You Share Your Gut Microbiome With Your Friends

IRA FLATOW: This is Science Friday. I’m Ira Flatow. The microbiome, the network of tens of trillions of microbes that live inside and on our bodies. Helps us digest food, protects us from diseases, and depending on what species of bacteria you have, your microbiome could impact your stress response, your decision making, and how likely you are to develop arthritis and depression. Scientists have known that your microbiome is partially shaped by your environment and the people you spend time with, but they haven’t had a lot of clarity on how exactly your social networks outside of your home and family impact your microbiome makeup.

To learn more, my next guest and his team mapped close to 2,000 social connections in isolated villages and compared the microbiomes To see just how exactly their social closeness impacted their gut bacteria. And it turns out we are more connected to people in our lives than you may think. Their research was published in the journal Nature. Here to tell us more is Doctor Nicholas Christakis, a sociologist and physician at Yale University, where he directs the Human Nature Lab. He studies the biology of human social interactions and was an author on that research. Welcome back to Science Friday.

NICHOLAS CHRISTAKIS: Thank you so much for having me back, Ira.

IRA FLATOW: Nice to have you back. What were the big questions you wanted to answer in this study?

NICHOLAS CHRISTAKIS: Well, overall, we wanted to understand where does the microbiome in our bodies, in our guts in particular, come from? These bacteria are optimized to live in the anaerobic environment inside our intestines. They don’t really survive very long outside our bodies. So how do we get them? Where do they come from if they don’t survive outside our bodies? And it turns out to a large extent we get them from other people. These normal bacteria that live inside our bodies spread from person to person, and we wanted to really deeply understand that.

IRA FLATOW: Well, we’ve talked about spreading it among families and household members, but your team found out something in addition to that, right?

NICHOLAS CHRISTAKIS: Yeah. It’s not too surprising that the microbiome might spread from mother to infant or from spouses or between people who live in the same household. But we wanted to go beyond that work and understand how broader social interactions, for example, among friends, how the microbiome might be shared between people who are not biologically related to each other. So they have different genes and don’t live together. How could that happen?

And in order to do that, we needed a very particular kind of data. So to really study this question, we had to collect data of a very unusual kind. We needed many different kinds of information, ideally from a constrained and isolated set of people. So we have these 18 villages up in the jungle in the Western Highlands of Honduras, and we carted liquid nitrogen into those villages and we persuaded people to poop into little cups for us, which was also not an easy thing to do. And then we also photographed everyone in the villages and we asked them to identify who their friends were, not just who their siblings and relatives were, and we mapped their social networks in detail.

And then we send all of those specimens and data back to the United States, and the specimens then needed to be processed using modern genetic technology to identify all the constituent species in every individual. And then once we had all of that data together, we could really study the extent to which people who are socially connected also had similar microbiomes.

IRA FLATOW: And your results, were they surprising?

NICHOLAS CHRISTAKIS: So the fact that people shared microbes with their friends was, in fact, what we had imagined would be the case. So that wasn’t surprising per se. But the extent to which it happened was a little surprising to us. So what we found was that connected people had similar gut microbiomes. And this occurred even for unrelated individuals who lived in different households. And what we did is we quantified something called the strain sharing rate. So we had 2.543 different species that we identified and 339,000 strains that we detected. So each species might have several strains defined by very small differences in their DNA sequence, two different strains of the same species, for example.

And we found that partners shared about 14% of the same strains. So me and my wife, for example, if you looked at the E coli and the Prevotella copri and all the various gut microbiome species in us, she and I would have a 14% chance of having similar strains. And that’s not so surprising. But then we also found that with my friends I might share almost as much, 9% to 10% of strains, even with people that I don’t live with, but with whom I interact.

IRA FLATOW: Wow. And so does that mean you could identify who is friends with someone else just by looking at their poop?

NICHOLAS CHRISTAKIS: Yes. Actually, we show that we could predict your friends based on your poop. In other words, more than looking at, for example, how similar in age or sex or wealth or education or religion you were to your friends, if I could take a specimen of poop from you and a specimen of poop from your friends and compare them, that comparison was better able to identify who your friends are.

IRA FLATOW: Wow. So what’s the big-picture takeaway from this research then?

NICHOLAS CHRISTAKIS: Well, there are a number of implications of it. First of all, we were also able to show that these effects are obtained between me and my friends. But actually my microbiome is similar to my friends and to my friend’s friends. It extends out to two degrees of separation in the network.

IRA FLATOW: Wow.

NICHOLAS CHRISTAKIS: And furthermore, we found that there are clouds of microbes in clusters of people, a little bit like Pig-Pen in that old peanuts cartoon. People have these little clouds of microbes that exist in these little niches in the social network where nearby individuals all share it. It’s like the microbes treat our social networks as the extended environment in which they live. They traverse these social ties, these microbes do, in the furtherance of their own Darwinian interests. So that’s pretty amazing to me.

And we also found that people living in different regions of the social network had different species. So one cluster of people over here would share one set of microbes, and a different cluster of people elsewhere in the village would share different microbes. So it’s not just that these effects were between pairs of people, between me and my friends. Whole groups of interconnected people shared similar microbial species. I want people to have in their mind’s eye a kind of a social network. So everyone has friends, and those people in turn have friends, and you can map out this web of connections. And then having mapped out that web of connections, you might imagine that there are regions of the social network or clusters not defined by geography, there’s not neighborhoods, it is network clusters. And within those little network clusters, you should be able to have the intuition that there are going to be microbes that might specialize.

So you and your friend group might have a distinctive set of microbes that, just like being in your friend group, because of the coincident appearance of other microbes, for example, or because of the nature of your social interactions, whereas a different group of interconnected people, also within your same village or the same population, might harbor a different set of microbes.

And in fact, that’s what we were able to show, the different regions of the social networks had different specialized microbes, as if you imagine there was a plot of dirt in your garden, and there were some microbes that really liked being under the gutter, where there was not much sunlight and not much leaf debris. And then there were different microbes in a different part of your garden, where there was not as much water and much more sunlight and much more leaf debris. So these microbes would specialize for those niches. And just like that, I think, is what happens in human social networks. You and your friends have a kind of distinctive profile of the microbes inside your intestines.

IRA FLATOW: Wow. So what can you do with this knowledge?

NICHOLAS CHRISTAKIS: I wanted to say that these findings took 10 years of my life. And actually, they were prompted by some work we did in 2007 that showed the social contagion of obesity in complex networks. And in that paper, when we published it, we mentioned, look, here in this paper we’re exploring the social contagion of obesity, but there could also be a biological contagion. At the time back then, nearly 20 years ago, it was known that some microbes could affect your risk of obesity, but it wasn’t known whether those microbes could spread from person to person. And if they do spread from person to person, then maybe part of what we observed back in 2007 with respect to the social contagion of obesity was actually a biological contagion.

So I have microbes in me that affect my body size, then they spread to you, and then they have a similar effect on your body size. So that sort of leftover idea from 2007 was what we really wanted to explore in this effort. And it took a long time to be able to do this work properly. but the most radical implication of these findings, to the extent that they are true are the following. Because if it’s true that the microbes in us affect our risk for certain conditions like obesity or depression or arthritis or diabetes or hypertension, and it is true, there’s a lot of evidence. In other labs that the microbes in us affect the risk for those conditions.

And if it’s true, as we show in our current paper, that these microbes can spread from person to person, then it means that these conditions, which were previously thought to be non-communicable, like obesity and depression and hypertension and diabetes and arthritis, could actually be communicable. They could actually spread from person to person to some extent because of the biological contagion of these microbes within our bodies, and not just because as we had previously shown, the social contagion.

IRA FLATOW: So that means that if there’s a real biological contagion, maybe there’s a real biological cure.

NICHOLAS CHRISTAKIS: I think that’s possible. I think it’s possible that we may find ways to, well, I won’t say necessarily to interrupt the spread of these microbes because I think that’s quite natural. But to the extent that we understand that this is happening, that, for example, maybe a cluster of people is becoming depressed because they’re sharing microbes with each other, or spouses are both becoming hypertensive because they’re sharing microbes with each other, it might open up new vistas on different kinds of interventions and treatments we might employ.

IRA FLATOW: In other words, you might be able to change someone’s unhealthy microbiome to a more healthy one and treat those illnesses.

NICHOLAS CHRISTAKIS: That’s the hope that many scientists have. Of course, our work is focused on the transmission dynamics within social networks, not so much the therapeutic implications in individuals.

IRA FLATOW: You’re not saying that we should be spending less time with our social network out of the fear that we’re going to catch someone else’s microbiome, are you?

NICHOLAS CHRISTAKIS: No, no. And quite the contrary, most of the things that we catch from others are good for us. Remember, we co-evolved over hundreds of thousands of years with these microbes. And I believe that these microbes actually have many beneficial effects on our bodies and on our minds, actually. And so I don’t think we should be interfering willy nilly with this sort of system in equilibrium of us and our microbes.

IRA FLATOW: And so where would you go next with your research? What do you need to know more?

NICHOLAS CHRISTAKIS: Well, we’re taking our work into a number of further connections. First of all, we remain deeply engaged and interested in the fundamental reality of social contagion and complex networks. So we’re very interested, of course, in the social dynamics here, not just the biological dynamics. Second, we are going to try to see whether we can find more evidence that the spread of the microbes might actually have some effect on the spread of some of these other phenomena. This is going to take much more work.

And last, in some very new work that we’re just starting, we’re interested in the extent to which these microbes also affect our odors. So the way, you know, human beings have very distinctive smells, we actually smell each other. And to some extent, those smells relate to the microbes in us and on us. And to the extent that’s the case, there’s a relationship between the microbiome and human chemo signaling. So we have some new work that we’re beginning to think about that will take us in that direction as well.

IRA FLATOW: This is Science Friday from WNYC Studios.

What about– If this study was done in such an isolated, isolated villages, as you say, can that stuff be then, that knowledge be used in a much less isolated situation that we all live in? I mean, is it that useful to be able to project that?

NICHOLAS CHRISTAKIS: You’re right. I mean, we, for experimental reasons, deliberately wanted to do this work in isolated villages for a couple of reasons. First of all, we wanted to be able to define the universe of social interactions these villagers have. So there’s a village of 500 people, and most or all of the social interactions that people in that village have are with other people in the village. So we could bound those social perimeter, we know it, and map the network of that village.

Second, we wanted a situation in which people lived in a relatively traditional way, had face-to-face interactions with people so when they said someone was their friend, that person, they saw them face to face. And they also consumed a traditional diet and were relatively devoid of antibiotics and other pharmaceuticals. So we wanted social isolation. We wanted a sort of old-fashioned lifestyle in order to be able to let’s say, find the more natural way in which the microbes in our social networks interact.

Having said that, and having discovered that, I do believe that the things that we found apply outside that as well. Now they’re much noisier in modern life. You can be friends with someone that’s 1,000 miles away, and think of them as your friend and communicate with them regularly, but you don’t see them. That’s noise for the point of view, from my point of view. Or you could interact with many strangers that you don’t know in a kind of modern city. That’s also noise. But none of those things efface or negate the fundamental reality that these microbes do spread through social connection, or can spread through social connection. And that when you do interact face to face with people, these microbes are well adapted and we are well adapted for that kind of spread.

So you could then compare the situation I just described in these isolated villages with, let’s say, a more modern lifestyle where we’re consuming lots of antibiotics, we are eating processed foods, we interact frequently with strangers in a big city. We maintain friendships with people we never see face to face, thousands of miles away. And it’s true that all of those things would affect the extent to which we would find that you are sharing your microbes with your friends, or the extent to which you are sharing them with strangers because you worked with them or shook their hands or so on, in a business or a commercial setting, for example.

But from a scientific point of view, all of those things would just be sort of noise, making it more difficult for us to see the true underlying reality, which is that indeed it is possible for these microbes to be shared between individuals because of social contact, even if it’s more hidden in the modern way that we live.

IRA FLATOW: Right. Well, so much more to talk about. You’ve brought up one of our favorite subjects on Science Friday, the microbiome. I want to thank you for taking time to talk with us. And come back when you’ve got some new stuff.

NICHOLAS CHRISTAKIS: Thank you so much for having me, Ira.

IRA FLATOW: Doctor Nicholas Christakis, sociologist and physician at Yale University, where he directs the Human Nature Lab.

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