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From NPR News, this is All Things Considered. I'm Melissa Block. If you have always been chubby, or too skinny, or prone to certain diseases, who gets the blame? Well, often, it's Mom and Dad. We blame our genes. But as NPR's Robert Krulwich is about to explain, we have inside our guts a second set of genes that come from other, much tinier sources.

ROBERT KRULWICH: I heard about some mice at Dr. Jeffrey Gordon's lab in St. Louis who eat, and eat, and eat, and eat...

Dr. JEFFREY GORDON (Biologist, Washington University in St. Louis): They consume much more food than their counterparts.

KRULWICH: Thirty percent more food than a second set of mice, a control group. And yet for some reason, these big eaters, the glutton mice, when you weigh them, they have 42 percent less body fat.

Dr. GORDON: They remain very lean.

KRULWICH: So how do these big eaters stay so skinny? Well, for one thing, says Jeff...

Dr. GORDON: They're raised in special plastic isolators, separated from the microbes in the world.

KRULWICH: Where they are never touched by other mice or by humans. Their food and their air is always filtered, so they are completely, as Jeff puts it...

Dr. GORDON: Germ-free.

KRULWICH: Meaning, that these skinny mice have no bacteria in their mouths, in their tummies, in their intestines, in their colons until Jeff takes them out of their plastic bubble world. And once they are exposed to germs, they gain weight just like normal mice. So what is this connection, then, between bacteria and weight?

Dr. GORDON: We're very interested in that question and are pursuing it.

KRULWICH: Because by the way, you listening, you humans, just before you were born, you had no bacteria in you either, according to British microbiologist Glen Gibson.

Dr. GLEN GIBSON (Microbiologist, University of Reading): We're all sterile until we're born. We haven't got anything in us right up until the time we come into this big, bad, dirty world, if you like.

(Soundbite of baby crying)

Dr. GIBSON: And then we pick up bacterial cells from the environment or the room we were delivered in, or whatever.

KRULWICH: And when you're put on your mother's breast and you start to drink, in comes her bacteria. When you suck your thumb after touching your brother, or your dog, or your toes, that's more bacteria.

Dr. GIBSON: It comes in the mouth.

KRULWICH: Through the soft food you eat, into your body, concentrating their enormous populations of bacteria in the large intestine.

Dr. GIBSON: They're pretty happy in there.

KRULWICH: So they multiply and multiply, so by the time you are one year old, you've got a lot of them.

Dr. GIBSON: Trillions and trillions, yeah.

KRULWICH: And what are they doing in there?

Dr. GIBSON: Their job is to help us digest the food we eat every day.

KRULWICH: So if I eat, say a Cheerio - a Cheerio, it's a crushed oat, which is really a plant seed. Now birds, they eat plant seeds. But we humans, with the genes that we got from our moms and dads, we aren't designed to fully digest seeds. So, says Jeff Gordon...

Dr. GORDON: If you want to harvest energy, calories from that Cheerio, you're going to depend upon your gut bacteria.

KRULWICH: Because your gut bacteria, they have their own genes and their own talents. Some of them are very good at eating seeds or oats so I can swallow a Cheerio. But I'm going to use their genes to mash that Cheerio into a soup that my cells can...

(Soundbite of slurping)

Dr. GORDON: So they're serving a very, very crucial role in helping us digest food.

KRULWICH: After all, if you eat something and you can't break it down in your intestines, then it just kind of goes through you like a pebble; you can't turn it into calories. The reason Jeffrey Gordon's mice could eat, and eat, and eat and not get fat is because they didn't have gut bacteria to help them digest.

Dr. GORDON: So they eat more, about 30 percent more, yet they remain very lean.

KRULWICH: And now comes the even bigger discovery.

(Soundbite of music)

KRULWICH: Imagine for a moment a baby born in Buenos Aires, Argentina to a mom who eats lots of steak, and lamb, and corns, and beans, and empanadas, and drinks some wine.

(Soundbite of singing)

KRULWICH: And now, imagine a second baby, this one born in Alaska to an Inuit or to an Eskimo mom who eats lots of caribou, and seal, and cod, and she likes a beer or two. Two very different babies with two very different moms. It turns out, says Jeff...

Dr. GORDON: People living in different environments with different cultures and different forms of cookery...

KRULWICH: Will give their babies different kinds of gut bacteria. And there are many kinds.

Dr. GIBSON: At least 500 different types, different species.

KRULWICH: With different talents. And now, scientists can look and see what kind of bacteria you find inside Argentines or inside Inuits.

Dr. GORDON: And that's what is done. A sample is procured.

KRULWICH: What kind of sample?

Dr. GORDON: Take those things that we donate to the world every day.

KRULWICH: I know those things.

Dr. GORDON: That we customarily discard, and they are representative of the gut community.

KRULWICH: Oh, how easy is that! I see. You just catch it coming out.

Dr. GORDON: Exactly. OK.

KRULWICH: And what scientists discovered is that yes, different people have very different bacteria in their guts, which creates consequences. For example, let's say our mom in Argentina swallows a Cheerio. When a Cheerio gets down into her intestine, her gut bacteria, here they are, they're a mix of species that kind of like Cheerios. They see a Cheerio, and they go...

(Soundbite of talking)

KRULWICH: So they gobble up about 80 percent of the oat, which isn't bad. But when an Inuit mom, when she eats a Cheerio, her bacteria are a different mix. Her species adore oats. They're ravenous for oats because they've got oat-loving genes.

(Soundbite of excited chattering and cheering)

KRULWICH: So in the Inuit mom, her bacteria dissolve 98 percent of the Cheerio. With the result, you could have two people sitting down to a bowl of Cheerios. They could each eat the same number of Cheerios. But because of a difference in their gut bacteria, one will get more calories than the other.

Dr. GORDON: So that one person extracts 95 calories from a serving of Cheerios, and another person, 99 calories. And that these slight differences, over time, add up.

KRULWICH: In fact, Jeff found you could feed two sets of mice the same amount of food, and the mice with lots of oat-devouring, oat-crunching bacteria, they will get fatter because they use more of each Cheerio and it goes to their fat cells.

Dr. GORDON: The obese mice harbor a gut community that's more capable of breaking down components of a Cheerio.

KRULWICH: So it's possible the mix of bacteria in your guts right now could be making you a little fatter or a little skinnier. You have trillions of guests in your belly, and their genes and their talents are making you different. They affect your weight, maybe your health, but what if you could change the combination of bugs in your intestine? Add more bugs to make you skinny, say.

Then, like Jeff's mice, you might eat, and eat, and eat and gain no weight, which would be so fine. So scientists are trying to figure out once you've grown up, can you change the flora, as they call bacteria, in your gut?

Dr. GIBSON: And there is evidence, that say when you grow up and you migrate from - I don't know, the States and go and live in Australia or somewhere, your gut flora will change because you will then go into the kind of Australian diet, and that makes a difference to the type of bacteria which one has in the gut.

Dr. GORDON: That's right. But also even within a given country, maybe even within a given community and in a given family, there is considerable variation in terms of who's there.

KRULWICH: So not only do people have different kinds of bacteria inside them, it seems we can, up to a point, change the mix, which leads to the much more profound thought: If we can change our bacteria, could we eliminate obesity? Could we get rid of some diseases? Could we live longer? Because now we know that we humans have a double inheritance.

Dr. GORDON: We are given many things by our parents.

KRULWICH: From our mom and dads, we get eye color, and height, and talents from their genes. But from the...

Dr. GORDON: Trillions.

KRULWICH: Yes, trillions of bacteria who live inside us like guests in a hotel, we get a whole different set of genes which can affect our weight, and who knows what else.

Dr. GIBSON: And of course, the obvious implication here is that if you find out bacteria which are responsible for diseases - and you can include obesity in this - you then target them to reduce the risk of that disease.

KRULWICH: So this is a whole new field. We've had a human genome project.

Dr. GORDON: Now, we're turning to the microbial parts of our cells.

KRULWICH: And saying who's inside us? Who's in there helping? Who's hurting? Who should stay and who should go? Robert Krulwich, NPR News.

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