11/27/2020

Why We’re Giving Thanks To Microbes For Stinky Cheese

16:57 minutes

a wheel of white camembert cheese next to petri dish of mold
A petri dish containing the mold Penicillium camemberti (right) used to make Camembert cheese (left). Credit: Adam DeTour

‘Tis the season for some of the best food smells of the year, whether it’s the aroma of turkey browning in the oven or the scent of freshly baked apple pie. But if you’re a cheese-lover—or a microbe—perhaps you prefer the inviting, smelly odor of a bloomy rind brie or camembert. 

New research published in the journal Environmental Microbiology suggests that the bacteria living on the cheese rind love those funky fumes as much as we do. Ira talks to cheese researchers Benjamin Wolfe and Casey Cosetta from Tufts University about why you should be giving thanks to microbes for stinky cheese this holiday season. 

a close up cross section of a thick layer of mold on cheese
A cross-section of a bloomy rind cheese (Camembert) showing the white rind that forms on the surface of the cheese. Credit: Benjamin Wolfe
closeup of fungi that look like veiny discs
Fungi isolated from a bloomy rind cheese. These fungi produce volatile organic compounds (VOCs) that bacteria can use as a food source. Credit: Benjamin Wolfe

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Segment Guests

Benjamin Wolfe

Benjamin Wolfe is an associate professor in the Department of Biology at Tufts University in Boston, Massachusetts.

Casey Cosetta

Casey Cosetta is a post-doctoral researcher in the Department of Biology at Tufts University in Boston, Massachusetts.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. What’s your favorite holiday smell? Is it the aroma of nicely browned turkey, the scent of cookies in the oven, or the wafting fumes of mulled wine? Well, for some people when it comes to a unique aroma, the cheese stands alone.

If you’re a cheese lover, perhaps you prefer the inviting smelly odor of a bloomy rind Brie or Camembert enjoyed by the fire. That’s certainly true if you’re a microbe. Because new research suggests that the bacteria living on the cheese rind love those funky fumes as much as we do. And we’ll gobble them up.

My next guests will talk about why that’s important. And about a surprising discovery about the wild blue fungi found on the walls of French cheese caves. Dr. Benjamin Wolfe is Associate Professor in the Department of Biology at Tufts University. Dr. Casey Cosetta is a postdoctoral researcher at Tufts and first author on some of this new research. Welcome to Science Friday.

BENJAMIN WOLFE: Thank you, it’s exciting to be here.

CASEY COSETTA: Hi, Thanks for having me.

IRA FLATOW: Nice to have you Ben, let’s start with the microbes that live on cheese rinds. Tell us what is the cheese microbiome made out of.

BENJAMIN WOLFE: So the typical cheese that you would see in a artisan or gourmet cheese market would be something like a Camembert or Brie. And so that white fuzzy surface that you see is a community of microbes that includes mold– so the fuzzy white surface– as well as yeast and bacteria. So it’s a mix of many different types of species. It could be anything from two species, all the way up to 15 or 20 different species of microbes living together in what we call microbial playgrounds on the surface of cheese.

IRA FLATOW: Playgrounds, I like that. What benefit does this fungi and bacteria give the cheese?

BENJAMIN WOLFE: So one of the main reasons that cheese makers would grow a rind on the surface is to add additional flavor– to really bring out aromas, to bring out various tastes when you bite into that cheese that you wouldn’t get from something like a standard plastic-wrapped cheddar. So that the rind is doing essentially what you see when a log is rotting in a forest, it’s decomposition.

But in this case, it’s delicious rot– that the cheese maker is controlling the rotting process as those microbes break up the proteins, the fats, and the other components of the cheese– the curd, the milk– they release all these flavors that we smell and that we taste.

IRA FLATOW: Casey, let’s talk about these smelly cheeses. I love the Bries, the Camemberts, the bleu cheeses. Because you discovered that one of the bacteria on cheese is actually attracted to this smelly stuff released into the air by the fungi. Is that true?

CASEY COSETTA: So when these fungi are breaking down these components of the cheeses, they’re releasing what we call volatile organic compounds. And those are the smells, and the taste that they release into the cheese and into the air. And that’s what some bacteria on cheese are actually attracted to, or eating up in a sense.

IRA FLATOW: So the fungi are actually feeding the bacteria then.

CASEY COSETTA: Yeah, so the cheeses aren’t just tasty for us, they can be tasty for the bacteria.

IRA FLATOW: And so why is that important for the bacteria?

CASEY COSETTA: It’s really important for one bacteria in particular, called vibrio, and that’s because volatile compounds that the fungi are producing, that’s the major source of nutrients that this bacterium, vibrio, uses to grow. And without it, it grows very little.

IRA FLATOW: And what happens to the bacteria after they eat the smelly compounds? Do they ingest it and it’s really, as you say, food for them?

CASEY COSETTA: Yeah, so we think that they’re somehow taking it up and metabolizing these compounds, using it as a source of carbon and then really taking off and blooming. They’re not only taking up food for themselves, but they’re taking over the whole rind, the whole microbial community.

IRA FLATOW: So if you can control the release of this smelly compounds, then you can control the growth of the bacteria and perhaps the flavor of the cheese?

CASEY COSETTA: In theory, yes. That would be an application of what we’ve learned here. That if we can kind of use these smells as a means of control or as a means of flavor, that would definitely be very important for some people like cheese makers or even cheese makers at home.

BENJAMIN WOLFE: So some other work in our lab shows that the types of bacteria and fungi that you have in a community living on the rind can dramatically impact the aromas that are coming from that cheese. And so what this study shows is that these volatile organic compounds are a way for cheese makers to dial up or dial down certain microbial members and then impact the overall quality of the cheese. And this is something that we’re learning in cheese, but it’s also potentially applicable to other microbiomes where you may want to manipulate or manage the community of microbes.

IRA FLATOW: Ben, your previous work discovered how the wild blue fungi found on French cheese caves, literally mutates before your eyes to create the creamy white rind we eat on say, a Camembert. Can you tell us about this?

BENJAMIN WOLFE: Yeah, so when we look at a wheel of Camembert, we see this white fuzzy mole that’s really iconic. It creates that amazing appearance and also contributes to the taste of that cheese. And what’s fascinating about that microbe is it’s domesticated. We think of animals like cows being domesticated and corn being a domesticated plant, but we don’t really think of microbes as being domesticated.

And so what my lab, and also researchers in France, are really interested in where did that microbe come from? How did we train that microbe to become the important cheese fungus that it is today? And so through a variety of experiments in my lab, we were able to show that just over a period of a few weeks you can train this mold from being this blue musty– sort of also slightly toxic– microbe that lives out in the wild– maybe you even accidentally have grown it on a piece of cheddar in your fridge– to something that is really tame and white– it produces more mushroom volatile compounds– and is no longer toxic.

And so as the fungus is adapting to the cheese substrate, it lets go of some of these wild traits that it has out in nature and is essentially tamed through the process of domestication.

IRA FLATOW: Is this the same blue fungus that’s found in blue cheese that we eat? And why does that fungus not mutate and turn color white also?

BENJAMIN WOLFE: It’s not the same species but it’s a close relative to penicillium roqueforti which is the blue veined fungus that you find in blue cheese. So most of the fungi that are used in cheese production, they start from a starter culture– from a microbe that you can actually purchase from a company. And so what these starter cultures are used for is to have stable production of cheese. So essentially to prevent evolution from happening.

But what our work is showing is not necessarily what’s happening intentionally from cheese makers right now, but historically what may have happened to give us the cheese microbe that is on Camembert. So we’re trying to figure out where did this thing come from historically in the domestication of these molds.

IRA FLATOW: Casey, if the bacteria are chowing down on the smelly compounds, does that mean that you won’t find this bacteria on the less smelly cheeses?

CASEY COSETTA: Not necessarily. It’s only at a very low concentration where these compounds can have an effect on the growth of these bacteria. So it doesn’t have to be a lot of smells. And it doesn’t necessarily have to be a particular smell. It’s really just a whole combination of the smells, in general, that can allow this bacteria to grow and flourish.

IRA FLATOW: So is there a relationship then between the smell and the amount of bacteria?

CASEY COSETTA: That’s a good question. That’s not something that we necessarily looked at. But there could be a correlation between the amount of volatiles present or a particular kind of compounds present. And maybe this bacteria, vibrio, that I mentioned that is really responsive to these volatiles. There is a correlation that we can find in the lab, in terms of concentration in this compound. And we might see that out in real cheeses as well.

BENJAMIN WOLFE: When you walk into a cheese cave– when you walk into a vault in a cheese cave– the air is full of all these aromas. And so what we’re thinking, and what we’re going to be testing out in the real world of cheese caves, is that even a cheese that’s on the other side of the room from another cheese where these volatiles are being produced, could be impacted by these volatile compounds. They can travel really far. And in most cheese aging environments, they’re at pretty high concentrations.

And so we think perhaps cheese makers are inadvertently creating these feedbacks. Where the age of cheese in a room, it creates more of the volatiles, and that promotes more of the growth of this bacteria. And you get this cycle over and over and over again.

And I’ll just point out, we sort of started this work being puzzled by this bacterium, vibrio casei, that Casey is talking about. It’s really a marine bacterium. A bacterium that typically lives out in the ocean. And we have been trying to figure out for a long time, why is it on cheese? Why is it loving cheese so much? And we really do think that this volatile feeding mechanism is one potential explanation for why this marine organism is just loving living on a wheel of Camembert or Limburger.

IRA FLATOW: Well, how does a marine organism make its way all the way from the ocean into a cave?

BENJAMIN WOLFE: That’s also been a question that keeps us up at night. And we don’t have all the dots connected yet. But here is our current hypothesis. Sea salt, so when a cheesemaker is making cheese, they add salt. Salt plays an important role in taste, but also in controlling the microbial community that develops on a cheese.

And many minimally processed sea salts– you just take the ocean and you dry it out. You just take big buckets of salt water and you dry them out in various conditions and you get salt. And during that process, any microbes that are present in the water that can go dormant can essentially hang out in a bag of salt. And we’ve played it out– sea salts from all around the world in the lab. And we find microbes. We find viable microbes hanging out in the salt.

And so what we think happens is using this sea salt that has these microbes, the cheese makers are inoculating their cheese with these ocean microbes. And we find vibrio in cheese made in Wisconsin in the middle of the continent where it’s very far from the ocean.

IRA FLATOW: Ben, some cheeses like Stilton, bleu cheese, they don’t have a rind but they still smell. Where do they get that odor from? What’s the relationship?

BENJAMIN WOLFE: So all cheeses will have some aromas coming from them. And that’s because microbes live in the paste– and these are lactic acid bacteria. They’re used during the part of the milk collection process and culturing the milk– when the milk goes from a liquid to a solid. And there’s a fermentation process there. And those microbes play an important role in releasing aromas as well.

And then in a bleu cheese, that bleu fungus that makes the veins inside of a bleu cheese is also decomposing the cheese and releasing aromas– breaking down fats and making free amino acids and free fatty acids from fats and proteins. And so all cheeses will have an aroma profile to them. But a cheese like Camembert or funky washed rind cheese like Limburger, will have a lot more volatile organic compounds being released– or VOCs being released– just because there’s so much more microbial activity and decomposition of the cheese.

IRA FLATOW: What else do you want to understand? Are there other fungi bacterial interactions you are interested in?

BENJAMIN WOLFE: Yes, so a big part of the last 5 to 10 years of research in my lab and others has really just been figuring out the biodiversity of cheese. What microbes are out there? It’s been like a natural history survey for us. And now what we’re trying to do, once we’ve figured out who’s out there, is trying to understand why are they there and what are they doing. How are they interacting each other in these microbial playgrounds?

So my lab is really interested in understanding how microbes interact in these microbial playgrounds. And a lot of the work that we’re doing is looking at how fungi and bacteria interact with each other. And in some cases, we see cooperation. So for example, we’ve seen that fungi can make these networks. The mold, as it creates that fuzzy layer across the cheese, is essentially laying out a highway that bacteria can use as an HOV lane to spread across the surface of the cheese.

In some other studies in our lab, we’re finding that fungi can produce antibiotics that can wipe out certain bacteria. So it’s both microbial war and peace on these rinds of cheese. And at the end of the day, what we can figure out by understanding how these parts fit together in this microbiome, we can then understand what is the assembly manual. If we can understand the parts and how they fit together, we can have a blueprint for how to better design and control these microbiomes.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. In case you’re joining us, we’re talking about smelly cheeses and why they taste so good. You mentioned before about the historical significance of cheese making. It seems to me that the cheese mongers of old had no idea of the chemistry that was going on inside, yet they were still making good cheese. Weren’t they, Ben?

BENJAMIN WOLFE: Yeah, in many ways cheese making and a lot of fermented foods are just happy accidents. We happen to get the right types of microbes growing on the right substrate at the right time. And if it didn’t kill you, it tasted great, and we kept making that over and over again. And so through the process of these happy accidents, we’ve figured out what microbes can make delicious cheese. And we then isolated those microbes and now work with them in a more controlled way to inoculate cheeses.

We’ve also figured out that there are important parameters, like temperature and the right amount of salt that you can add, to control that microbiome and develop a delicious and high quality cheese. But there’s still a lot to learn, right? We’ve only, in the last five or 10 years, really discovered that full biodiversity that’s there. And now we’re still trying to figure out what are we missing. What are what are other components of the cheese system that we maybe don’t understand?

Viruses, for example, there’s a lot of viruses that naturally live in these microbial playgrounds that, we’re beginning to learn, can attack bacteria and eat up certain bacteria. And those may play a really important role in these ecosystems.

IRA FLATOW: The phages,

BENJAMIN WOLFE: Phages, yeah we’re really a big fan of the roles that phages could play in these systems.

IRA FLATOW: Casey, are you going to make cheese your research specialty now? I mean think of food tasting you’ll have to submit yourself to. Oh, my goodness.

CASEY COSETTA: I know that seems like such a tough job. I’m continuing some cheese research right now in the position that I’m in. But I’m hoping to branch out and translate what I learned from cheese and the cheese microbes out into other systems as well.

IRA FLATOW: Such as? What kind of things can you take from this?

CASEY COSETTA: There’s a lot of things that are very similar in cheese systems to other systems. We see a lot of similar microbes that we find on cheese on our own human skin. So some of those interactions, or some of those cooperations, or competitions that we find on cheese, maybe we can translate to other systems. And it could be a clinical sense. It could be an industrial sense. It’s really pretty diverse what we could potentially translate.

IRA FLATOW: Ben, one of our favorite topics is the microbiome on Science Friday. And we are so happy to hear that it’s everywhere, even on the cheese.

BENJAMIN WOLFE: Yeah, and really our lab is a microbiome lab. We happen to study cheese but we’re generally interested in understanding how microbiomes work. And this is a big question, right now, in many labs that study humans, or plants, or soil. And we’re all trying to figure out these assembly rules, the guidebook to help us figure out how microbiomes come together.

And once we can figure out these ways that microbial parts fit together, we’ll be much better at managing diseases or maybe making probiotics more successful for humans or for agriculture. And this microbiome living on cheese is a great model system to do that. It’s relatively simple. We can grow the microbes easily in the lab. And so it’s been a great lab rat for the microbiome worlds to understand basic design principles for microbiomes.

IRA FLATOW: We’ve run out of time. I want to thank my guests, Dr. Benjamin Wolfe, Associate Professor in the Department of Biology at Tufts University, Dr. Casey Cosetta, a postdoctoral researcher at Tufts and first author on this new research.

Thank you both for taking time to be with us today and happy holidays to you.

BENJAMIN WOLFE: Thank you so much.

CASEY COSETTA: Thanks.

IRA FLATOW: And if you want to see some of those bacteria cheesing for the camera, we have pictures and videos up on our website at sciencefriday.com/cheesemicrobe.

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