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Eating Yogurt Is Not Enough: Rebalancing The Ecosystem Of 'The Microbes Within Us'


This is FRESH AIR. I'm Terry Gross. You may think you know who you are, but you don't really unless you're aware of the fact that there are trillions of microbes living on and in you. They affect your digestion, immunity, body weight and overall health. Scientists are researching what those microbes do and how we might be able to improve our health by manipulating the balance of those microbes. The ecosystem of microbes within humans and animals is the subject of the new book "I Contain Multitudes" by science writer Ed Yong. He writes about science for The Atlantic and writes a science blog hosted by National Geographic called "Not Exactly Rocket Science."

Ed Yong, welcome to FRESH AIR. So as you say, our body is really an ecosystem. What are some of the different communities of microbes on and in our bodies?

ED YONG: Well, there are tens of trillions of them all together, but they vary from one part to the other because, for example, my forearm is a quite a dry environment. It's a bit parched like a desert whereas something like my mouth or the inside of my nose is a hot and humid environment that's more like a rain forest. And each of these places harbors a very different community of microbes.

GROSS: And a lot of those bacteria - a lot of those microbes are good guys, helping your body do what it needs to do. But as you point out, if one of the good guy bacteria gets into the wrong spot, it becomes a bad guy.

YONG: That's right. We've had this long-standing idea that microbes are germs, are enemies that we need to destroy lest they destroy us. But actually we're coming to realize that many of these microbes are profoundly important for our lives. But I wanted to get across in the book that there isn't really any such thing as a good microbe or a bad microbe. They just live with us. They are our partners in life and they can often do us tremendous amounts of good. They can help to digest our food and tune our immune system and protect us from disease. But if they get in the wrong place or if our relationship with them breaks down, then they might also do us harm.

GROSS: So when we're talking about the microbes in the microbiome, obviously we're talking about bacteria. What else are we including in that?

YONG: So bacteria form the majority of them, but there are also another group of microbes called Archaea, which look very similar but are actually very, very different. There are single-celled fungi. There are protists. There are viruses, too, in huge numbers. Some people might not think of them as part of the microbiome, but they are there, too, and they are incredibly influential.

GROSS: I want to talk with you about probiotics. So many people take probiotics in the hopes of having a healthy gut. And among scientists who actually study the microbiome, which is the microbes living in the gut, it's actually pretty controversial whether probiotics accomplish anything. So let's start with what are probiotics supposed to do?

YONG: So probiotics are products that contain beneficial microbes, and they have all sorts of health claims attached to them, but actually if you look at the evidence behind these products, they tend to be a bit medically underwhelming. So they're good for certain types of infectious diarrhea but not a huge amount else. And even regulators in the U.S. and the - and in Europe have taken a dim view of many of these health claims. And I think that's because probiotics contain typically small numbers of microbes, much fewer than already exist in your body.

A lot of these species are being chosen for historical reasons because they're easy to manufacture and package, not because they're excellent at establishing themselves in the gut. And often they're very industrialized, very proprietary strains. You know, they enter and then they disappear or they pass through. They don't have a huge amount of impact. And yet, the idea behind probiotics, the idea that we might be able to give ourselves beneficial bacteria that improve our health, that idea has a lot of potential, and it is solid.

But we are still in the very early stages of working out how to do this. It is about engineering an entire ecosystem. It's about - it's as complicated some say, like, changing a coral reef or a grassland. It's a - it's about sculpting a world.

GROSS: So when we take a probiotic that we bought in a store, are the bacteria in that probiotic likely to repopulate in our gut?

YONG: No, they're not. Typically, they don't manage to establish themselves, and they don't last for a long time, so they're not very good colonizers.

GROSS: And are they microbes that typically live in our gut?

YONG: They're microbes that can be found in the gut. But they're not - they're not the A-listers. They're not the ones that are especially common or especially good at colonizing the gut.

GROSS: What about yogurt, which usually has active cultures in it, probiotic cultures?

YONG: Same thing. Studies have shown that you swallow them and those bacteria then disappear. They - I think of them as, like, a breeze that blows between two open windows. It might rattle a couple of objects in the way, but it doesn't have a lasting impact.

GROSS: You're depressing me.


YONG: I'm sorry.

GROSS: OK. When you take antibiotics, the antibiotics kill off a lot of the bacteria in your body, including ones that you would prefer to have and some of those bacteria that you'd prefer to have live in your gut. And I think that's at least one of the reasons why taking antibiotics can lead to diarrhea because it kills off some of the bacteria that keeps your gut in balance. And that's usually a time when even doctors who don't especially believe in probiotics still recommend that you take it. So what's the common wisdom now among scientists who are actually studying this about the value of taking probiotics as an antidote to antibiotics?

YONG: Well, so some types of antibiotic-associated diarrhea involve this very nasty, weedy bacterium called Clostridium difficile, or C. diff, which is very good in invading guts that have been repeatedly assaulted with antibiotics and can cause recurring intractable cases of diarrhea. And actually one of the best treatments for C. diff - oh, and in fact, one of the most successful microbiome-based treatments we have is a thing called a fecal transplant. A fecal transplant is almost exactly what it sounds like. You're trying to take the stool from a healthy donor and implant it into the gut of someone who's sick. And you are doing that almost as a way of resetting their inner ecosystem, of replacing a community that has been faltering, much like trying to re-turf a lawn that's been overrun with dandelions or other weeds.

GROSS: And how is that being done now? It's been in its experimental stages, but I think they're kind of perfecting ways of doing it without being - without it being as gross as it sounds.

YONG: So the traditional way is to use actual stool and it can be delivered either up the bum or down the nose via a tube. And, you know, it does have this ick factor attached to it. But it is also spectacularly effective at treating infections of Clostridium difficile - C. diff - this very invasive, weedy bacterium that can cause recurring severe diarrhea. In one now classic study, some 20 - or 27 percent of people who took a common antibiotic were cured of C. diff infections whereas those who receive fecal transplant, about 94 percent of them were cured. So we're in this weird position where the number one treatment for C. diff is a number two. And...

GROSS: (Laughter).

YONG: ...Fecal transplants are - have been - have done wonders for this particular condition. Whether it works for other diseases - so in - so following the successes with C. diff, scientists and doctors have tried fecal transplants for a large number of other conditions that have been linked to the microbiomes, things like inflammatory bowel disease or irritable bowel syndrome.

And there, the results are less - certainly less successful, a bit less consistent. And that's probably because those types of diseases - those ecosystems inside us - are more difficult to reset. So others - other researchers are now trying to take the concept behind fecal transplants and refine it to pull out the bacteria and other microbes that are conferring the benefits to the recipients without having to bother with the gross fecal bit.

So several companies have developed pills that are - you can think of them either as, like, pared-down stool or souped-up probiotics, right? So they are communities of microbes that are meant to replicate a lot of the benefits of doing a stool.

But recently, one of the most leading contenders for these fake poop pills flunked out of a major clinical trial, which I think emphasizes how little we know still about why this age-old procedure actually works and how early we are in the process of actually manipulating our microbiomes in order to improve our health.

GROSS: Did you say this was an age-old procedure?

YONG: Yeah. Well, it first started - I think the first records were from China several centuries ago. And people have sort of rediscovered it time and again. And it's only become - I'm not going to say popular - but certainly more prominent in the last 10 - maybe even five - years or so.

GROSS: So why is it that a fecal transplant might be more effective than a probiotic pill?

YONG: So there, you're using a much wider community of microbes. And you are giving them in large numbers. And you're using the ones that are naturally there in the gut anyway. So it's a much more realistic community to implant in a person. But, you know, the real answer to this question is we still don't entirely know.

We don't really know which components of stool - which members of that community - are really doing good, how they might interact with the native microbes in a recipient's gut or with the gut itself, with the immune system of the host.

There are still so many questions that we have left to answer. All we know is that for this one particular condition, C. diff infections, it seems to be a very, very good treatment with few known side effects, if any. And that's very cheap.

GROSS: If you're just joining us, my guest is science writer Ed Yong. His new book is called "I Contain Multitudes: The Microbes Within Us And A Grander View Of Life." Let's take a short break. And then we'll talk some more. This is FRESH AIR.


GROSS: This is FRESH AIR. And if you're just joining us, my guest is science writer Ed Yong. His new book is called "I Contain Multitudes: The Microbes Within Us And A Grander View Of Life." He also writes about science for The Atlantic and has a science blog for National Geographic called "Not Exactly Rocket Science."

Microbes can contribute to our immune system. And you describe how microbes can trigger inflammation. And microbes can also suppress inflammation. Can you give us a sense of the interplay between those two sets of microbes and how they regulate our immune system, for better and for worse?

YONG: Right. So we typically think of the immune system as this militaristic force that defends us from the microbes in the world around us. And yet we see that certain types of microbes help to build parts of the immune system. And then they also calibrate it.

So they strike this balance between reacting - between protecting ourselves from infections - but also not overreacting. So they help - some species help to stop the immune system from going berserk at harmless things in our environment like pollen or dust, things that might trigger allergies.

And that balance between inflammation and anti-inflammation is very much affected by our microbes. They set that tipping point, much like the thermostat in my house might control the temperature of a room and keep it stable.

GROSS: So this interaction between microbes that suppress inflammation and those that trigger it are being studied in germ-free mice, mice that are meant to have a controlled environment within them because they're germ free. How do you create a germ-free mouse?

YONG: You raise them in the sterile isolators, these plastic bubbles that are completely disconnected from the microbial world. The mice are the only things living inside them. They have sterile food, sterile water. Anything that's put in gets sterilized before it's connected to these chambers.

And I think that makes these mice some of the strangest animals on the planet. I visited one of the facilities that raises them. And I held one not with my bare hands but with this black rubber glove that protrudes into their plastic bubble. And this is an animal that, unlike anything else - unlike you or me or an elephant or an - anything - has no microbe.

It's like a silhouette that hasn't been filled in yet. And because of that, it has a lot of problems. It has different - it has problems with its bones, its blood vessels, its gut, its behavior. It testifies, I think, to just how important microbes are for sculpting and shaping our bodies and how much can go wrong if we live our lives in their absence.

GROSS: So what kind of experiments on immunity and inflammation are being conducted on these germ-free mice?

YONG: And so, for example, you can give them specific microbes and see which types of immune cells rise to the fore, like what - whether you see an inflammatory or anti-inflammatory response. And we can see that certain microbes can actually tune down inflammation which is an odd concept because I think we think of microbes as foreign, as other, as things that trigger information. Whereas we see with rodent experiments that some species that live with us are actually capable of quenching inflammatory fires, so they are capable of calming the immune system and saying I come in peace.

GROSS: What are the implications of this for autoimmune diseases, diseases in which the body attacks healthy body cells?

YONG: So theres a growing body of evidence suggesting that early exposures to a wide range of microbes is important for carefully building and calibrating our immune systems to strike that balance between immune reactions and not going nuts at the things in the world around us. And, perhaps, by avoiding contact with microbes either by living in urban environments or by sanitizing our worlds too much, we are breaking our contact with all of these old friends that we rely upon to tune our immunity, leaving us ill-prepared for dealing with all the challenges of the wider world, for dealing with all the stuff that might then trigger an immune response.

GROSS: What you're talking about, too, here is the theory that when children are born - when babies are born and they pass through the vaginal canal, they get exposed to a whole set of microbes that a baby needs to develop a healthy microbiome and that babies who are delivered through a C-section do not get exposed to the same microbes. Can you talk a little bit about that and what the latest thinking on that is?

YONG: Sure. So the womb is a sterile environment and we - and babies first encounter microbes when they pass through the vaginal canal during birth, and they set the tone for all the other microbes that then colonize the child. If you are born through a C-section, you encounter a different set of microbes that might come from say the skin or the hospital environment. So we see different communities in babies that are born from different roots.

Now, what that means for a child's future health is still very much unclear. We see from studies in adults that people who were born through C-sections or through normal vaginal roots don't actually end up with vastly different communities of microbes on their bodies. So it seems that in the long run, a lot of these differences shake themselves out. But whether those temporary differences can affect our health, whether they change the way our body develops at a very crucial point in our lives is very much an open question. We don't know the answers to that yet.

GROSS: Well, I'm glad that the C-section babies seem to catch up because a lot of women have to have C-sections, and I'd hate to think that they have to feel guilty that their babies aren't going to have healthy microbiomes as a result of that.

YONG: Right. I don't want to demonize anything, and I don't think microbiome science is in this - is in a mature enough state to make people worry or feel guilty about their health choices yet or their life decisions. And it's helpful to think of the body as these sets of grand ecosystems. You know, if you change a jungle or a grassland, you know, it's not going to go wildly off course through simple means.

Often, you need to ding these things multiple times from multiple stresses, say, pollution or fire or whatever before they go - they shift into an irreversible new state. So ecosystems are resilient. They can absorb a lot of change, and the same is true for the microbiome. If you ding it with, say, antibiotics, if you change it through, say, a C-section. It will move. It will change, but often it bounces back. And the big questions are, you know, what sorts of changes do you need in order to really change it? And do those little changes matter in the short term? And I think we don't know.

GROSS: My guest is Ed Yong, author of the new book "I Contain Multitudes" about the trillions of microbes that live in and on us. After we take a short break, we'll talk about the microbes in breast milk and how bacteria are being used to prevent mosquito-borne illnesses like Zika and dengue fever. And as the Olympics near the conclusion, Milo Miles has a review of an album from a Brazilian bossa nova ensemble. I'm Terry Gross, and this is FRESH AIR.


GROSS: This is FRESH AIR. I'm Terry Gross back with Ed Yong, author of the new book "I Contain Multitudes: The Microbes Within Us And A Grander View Of Life." It's about the trillions of microbes that live in and on humans and animals and how this ecosystem affects our digestion, immunity and general health. When we left off, we were talking about how this ecosystem is affected by the microbes you're exposed to as an infant. For example, scientists are studying how babies are colonized by bacteria during childbirth as the babies pass through the vaginal canal.

I think scientists are also investigating the difference between breast feeding and bottle feeding when it comes to developing a healthy microbiome in an infant. What are some of the differences between the bacterial makeup or the microbe make - microbial makeup of breast milk - human breast milk - versus cow's milk?

YONG: Well, so human breast milk seems like a very straightforward way of nourishing a baby. But actually about 10 percent of breast milk consists of these sugars, these complex sugars that babies can't digest at all. And they are there to nourish the microbes in the baby's gut. There are specific strains, including one called the...

GROSS: I'm going to stop you here, so part of breast milk is to feed the microbes (laughter).

YONG: That's right, yeah.

GROSS: That's kind of fascinating.

YONG: It's amazing, isn't it? I think this is why I love the science of the microbiome because it takes very familiar things like breast feeding and casts them in a completely different light. So breast milk isn't just a way of nourishing an infant. It's a way of nourishing babies' first microbes. It's really a way of engineering an entire world inside a baby's body. You know, breastfeeding mother is a sculptor of ecosystems.

GROSS: And cow's milk doesn't have that?

YONG: Human milk, for reasons that are still unclear, seems to have a much wider range of these sugars than the milk of any other mammal. There are very specific strains of microbes, including one called B infantis that have evolved to digest these sugars with great efficiency. So in the guts of breastfed infants these microbes tend to dominate.

And they have benefits for the baby, too. They feed the baby's gut cells in return. They help to seal up the lining of the gut, and they reduce inflammation. So they have - they have tremendous benefits. And by nourishing those microbes, mothers are also helping their own kids.

GROSS: I want to get back to talking about the germ-free mice and the use of mice that are living in germ-free bubbles so that scientists can experiment on them, exposing them to certain microbes to see what the response is. One of the things that scientists are studying in this kind of germ-free mouse is obesity and body weight. Would you give us a sense of the kinds of experiments being done relating to body weight with these germ-free mice?

YONG: Sure. So about 10 years ago, in some very classic experiments, scientists took microbes that were from obese and lean individuals and put them in the sterile mice. So the mice that received the obese community's put on more weight than their counterparts who received the lean ones. And that was a seminal finding. It really showed that perhaps these microbes aren't just, you know, going along for the ride. Maybe they're actually grabbing the wheel now and then. Maybe they are seriously influencing our weight, the way we deal with the energy in our food.

The thing is that the links between obesity and the microbiome are actually very complicated. And even though there's been 10 years of research, there are still a lot of things we don't know. So people have found differences in the microbe communities between people who are fat and lean. So you get different species that rise to the fore in different groups. But there aren't consistent differences between studies. So it's still really hard to say what, say, a, quote, unquote, "obese microbiome" looks like, let alone how you might want to manipulate it.

And one critical study that was done, again with these germ-free mice, showed that if you take the microbes from a lean individual and put them in a rodent that has put on too much weight, then that rodent will lose weight but only if it also eats a traditionally healthy diet that's full of plants and fiber. And I think that shows that the microbiome might be deeply influenced in our lives, but it doesn't provide any easy solutions to health problems. When the first wave of studies came out about 10 years ago, headlines were full of things like, you know, blame microbes for your weight or maybe we'll be able to take some probiotic that immediately causes us to lose weight. And I think it's nowhere near that simple.

The microbiome is not a panacea for our health problems. It doesn't explain them all in their entirety. I think it just shows how important microbes are in our lives and what a strong influence they have.

GROSS: And this is why someone - and I forget who it is - gives an award that's named the Overselling the Microbiome Award (laughter).

YONG: (Laughter) That's right, yeah. His name is Jonathan Eisen. He's a wonderful microbiologist. He's a true champion, an evangelist for microbes. But he's also very skeptical, and I think with good reason for a lot of the claims that are surrounding the microbiome. This is one of the most fashionable areas, the hottest areas of modern biology. And the microbiome has been linked to all sorts of health problems.

But a lot of those studies are still correlative. We still don't know whether the microbes are leading to the disorders, whether they're a consequence of them, whether it's both or neither. And we are still at this very early stage of understanding our partnerships with the microbes that share our lives. We know they're important. But how important are they? And in what ways, and how might we be able to influence them?

GROSS: So when it comes to microbiome research, it seems like we're still kind of in the infancy of it. We know enough to know this is a really promising field. We might learn a lot about foods to eat and not eat, about things to expose ourselves and not to expose ourselves to. We might find new cures for autoimmune diseases. But right now, it seems like it's still in the formative stages, very experimental.

YONG: Yeah, very much so. We've known about microbes for over 350 years. But the science of the human microbiome has really only taken off in the last decade or so. And there's so much we don't know. We don't still have a full catalog of the species that live inside us. We still don't really understand what makes my microbiome different to yours. What explains those variations between people - much less what a good microbiome looks like or how you might get to that stage.

But I think the potential there is huge because the microbiome is theoretically very flexible. We should be able to change it. We just don't quite know how yet. And maybe one of the quicker wins with microbiome science might be to use the microbiome as a sort of diagnostic tool, of looking at it and say what you're more likely to develop - this disease or that disorder - or to use it to influence our treatment. So we know that microbes within us affect how well medicines work in our body. We know that a lot of different drugs, from the hottest new cancer immunotherapy drugs to obvious things like acetaminophen can work in some people and not in others because of which microbes they carry.

And maybe we might also be able to find ways of influencing the microbiome without needing to add or subtract strains. So there's one group that does a lot of work on heart disease. And we give - if you have a high risk of heart disease, your doctor might prescribe you a statin, which is a drug that's designed to target enzymes involved in cholesterol metabolism that are made by your own body. But bacteria can also affect the way you deal with cholesterol, and they could also contribute to the risk of heart disease in that way. And now some - one group of scientists has developed a molecule that can stop these bacteria from leading to rises in cholesterol. So maybe in the future you might get a combination of two drugs from your doctor, a statin to treat the human half of you and another chemical to affect the microbial half.

GROSS: If you're just joining us, my guest is science writer Ed Yong. His new book is called "I Contain Multitudes: The Microbes Within Us And A Grander View Of Life." Let's take a short break. Then we'll talk some more. This is FRESH AIR.


GROSS: This is FRESH AIR, and if you're just joining us, my guest is science writer Ed Yong. His new book is called "I Contain Multitudes: The Microbes Within Us And A Grander View Of Life." He also writes about science for The Atlantic and does a science blog for National Geographic called "Not Exactly Rocket Science." Not everybody has a favorite microbe, but you do (laughter).

YONG: Yes.

GROSS: Tell us what it is.

YONG: I do. It is called Wolbachia. It was discovered in 1924, and it was found inside a common brown mosquito by two scientists, Hertig and Wolbach. And for decades, no one knew what it did or how common it was, whether it was important. It was largely neglected. And then scientists realized that actually it's incredibly common. It's not found in humans, but it is found in some 4 in 10 species of insects and other arthropods, which are themselves the most numerous and most diverse animals on the planet.

So Wolbachia is extraordinarily successful, and it does all sorts of incredible things. So in some hosts, it is a parasite that has a thing out for males because Wolbachia only passes down from mothers to daughters, and so males are a complete dead end to it. In some - in some species, like woodlice, it converts males into females. In others, it kills males outright so that the females outnumber males by 100 to 1.

And in some wasps, Wolbachia allows them to reproduce by cloning themselves, so they have no need for males whatsoever. But in some cases, this thing is also an ally. It is a mutualist. It allows some caterpillars that eat leaves to stop the leaves from turning yellow. It actually holds back the progress of fall so that these - its hosts can have more to eat. And it has properties that are really important to us humans.

So Wolbachia is found in parasitic worms that cause debilitating tropical diseases like elephantiasis and river blindness. And by finding drugs that can kill Wolbachia, scientists are finally looking for and discovering effective ways of curing and treating these illnesses.

GROSS: But what about mosquito-borne diseases like dengue and Zika? There seems to be some relevance there, too.

YONG: That's right. Wolbachia turns out to be a really powerful way of controlling mosquito-borne diseases. So scientists in Australia have spent about 25 years trying to load this bacterium into Aedes aegypti, the tiger mosquito, the one that spreads dengue and yellow fever and infamously now Zika. And they've been trying to do this because Wolbachia seems to prevent these insects from transmitting the viruses behind these diseases.

And Wolbachia is so good at spreading through a wild population that if you release a small number of these infected mosquitoes, they ought to fly off into the distance and then within months Wolbachia should be everywhere, thus rendering this entire community of insects unable to spread disease. They - the bacterium changes them from vectors into dead ends. And this was tried for the first time in 2011 in several suburbs of Australia. And this team showed indeed that Wolbachia rises to dominance in these natural populations, and they're now trying this approach in big megacities around the world, in places like Brazil and Colombia and Indonesia and Vietnam. And there are so many...

GROSS: Places with mosquito issues.

YONG: Places with mosquito issues, places with severe dengue problems. The approach was initially developed in order to eliminate dengue fever, but it seems to work against Zika, as well against a lot of other mosquito-borne diseases, and it has tremendous advantages. The - unlike, say, insecticides, which are toxic and need to be continuously resprayed, Wolbachia in these mosquitoes seems to have no problems, no safety risks. And it sustains itself. Once you release the insects, they go off on their business and you don't need to keep on replenishing them.

GROSS: So you're not messing with the mosquitoes' genetics. You're introducing a bacteria?

YONG: That's right. There is no genetic modification. These mosquitoes are not GMOs to speak of. They've been simply united with a bacterium that is already present in a large proportion of other insects.

GROSS: So now that you see your body as housing huge communities of microbes, how has that changed your life? Has it changed, like, what you eat or how you behave or what you expose yourself to?

YONG: I'm certainly less fearful of the microbial world than I would have been 20 years ago. You know, I don't - I wash my hands of course, but I don't over-scrub them. I don't use sanitizers. But, you know, really what writing this book has given me is a broader perspective of the world around me. So I always loved the natural world. You know, I was that kid who went to zoos all the time. I would watch wildlife documentaries and read books about nature.

And now I understand that all this biology which I thought I knew, all these creatures, these elephants and hawks and fish that I was fascinated by, these things I could see with my eyes, are actually deeply and profoundly influenced by things that I cannot see. And I know that if I go to a zoo now that every animal and every visitor in that zoo is in fact a zoo in its own right.

GROSS: Give us an example of an animal that you see differently now.

YONG: There are - in the very deep oceans, there are environments called hydrothermal vents, these belching chimneys of superheated water. And there's a worm that lives in these vents called riftia and it's - it can grow a few metres long, and it has these beautiful red gills that look like someone's pushed a tube of lipstick out all the way. And this worm lives in this environment, which just hasn't got any food, but it has microbes in its body.

A lot of its body, probably the majority of it, consists of bacteria. They grab minerals from the vent environment and use these to create nutrients. And that is entirely how the worm sustains itself. This animal has lost its mouth and it's gut because why bother having a digestive system at all when you don't need to eat when you have microbes inside you to provide you with all your energy? And I think that's just a stunning example of how intimate the animals can evolve to rely upon their microbes. This is a creature that would not be able to survive at all without the multitudes inside it.

GROSS: Don't you wonder what the world looks like from the point of view of the microbes that you write about, like, if one of the microbes was writing this book, how different it would be (laughter)?

YONG: I feel like we wouldn't even get a mention.

GROSS: (Laughter).

YONG: We are just this footnote, this icing on the - on this gigantic microbial cake. You know, in the book, I talk about how if you condense the history of the Earth into a single calendar year, so the Earth was created in the first of January and we are now in the 31 of December just before the stroke of midnight, then humans arose about 30 minutes ago. And even all the multicellular creatures, the things we can see, all the animals and plants we're familiar with, only arose a couple of months back whereas bacteria probably first evolved, say, at the start of March. So for the vast majority of life on Earth, everything was microbial.

You know, bacteria have been the rulers of the planet for most of the Earth's history, and they are still the rulers of the planet. They still govern the movements of chemicals all around the world. They are in every possible habitat. They are omnipresent, and they are hugely influential. And in many ways, our bodies, the bodies of every other animals or our cities that we are so proud of, all of these are just more habitats for bacteria and other microbes to inhabit.

GROSS: Ed Yong, thank you so much.

YONG: Thank you very much for having me.

GROSS: Ed Yong's new book about the microbes within us is called "I Contain Multitudes." After we take a short break, Milo Miles will review an album by a Brazilian bossa nova ensemble. This is FRESH AIR. Transcript provided by NPR, Copyright NPR.

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