IRA FLATOW: What am I thinking about here? Here are the clues. Without it, you couldn’t blink, swallow, smell, or taste. You could not digest your food either. In fact, you wouldn’t even exist. The seemingly miraculous reason for your survival is your mucus.

Now, this natural goo, it’s ubiquitous. It’s found in jellyfish and hagfish. It coats corals and oozes from vegetables. Okra, anybody? And far from a waste product, mucus is an amazing natural substance that ensures our bodies keep working and tames the mighty microbiome. After all, somebody needs to do the job, right? Well, mucus gets it done.

Let us know what you want to talk about, about mucus. What do you need to know? What do you want to know about mucus? Our number is 844-724-8255. You can also tweet us @scifri. 844-724-8255. And our trusty guide through the magical world of slimy stuff is Kathari Ribbeck, a biochemist– Katharina– I’m sorry– Ribbeck, biochemist and a professor at MIT in Cambridge. She runs the Biogels Lab in the Department of Biological Engineering. Welcome to Science Friday, Katharina.

KATHARINA RIBBECK: Thanks. Hello, Ira. Thanks for having me.

IRA FLATOW: Why do we have mucus?

KATHARINA RIBBECK: Why do we have mucus? Well, you just gave us this really great introduction. It is a material that, really, most animals have– animals and also sponges, all sorts of creatures. It helps protect your surfaces.

So we have two types of surfaces– the skin we typically see and think about our skin a lot more. Then our internal surfaces, those are the moist surfaces. And that’s a much bigger surface– 2,000 square feet in the intestine alone. And those surfaces are very tender, they’re fragile. They contain our sensory organs. They need to absorb food. And they need at all times to be coated with mucus. They need to be moisturized, and that is what mucus, the gel does. It helps preserve moisture in all these epithelia.

IRA FLATOW: You were just talking about our intestines were one of our favorite topics the microbiome lives. Is it mucus that puts the barrier between those trillions of microbes and our intestines, so sort of protects them from the bacteria there?

KATHARINA RIBBECK: Well, here’s the thing. Trillions of microbes are inside the mucus. Yes, it provides a barrier from infection, but the way it works is really that it maintains a broad variety of microbes, both beneficial and pathogenic. And it is really fascinating to think about how it can manage to accommodate them without the problematic pathogens causing really damage to the body.

IRA FLATOW: That is amazing. I’m Ira Flatow. This is Science Friday from WNYC Studios, talking with Katharina Ribbeck, who’s a biochemist and professor at MIT in Cambridge. I mentioned at the top of the segment that you couldn’t blink without mucus. You know what I’m thinking, that there’s tears in our eyes. Is it mucus that’s in our eyes?

KATHARINA RIBBECK: Yeah, exactly. So tears are some form of mucus, very thin mucus. The polymer– those are thread like molecules that entangle with each other– reappear in different surfaces of the body, and the eye’s surface is one of the places, yeah. And they form this very ever so thin sheet of lubricious layer with which you can blink effortlessly. And when it is missing, that can cause a lot of damage and discomfort and infections.

IRA FLATOW: So those little crusty things at the corner of your eye when you wake up is crusty mucus?

KATHARINA RIBBECK: That is collected mucus and the dried out mucus, yeah.

IRA FLATOW: Is there any way to measure how much mucus we make each day?

KATHARINA RIBBECK: Yeah, you can estimate. It’s probably a couple of liters a day. So the body invests a significant amount of energy into building that material.

IRA FLATOW: So what is mucus? Is it a solid or a liquid or something in between?

KATHARINA RIBBECK: Yeah, exactly. It’s something in between. It has liquid like properties, but it also has solid like properties. And that is because it forms a very, very thin gel. These gel forming properties can change depending on where the material is in the body. In the eyes, it’s very thin. In the mouth, it’s very thin. In the lungs, it’s thin.

But then in the stomach, the place where mucus protects your stomach really from digesting itself, it forms a thick, almost rubber like material. And yeah, in the reproductive tract, the properties of mucus can change depending on pH, for example. And that allows passage of sperm during ovulation, for example.

IRA FLATOW: Do we change how much mucus we make as we age? Does it change?

KATHARINA RIBBECK: Yeah. Yeah, so it can change, and also, the properties, the characteristics of mucus can change. And that can lead to suboptimal mucus moisture. For example, dry eye, a lot of– 20% of the elderly population suffer from dry eye symptoms, dry mouth. Yeah.

IRA FLATOW: And we had at the top of the show when we asked on our SciFri VoxPop app about mucus, and somebody mentioned that it was just snot. Is that true?

KATHARINA RIBBECK: The dry– the tear that dries–

IRA FLATOW: When it comes out of your nose, it’s snot.

KATHARINA RIBBECK: That’s not exactly– yeah, yeah, yeah. But that’s really the waste part that’s mucus when it’s done. It’s like an air filter that you exchange. And when it’s full and dirty, you gotta take it out, and that’s what you do when you blow your nose.

IRA FLATOW: That’s a good time to take a break here. [CHUCKLES] We’re talking here about mucus with Katharina Ribbeck, a biochemist and professor at MIT. Our number, 844-724-8255, is our number. It’s not what you think today on Science Friday. And we’ll prove it when we get back. Stay with us. We’ll be right back after this break.

This is Science Friday. I’m Ira Flatow. We’re talking with MIT biochemist Katharina Ribbeck about mucus, how it evolved, how it helps us, how it subdued problematic pathogens. And we’re taking your calls, too. Let us know what you want to know about mucus. Our number is 844-724-8255. Also, you can tweet us, @scifri.

And we had a lot of listeners submitting questions about mucus on our Science Friday VoxPop app, and I’d like to address a few of those questions. Here’s a question from Mark in Montana.

MARK: I live in the west and run in the mornings around 5:00 to 5:30. And anytime the temperature is below 55 degrees, my eyes and my nose start to run. Why is that?

IRA FLATOW: Dr. Ribbeck, you must know that.

KATHARINA RIBBECK: Well, that’s a really good question. The water vapor, the air in your nose is warm, and when it hits a cold air as you breathe in, the vapor reaches its dew point, and it comes to some sort of condensation where the cold air forms liquid drops of water, which then come out of your nose.

IRA FLATOW: Now David in California had a very popular question.

DAVID: Is this healthy or unhealthy, to eat snot? I’ve heard both. Do you have any comments about this?

KATHARINA RIBBECK: Yeah, interesting. This is a question I get a lot. Snot, I wouldn’t recommend. Maybe eating– though kids do it a lot of course. But in the ’30s, a related form of mucus was tested out to treat ulcer patients. So a clinician purified mucus from the stomachs of hogs, and he gave that to patients who had uclers. And these patients, the symptoms went away. And they didn’t reoccur. So there is, I think, a reason to believe that eating mucus or supplying mucus supplement to fortify the natural mucus barrier can help, indeed, when it is dysfunctional.

IRA FLATOW: Here’s a tweet from RC who asks, “When mucus changes color, does it mean it is dead?”

KATHARINA RIBBECK: So mucus itself is a material. It’s like rubber. What gives it the color is what lives inside it and what is deposited, so for example, dirt that you breathe in or toxins that bacteria secrete that live inside. They can have colors. Green is one that, for example, bacterium Pseudomonas aeruginosa can secrete. And that can show up as green as this snot-like color that we know.

IRA FLATOW: Let’s go to Mark on the phone in Des Moines, Iowa. Hi, Mark. Welcome to Science Friday.

MARK: Hi. Specifically, I was interested in the mucus of the intestinal lining. And I was wondering, are there any specific activities that you can do to either damage that lining or to improve it?

IRA FLATOW: Hm, good question.

KATHARINA RIBBECK: That’s a great question. Unfortunately, we don’t have tools yet to regulate mucus production at will. So the best I think you can do is eat healthy, and yeah, take care of your nutrition. Our options to really modify mucus and influence it right now are really limited, but it’s a big, big piece of research in the future that we want to address.

IRA FLATOW: Let’s go back to our Science Friday VoxPop. Here is Beckett from Virginia asking this question.

BECKETT: This is Beckett, and I was wondering why spicy foods make your nose run.

IRA FLATOW: That’s a good question.

KATHARINA RIBBECK: That’s a really good question. Also, yeah, the molecules that make your food spicy, they have the ability to bind to pain receptors. So that’s a binding event where the pain receptors then signal to the body that there’s some inflammation or damage to the tissue, which in turn stimulates mucus production, which has a protective barrier function.

IRA FLATOW: Here’s a tweet from Anne. She wants to know via Twitter, does constant spitting of mucus or saliva by men cause health issues?

KATHARINA RIBBECK: I’m not sure the study has been done. Yeah, I am not sure.

IRA FLATOW: Well, do we know why people spit like that? Is there accumulation of mucus or–

KATHARINA RIBBECK: Yeah, sometimes when the mucus is too thick or if you have allergic reactions, it can feel uncomfortable in the mouth if it’s too thick. Your mouth is sensitive to the gel properties. If they’re not right, then you want to get rid of it.

IRA FLATOW: Your team had a recent study out in the journal Nature Microbiology about how mucus tames microbes. You mentioned it just a bit. Can you give us some more details?

KATHARINA RIBBECK: Yeah, so the question we addressed here was the following. You have trillions of microbes growing on and inside of your body. And the vast majority of these microbes live inside the mucus of your digestive tract. And many of these microbes are beneficial. They help you digest food. They help you build vitamins. And some also protect you from potential other pathogens.

But in this mix are also problematic pathogens. And in fact, it is a wonder that we are alive. So over millions of years, mucus has evolved the ability to keep those problematic pathogens in check. And we wanted to get to the bottom of how exactly the material does that. And what we’ve found is that mucus does that with so-called mucin polymers. mucin polymers are thread-like molecules. They look like tiny bottle brushes, and they bind also components from the immune system, which protects your health.

And on these bottle brush molecules are lots and lots of sugar molecules, glycans we call them. Those are body produced sugars that can regulate microbial physiology. They tame the microbes. They take away their ability to build certain weapons, certain molecules, or strategies that they need to mount infections effectively.

IRA FLATOW: Interesting. Let’s go to Jenny, Jenny in Norman, Oklahoma. Hi, Jenny.

JENNY: We’d like to know what makes mucus smell.

IRA FLATOW: What makes it smell?

KATHARINA RIBBECK: What makes it smell?

IRA FLATOW: Jenny, does your mucus– well, do you think mucus smells?

JENNY: [GIGGLING] Sometimes.

IRA FLATOW: OK.

KATHARINA RIBBECK: What kind of mucus are you thinking about?

JENNY: You know, when you’re sick, and maybe you’re coughing up, coughing up gunk.

KATHARINA RIBBECK: OK, so that usually is byproducts from bacteria. That’s not the mucus itself. Again, mucus is not– mucus is just a scaffold. It’s like a sponge. And whatever grows inside of it can sometimes release factors, and the microbes can release factors. Underarm odor is a similar situation, where you have the molecules that can cause odor in the mouth also. These are usually products secreted by the microbes that live inside your mucus.

IRA FLATOW: Now that’s an interesting point you made because we have a tweet from Jonathan who asks, is my mucus the same as your mucus? And how is mucus catered to our body? Or is it specific to everybody’s body, their mucus?

KATHARINA RIBBECK: Yeah, that’s a great question. And we think this is a really active area of research now. We think there are, on one hand, many personal differences between individuals, but also even within an individual, adapting to the exact environmental conditions to incoming microbes or nutrients. So there’s a constant, we think, dynamic adjustment of the mucus properties. And sometimes they can go wrong, these adjustments, and cause disease. But yes, there is a lot of flexibility and dynamics, and we want to understand exactly how they come about and how we can regulate them if we don’t want them.

IRA FLATOW: That’s interesting. Let’s go to a Science Friday VoxPop from Ingrid in LA.

INGRID: Why aren’t we using mucus as an adhesive when it has such excellent bonding and polymerization properties?

KATHARINA RIBBECK: So that’s an interesting question. Mucus is, on one hand, adhesive because, indeed, it associates with the surface of the body. That’s how it keeps the surface moist, but at the same time, it has a very special property. And that is it is lubricious. That means it is actually anti-adhesive. That’s why it’s also called an anti-fouling surface because it prevents things, microbes and particles, from sticking to it. So it actually– as opposed to similar materials made by mussels under water that attach them to stones, and those are really adhesives. Mucus is a lubricious layer, a very soft gel that prevents adhesion more.

IRA FLATOW: Let’s talk about what happens when mucus goes wrong. Here’s a tweet. Jackie wants to know– she says, “I work as a nurse and have seen people actually die from overproduction of mucus. Why is the body unable to regulate mucus production at certain times?”

KATHARINA RIBBECK: So in the lung, for example, the way the lung surfaces are kept clean is the cilia. You have tiny protrusions on the surface of your body, which continuously sweep out the mucus that absorbs, again, like an air filter particles that shouldn’t go into your body toxins, and microbes also. So that’s how it’s kept clean. Those cilia continuously shuffle out, sweep out that used mucus. And sometimes when mucus gets too thick, and you have this, for example, with cystic fibrosis, these cilia becomes too heavy for those cilia, and they fail. That is one example.

IRA FLATOW: Interesting. To the phones, to Sarah in Madison, Connecticut. Hi, Sarah.

SARAH: Hi. I’m fighting a cold right now, and I’m wondering why the body overproduces mucus when you have a cold.

IRA FLATOW: Good question.

KATHARINA RIBBECK: Yeah, great question. So we think that has to do with its protective properties, that when it senses an inflammatory state, when it senses an acute exposure to pathogens that could be damaging viruses or microbes, bacteria, then it increases secretion of molecules that specifically can attend to those microbes, and hopefully, neutralize them.

IRA FLATOW: Hi, John in Cincinnati, welcome to Science Friday.

JOHN: Yeah, hi. Yeah, so I have a question. I’m not sure if saliva is considered a type of mucus or not, but when, let’s say, a dog either licks itself or a human being, what’s the impact of that? Is that a good thing, bad thing?

IRA FLATOW: Yeah, we hear the animals licking animals. They have some antibiotic effect. Is that true?

KATHARINA RIBBECK: Yes, that is absolutely true. That is also something we want to leverage. In part, this comes from the polymers themselves. But then the polymers are also loaded, charged with molecules from the immune system, which can additionally assist in fighting off of microbes. You see this very prominently, for example, on the skin of frogs, which they live in dirt, basically. And when they hurt themselves, they don’t get infections. That’s because they have this very potent mucus layer.

IRA FLATOW: Even underwater, right? Underwater, I remember I used to raise corals, and I remember that they were prolific mucus producers.

KATHARINA RIBBECK: Yes, yes, yes.

IRA FLATOW: And why?

KATHARINA RIBBECK: So just like us, they need mucus to engage with their environment in a way that enhances their function. For example, they attract food particles. They enrich food particles with this gel. They move the food particles around. But then what they also do is they select for themselves a range of microbes that can benefit their health.

And it is fascinating because these microbes are really distinct from the environment. It’s a very specific set, just like we have in our mouth, our intestine, succinct from not just other species, but then also from the environment. And some other– like squids, they use mucus to build light by selectively recruiting microbes that can develop bioluminescence. And that’s a survival strategy, so–

IRA FLATOW: I have to make note of that for Cephalopod Week coming up.

KATHARINA RIBBECK: Exactly, yeah.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios, talking about mucus with Katharina Ribbeck and all kinds of things. What about vegetables? Do vegetables need mucus?

KATHARINA RIBBECK: Yeah, we all, I think, have experienced either the goo in okra.

IRA FLATOW: But is it the same mucus that we’re making?

KATHARINA RIBBECK: No, I don’t know enough about the composition, but it could have a related function to fend off predators.

IRA FLATOW: Mm-hmm. I’m sorry, go ahead.

KATHARINA RIBBECK: No, that eats plants, so plants do also sometimes shed mucus that can protect them from–

IRA FLATOW: Mm-hmm. Is there any connection between mucus and it being used diagnostically to tell if you have a disease or not, besides just the cold that you get?

KATHARINA RIBBECK: Yeah, so that is, I think, a really exciting and promising area of research– however, really understudied, where one can imagine that changes in mucus properties can, if sensitively enough measured, can be used as a diagnostic tool to preempt, to identify susceptibility, for example, or already existing disease dysfunction.

IRA FLATOW: Mm-hmm. Interesting tweet from Kelly who says, isn’t mucus production very dependent upon hydration? If you’re drinking enough water or something.

KATHARINA RIBBECK: I would think so. I’m not sure. I’m reluctant to– the measurements.

IRA FLATOW: It makes sense, doesn’t it?

KATHARINA RIBBECK: It does.

IRA FLATOW: You’re not going to make mucus if you’re not drinking enough.

KATHARINA RIBBECK: Yeah, yeah, yeah. One would think there’s a connection.

IRA FLATOW: Well, what do you want to know about mucus that you don’t know about mucus?

KATHARINA RIBBECK: What I want to know?

IRA FLATOW: Yeah.

KATHARINA RIBBECK: I want to know– so here’s what I want to know. I want to know how it selects for microbes, because that is really, really powerful. That can equip us with potentially new functions. So the microbiome that we have in our mouth or in the intestine is specific. It’s diverse, but also really specific. When infants are born, initially, they don’t have a microbiome. And then they go through a phase where they have many more microbes growing, types of microbes growing and then eventually become permanent residents.

And that specification is exciting because if we understand that, not only can we maybe prevent outgrowth of unwanted microbes, but perhaps we can also invite new microbes that have special skills, such as giving a salt tolerance or helping us build food in our digestive tract.

IRA FLATOW: Well, if you’re a listener to Science Friday, you know that the microbiome is right in our wheelhouse. So we’ll have you back when you figure that out. OK?

KATHARINA RIBBECK: OK.

IRA FLATOW: OK, Dr. Ribbeck? Katharina Ribbeck is a biochemist and professor at MIT in Cambridge, and she runs the Biogels Lab in the Department of Biological Engineering. Fascinating work, Dr. Ribbeck. Thank you for taking time to be with us today.

KATHARINA RIBBECK: Thank you so much for having me.

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