Everywhere you turn, it seems, there's news about the human microbiome. And, more specifically, about the bacteria that live in your gut and help keep you healthy.
Those bacteria, it turns out, are hiding a big secret: their own microbiome.
A study published Monday suggests some viruses in your gut could be beneficial. And these viruses don't just hang out in your intestines naked and homeless. They live inside the bacteria that make their home in your gut.
These particular viruses are called bacteriophages. And, until recently, many scientists had sort of ignored the ones in the gut, says Mark Young, a virologist at Montana State University. Researchers didn't have good tools to study these phages, Young says, and understanding them hasn't been a priority, because they don't seem to cause problems.
"Most virologists are looking at how viruses cause disease," he explains. "We're flipping that around, and looking at the possible role for viruses in promoting health."
To begin their study, he and his team sequenced the genes of bacteriophages found in the guts of two people. They combined their data with evidence from a previous study, which had sequenced bacteriophages in more than 160 people from the U.S. and the U.K.
In the larger data set, about a third of the people had healthy guts. The others had a chronic intestinal illness — either Crohn's disease or ulcerative colitis.
From the combined group of people, Young and his team identified 23 bacteriophages that seemed to be associated with a healthy gut. These viruses were common in more than half the healthy people and were much less common in people with Crohn's or colitis, the team reports this week in the Proceedings of the National Academy of Sciences.
"We speculate — and clearly at this point, it is speculation — that these viruses help maintain your health," Young says.
Their conclusion is still speculative, he explains, because he and his team can't tell from their work whether the bacteriophages are contributing to a healthy gut or are just a result of that health.
Plus, the team's study was small, and it didn't look at a diverse group of people, says Jonathan Eisen, a medical microbiologist at the University of California, Davis who studies gut microorganisms and wasn't involved in the research.
"It's cool that the study found these common viruses in people," Eisen says. "But the caveat is that they really didn't survey the human population broadly. They didn't survey indigenous populations, [for example, or] people of different age groups and genders." What each person eats might have an influence on these viruses, too, Eisen says.
"There's incredible diversity among the viruses in our guts — way more diversity, actually, than you see in the gut bacteria," Eisen says. "It's this amazing, amazing world that has, so far, not been studied in a lot of detail."
Some scientists suspect bacteriophages may determine which bacteria get to dwell in the gut and which ones aren't allowed to stay, Eisen says. Bacteriophages are potent assassins.
"Bacteriophages can take over a bacterium," he says, "make a lot of copies of the virus, burst out of the cell and kill that cell."
Doctors may one day be able to turn that behavior to their patients' advantage, Eisen and Young say. Though researchers still have a long way to go in figuring out how all this works, we may one day be able to fight bacterial infections with viruses.
"That's certainly one of the avenues we're exploring," Young says. "We're certainly hoping that's the case."
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. Transcript provided by NPR, Copyright NPR.