ROBERT SIEGEL, HOST:
There's a lot happening right this moment inside your gut, inside mine, too, and, Ari, same with you (laughter). There's been a lot of talk lately about the microbiome, the millions of bacteria that live in our guts and help keep us healthy. But those bacteria aren't alone.
Scientists are just starting to understand something else that lives in our guts - viruses. NPR's Michaeleen Doucleff reports that they're more plentiful and possibly more potent than the bacteria.
MICHAELEEN DOUCLEFF, BYLINE: This is going to sound a little scary, but there are trillions, perhaps quadrillions of viruses hanging out in your intestines. Think of it like a soup. It isn't just a plain, boring soup made of one or two things. It's a soup that contains a huge variety of ingredients. In your gut, there are thousands upon thousands of different species of viruses.
JONATHAN EISEN: So there's incredible diversity, way more diversity, actually, than you see in the bacteria. The diversity is off the charts.
DOUCLEFF: That's Jonathan Eisen at the University of California, Davis. He studies microorganisms in our guts. He says some viruses are really bad. They give us the flu, stomach bugs or worse things like Zika or Ebola.
EISEN: Even the scientists who study them for many years have sort of assumed that we should view them basically as killers.
DOUCLEFF: But in fact, the majority of viruses are probably harmless. And according to a study out today in the Proceedings of the National Academy of Sciences, some of them could be beneficial.
EISEN: It's an amazing world that has so far not been studied in a lot of detail.
DOUCLEFF: Mark Young is a virologist at Montana State University. He and his team analyzed the viruses found in the guts of 64 people - some people in Chicago, some in Boston and some in London. They found 23 viruses that were common in over half of the healthy people.
But here's the important finding. These viruses were less common in people with intestinal diseases like Crohn's disease and Colitis. In other words, these viruses could be good. You want to have these viruses in your gut.
MARK YOUNG: We speculate - and clearly at this point it's speculation - that they help maintain your health.
DOUCLEFF: It's just speculation right now because it's a small study, and it's not a diversified enough group of people. And while the viruses were associated with healthy guts, the researchers don't know if they are actually contributing to health. For that, they have to figure out what the viruses are doing in the gut.
One hypothesis is that they are killing bad bacteria because here's the coolest thing about these gut viruses. Each one lives inside a bacterium in your gut, which means when they replicate, they're capable of killing their host bacterium.
YOUNG: Some viruses literally blow open the cells in which they're replicating. That's how they get out to infect another cell.
DOUCLEFF: So that means some of these viruses may decide which bacteria stay in the gut and which ones go. One day, Young says, we could fight bacterial infections with viruses.
YOUNG: That's certainly one of the avenues we're exploring, and we're certainly hoping that's the case.
DOUCLEFF: But he says there's still a long way to go before these friendly viruses show up in our medicine cabinets. Michaeleen Doucleff, NPR News.
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