"Bugs" in Our Guts Make Us More Than Human, Study Says

Mason Inman
for National Geographic News
June 1, 2006
The ever present armies of microbes in your digestive tract are so essential to your survival, a new study says, that you might consider yourself a super-organism—human plus microbes equals you.

These hordes of "gut bugs" perform digestive duties that the human body alone cannot, according to the first ever comprehensive study of these microbes' genes.

To reach their conclusions, scientists conducted the most intensive intestinal exit poll yet.

By sampling two people's excrement, the researchers have revealed just how the microbes help us ferment our food, produce vitamins for us, and break down toxic chemicals.

The study maps the genes of the estimated 500 or more species that live inside us. About a quarter of these genes appear to belong to unknown species.

(See our quick overview of genetics.)

A Whole New World

The bacteria primarily cling to the intestinal walls in the bowels of the bowels—the colon. They also hitch rides on chunks of undigested food, which researchers have nicknamed "whovilles," after the tiny villages in Dr. Seuss stories.

"We are discovering parts of ourselves we were not aware of," said microbiologist and study co-author Jeffrey Gordon of the School of Medicine at Washington University in St. Louis, Missouri.

"It's a whole other planet down there."

Our bodies carry ten times more microbial cells than human cells, and these microbes collectively contain at least a hundred times the number of genes in the human genome.

"Not only are we never alone," said microbiologist David Relman of Stanford University in Palo Alto, California. "Our partners contribute essential functions to our collective."

Relman is a co-author of the new study, which will be published in tomorrow's issue of the journal Science.

Gas Refining

By analyzing the microbes' genes, the scientists saw that gut bugs produce proteins that enable chemical reactions that would otherwise be terribly slow or even impossible for the human body to perform.

For instance, our bodies are rife with bacteria that specialize in fermentation, the study says. This process allows the body to break down the tough cell walls of vegetable matter.

But rather than making beer, these fermenters supply us with sugars, and lots of gas. So much gas—primarily hydrogen—that if it builds up, it can stymie the fermenting bacteria's further growth.

But another microbe, Methanobrevibacter smithii, which showed up prominently in the study, saves the day.

It can transform hydrogen into methane, a flammable, odorless gas which escapes us in the form of flatulence. This transformation keeps the fermenting bacteria chugging.

The study also showed that the microbes can make precursors to vitamins B1 and B6 as well as amino acids—including some necessary acids that our bodies lack the machinery to make.

Our microbes also manufacture a few not-so-essential amino acids, such as taurine, the stimulating ingredient also found in energy drinks.

Gut bugs also may protect our health through what Gordon calls bioremediation.

They can break down toxic chemicals such as tetracholoethene, a chemical common in dry cleaning, and caprolactam, used to make synthetic fibers like nylon.

Fast Food Koalas?

"This is a very important study," said biochemist Jeremy Nicholson of Imperial College London in Britain.

It shows that "our gut bugs definitely have significant positive contributions to our health," Nicholson said.

"But the bug system biology is so much more complex than we thought. We do not know yet how to be helpful to our good bugs," he added.

But with more understanding, we may open up the possibility of "reprogramming our gut microbes to contract or expand our energy harvest," Gordon said.

This could help the obese slim down or the malnourished gain weight.

To get an idea of the power of tailoring our gut microbes, take koalas. They carry only a tiny set of microbes, which is all they need to process their very limited diet of mostly eucalyptus leaves.

And yet, "maybe if we innoculated them with human microbes they could eat McDonald's," Nicholson said. "But maybe that would not be a good health option."

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