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It's ALL THINGS CONSIDERED from NPR News. I'm Audie Cornish.

Here's some news to think about while you're enjoying that Fourth of July picnic. It's about your digestive system, specifically the microbes that live in your gut. Researchers have found that we carry the same collection of bacteria with us for most of our lives.

As NPR's Rob Stein reports, this insight could change how doctors monitor our health.

ROB STEIN, BYLINE: Scientists have known for a long time that we all have helpful bacteria and other microbes in our guts. They help us digest food, make vitamins, they may even help fight off infections and regulate our metabolisms.

But Jeffrey Gordon at Washington University in St. Louis says there's always been one big question about these microbes.

JEFFREY GORDON: The question is: Once these communities are formed, how long do they endure? What is the stability in healthy individuals?

STEIN: Are they the same microbes? Or do they come and go? To try to get a sense of that, Gordon and his colleagues developed a new way to identify our microbes, a genetic analysis Gordon calls a bar code of life. He used it to monitor the microbes in 37 healthy women, year in and year out, for about five years.

GORDON: Using this method, this bar code of life, it was a way of classifying organisms represented in a given individual's gut community, at a moment in time and over time.

STEIN: In this week's issue of the journal Science, the researchers report that the microbes seem to change very little, at least in healthy people. And based on what they found, the scientists think they can project the long-term composition of our microbial community.

GORDON: These microbes, likely, are with us for most of our lives and, as such, are in a position to shape our lives, to promote our health or, in certain circumstances, to contribute to risk for disease.

STEIN: Contribute to our risk for disease by getting out of whack somehow. That might play a role in all sorts of diseases, like inflammatory bowel disease, maybe even things like obesity, diabetes. And if that's the case, then testing our gut microbes from time to time, a new kind of gut check could be very useful. Gordon says it would sort of be like getting a blood test as part of an annual physical.

GORDON: An individual comes in and not only do we get blood chemistries, but we also examine the representation of different microbes in his or her gut. This will give us a more comprehensive view of their health status and give us a way of understanding how things are changing.

STEIN: In fact, the researchers have already been able to measure one key thing about someone's health just by looking at their microbes. Jeremiah Faith helped conduct the study.

JEREMIAH FAITH: By looking at someone's intestine, we could pretty accurately tell, over a length of time, how much weight they had lost or gained without having to put them on a scale.

STEIN: Another intriguing finding was that peoples' microbes seem to run in their families, sort of like genes.

FAITH: For everyone that we checked, we were able to identify strains of bacteria that were shared between related individuals, which suggests that you had these microbes for a long time, because many of these people, you know, they live far apart from each other now.

STEIN: All this seems to reinforce the idea that the microbes in our guts are really important. And Eric Pamer at Memorial Sloan-Kettering in New York says that means we should be paying a lot more attention to them.

ERIC PAMER: In the same way that our genome defines who we are, one could say that the microbial populations that inhabit us define who we are.

STEIN: What remains unclear is exactly what we might be doing that disturbs our microbes and what that means for who we are and how we're feeling.

Rob Stein, NPR News.

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