This episode is brought to you by Squarespace.

Go to Squarespace.com/microcosmos to get a free trial, and 10% off your first purchase of a website or domain When you think of kombucha,  you might think of a nice,   refreshing, healthy drink, one that’s  exceptionally good for your microbiome. But what we here at Journey to the Microcosmos think of is different from that.

Our first thought is of a terrarium… a place where a whole ecosystem  can exist, trapped in glass. Kombucha is an old and mysterious drink. It's thought to have originated around 221 BCE in   Korea and China, though we had a hard time  tracking down the sources for that date.

There are also reports of kombucha being  consumed in Russia in the 19th century. But those two time points don’t tell  us much about how kombucha came to be. And that’s not unusual when  it comes to ancient foods.

Here on Journey to the Microcosmos,   we’ve ventured into the microbial world  of food like bread, yogurt, and cheese. And the processes humans have used to  make these foods go so far back and are   accompanied by so much lore that it is difficult to untangle exact histories. But no matter how murky the history is,   the microbial universe buried in  these foods is somewhat consistent.

There is an organism, maybe a  yeast or a bacteria or a fungus. It wants to survive, and so it  uses the resources around it— like sugar or the proteins in milk— to do that. It might even work with one or  two other organisms to accomplish its goals.

So James, our Master of Microscopes, thought  kombucha was going to be a very familiar sight,   something chill like brewers yeast. Or something like the feeling he gets  when looking at fermentation products   like yogurt under the microscope, which to him  resemble domesticated animals in a petting zoo. There’s a stable environment and plenty of food.

And the organisms within have  no predators and no competitors. But that's not what he saw when he looked at kombucha. The word that came to mind then was “wild.” It was so wild that at first he’d thought he  was looking at kombucha that had maybe gone bad,   but he quickly realized that this was not the case.

Kombucha is just wild like this. Now if you know anything about how to make  kombucha, then you’ve probably heard about   this weird thing that James is pulling  out of the top of his kombucha sample. This is the SCOBY.

It’s a strange word, but it stands for something “Symbiotic Culture Of Bacteria and Yeast”. So yes, that wiggly mass is actually the weird,   wild world of bacteria and yeast  that makes kombucha possible. We’re not in the business of giving  recipes here on Journey to the Microcosmos,   and besides, everyone has their own preferences.

So we’re just going to describe how  one might go about making a SCOBY. The process starts with some brewed tea,   usually black or green, which  is then mixed with some sugar. A bit of some prior kombucha is added to the mix,   providing a starter culture  of the bacteria and yeast.

Then the container is covered with  something breathable and set aside. Over the next few weeks, bubbles begin  to form on the surface of the mixture,   gathering together until they form a thin layer. That layer eventually  thickens into the solid SCOBY.

While the liquid used in this fermentation  process will ultimately be too vinegar-y to   drink, the SCOBY can be used in subsequent  fermentations with brewed tea and sugar. For those of you who are experienced kombucha  brewers, you might be filling in the gaps of our   description with the myriad of choices that  make your kombucha taste the way it tastes,   whether that’s the use of store-bought cultures  or the addition of flavorings in later steps. That’s the thing about kombucha: it is all so complicated, and again, wild.

The community of bacteria and yeast that  makes up the SCOBY can vary tremendously,   depending on where the  kombucha was originally made. And as fermentation progresses, the composition of  the community changes as well, evolving with the   culture and responding to microbes in the air  that might try to interact with the kombucha. So this community we’re looking at might  have looked very different a few weeks prior.

And the species might not be the exact same  species that make your favorite kombucha.   But the underlying processes they engage  in to make that kombucha are the same. When the starter culture is mixed  with the brewed tea and sugar,   the yeast use an enzyme called invertase to  break down the sugar into smaller monomers. This kicks off a whole series of  metabolic reactions, beginning with   the yeast and bacteria converting those sugar  monomers into ethanol and carbon dioxide.

The ethanol could be a problem, as too much  can be toxic for the residents of the mixture. Luckily, the bacteria are able to oxidize the  ethanol, producing acetic acid in the process. At the same time, the bacteria makes cellulose, which helps form the biofilm of the SCOBY.

These interactions demonstrate quite a bit of  cooperation between the bacteria and yeast,   and each carries out reactions that  make each other’s survival possible. They might break down sugar or lower  ethanol levels or create the molecules   that will help bind the community going forward. These are all examples of cooperation,  and a nice foundation for an ecosystem.

For a drink that is as associated with  wholesomeness as kombucha often is,   that is a nice image. But ecosystems aren’t marked  solely by cooperation. They’re also marked by competition, as  organisms vie for limited resources and space.

There are several ways we might see this  competition taking place in kombucha. For example, not all yeast can  make invertase, that enzyme that   kicks everything off by breaking down  sugar for the community to partake in. These non-invertase yeast are considered  “cheaters,” taking advantage of the   resources provided by other organisms  without contributing any of their own.

But one study of yeast grown with the  bacteria Escherichia coli found that   cheater yeast tend to not grow as  well as the yeast with invertase. In an environment where everyone wants sugar,   these cheater yeast don’t seem to survive  as well, perhaps because their non-cheating   counterparts are able to consume a tiny  bit of the sugar monomers they produce. And that competition between yeasts  might carry over to the kombucha.

Earlier, we mentioned that bacteria  oxidize ethanol into acetic acid,   lowering the pH of the environment overall. This acid is actually one of several acids found  in kombucha, leading scientists to wonder whether   acid in general serves as a protective quality  of the environment, deterring organisms that   can’t survive acidic conditions and creating a  gate against invaders that’s made out of that low pH. And there’s another potential shield  against invaders: the SCOBY itself.

The thickness of the biofilm acts as a  physical deterrent, further strengthened   by its composition, which makes it hard for other  organisms to diffuse their way into the kombucha. Now we should note that these  deterrents aren’t always effective,   and sometimes unwelcome organisms  can find their way into the SCOBY— at which point, you should  probably not drink that kombucha. It is this complex array of interactions, some  of which are better understood than others,   that makes us think of kombucha like a terrarium— a glass bottle full of an environment you  can watch brewing itself an ecosystem.

With every bit of SCOBY that bubbles to  the surface and attaches to each other,   we get to see this untamed landscape assemble. And with every SCOBY transferred to a new culture,  we can replicate this small, wild universe. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.

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Let's crack open a cold kombucha to  cheers the people on the screen right now. They are our Patreon patrons. They're the people who make it possible for us  to continue exploring all the weird and wild   ways our lives are influenced by tiny, tiny  creatures that we barely even know exist.

So thank you to our patrons for  allowing us to continue this journey. If you want to join them, you can  go to Patreon.com/JourneytoMicro. If you want to see more from our  Master of Microscopes, James Weiss,   you can check out Jam & Germs on Instagram.

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