for National Geographic News
The human intestine is a swirling and churning environment that is host to microbial communities as diverse as those found in the Amazon rain forest. And like the regions beneath the soils that carpet the rain forest floor, much of what lies within the gut remains unexplored.
A series of papers in the March 28 issue of Science delves into this scientific frontier and begins to unravel the secrets of the complex and highly evolved microbial communities that teem throughout the length of our intestines.
The human gut is home to at least 500 species of microbes. The number of bacterial cells may exceed the number of human cells our body. Some microbes are firmly rooted in place. Others are just tourists on a leisurely stroll through the gastrointestinal tract. The residents form communities that are dense and bursting with pockets of ethnic pride.
Their collective role is to process food and fight off disease, but like the complexity of the microbes that process nutrients in the forest floor, scientists are just beginning to understand the contribution of each species and identify what makes some of them go bad.
One of the papers in Science reveals the genetics of a dominant gut bug that serves humans well by breaking down otherwise indigestible food. Another paper details the inner workings of a normally benign bug that has evolved drug-resistance and turns traitor when its human host is weakened by disease.
These studies are a positive step towards revealing "the underlying basis for the highly coevolved relationship between humans and microbes," write Michael Gilmore and Joseph Ferretti of the University of Oklahoma Health Sciences Center in Oklahoma City, in an accompanying perspective article.
The Good Bug
One common bacterium in the human digestive system is Bacteroides thetaiotaomicron. Jeffrey Gordon, a molecular biologist with the Washington University School of Medicine in St. Louis, Missouri and his colleagues plumbed the bacterium's genetic material to find out what it does.
They discovered that much of the bacteria's genome, which is one of the largest examined, is dedicated to breaking down complex carbohydrates that enzymes in the human gut cannot otherwise process. The complex carbohydrates are called polysaccharides. They are common to foods such as vegetables and other plant materials.
The human host benefits, explained Gordon, because B. thetaiotaomicron also breaks down the polysaccharides into products that can then be absorbed by the body. "We absorb 15 to 20 percent of daily caloric intake this way," he said.
The bacteria also exhibit an elaborate environment-sensing system to help them thrive in the highly competitive community within the human digestive system.
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