microbes in an individual’s gut can do, so [individuals could have] more of a personalized nutrition of what they should consume,” said Gordon.

The microbes, which interact with genes in the storage of fat, could potentially help overweight individuals drop pounds.

“This gene product, which actually operates to limit the amount of fat stored, could become more of a potential therapeutic agent,” he said.

Despite the crucial role gut bacteria play in digestion and energy storage, the microbes’ genomes have only recently been addressed within the scientific community. While the first comprehensive description of microbes in the human gut was published in June 2005, Gordon explained that the study only included three adult humans.

“There was quite a large differences in types of bacteria present,” he said. “We don’t know if the membership to the community vary or operate differently. It may be that there’s a lot of redundancy among the types of bacteria even though they’re called different organisms, they may perform similar functions.”

Gordon and Ley hope to sequence gut bacteria first in mice and then compare and contrast microbes in people living in different regions of the planet, “both to understand how humans vary in respect to one another and how environment influences differences in genes,” said Gordon.

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