Thanks to Babbel, a language learning app, for sponsoring this episode.

If you’re interested in growing your language skills, Microcosmos viewers get up to 60% off with a 20 day money-back guarantee when you sign up using our link in the description. The microcosmos is home to many unusual partnerships.

There are species of algae buried inside of paramecium, exchanging their photosynthetic products for protection. There are cyanobacteria fixing nitrogen within the glass walls of their diatom host. Life is, after all, just relationships, individual entities fighting and tolerating and partnering in tiny little dramas, each of which build upon one another like strokes of paint in an epic tableau of ecology, epidemics, and yogurt?

Yes, yogurt. That plastic tub tucked away in the dairy section of your local grocery store is the product of one of the most fruitful microbial friendships that humans have unknowingly played matchmaker to, a friendship that goes back millennia. That relationship is playing out in front of your eyes right now, extracted from some yogurt that James, our master of microscopes, had on hand.

We don’t know how exactly yogurt was invented, just that it is an ancient food that takes on many names in different cultures. Humans began consuming dairy somewhere between 10,000-5,000 BCE, likely coinciding with our domestication of animals like cows and goats that could provide us with milk. And the challenge those humans faced is one many of us can relate to: if you leave milk out, it goes bad quite easily.

In the Middle East and Western Asia, nomadic herdsmen would carry milk in bags made from animals that retained some of their enzymes, which may have then fermented the milk and preserved it. And over time, yogurt spread through different cultures not just because it was effective as a milk storage method, but also because it seemed to be quite good for the stomach. According to one story, King Francois I of France was suffering from severe diarrhea.

To help him out, his Turkish ally Suleiman the Magnificentt suggested yogurt, bringing yogurt to the rest of Europe. At least, that’s one of the stories. Yogurt has a surprisingly mythic aspect to it, thanks to its longevity, popularity, and cryptic origins.

And in the early 20th century, one Bulgarian scientist sought to untangle some of the mysteries behind yogurt. His name was Stamen Grigorov, and in the year 1904 he was headed to the Medical University of Geneva to study medicine. And he brought with him a clay pot containing homemade yogurt.

Grigorov’s work came about fifty years after Louis Pasteur pieced together the role of yeast in the fermentation of alcohol. But the organisms behind yogurt’s genius were still unknown. And as a Bulgarian stocked with both his homeland’s yogurt and curiosity, Grigorov decided it was time to take a closer look.

He found two bacteria. The first was shaped like a rod, and it would be named Bacillus bulgaricus in honor of its origins. With time, the species’ name would be changed, first to Lactobacillus bulgaricus, and then to Lactobacillus delbrueckii subsp. bulgaricus.

Because that’s quite a lot to say, we’re going to stick with the slightly incorrect and older name: Lactobacillus bulgaricus. The other bacterium was rounder in shape, and it would later come to be known as Streptococcus salivarius subspecies thermophilus, though we’re going to call it Streptococcus thermophilus. Lactobacillus bulgaricus and Streptococcus thermophilus may look different from one another.

But what they share in common is that they thrive on the lactose sugars present in milk, digesting them and releasing lactic acid back into their surroundings. As the lactic acid seeps into the milk, the milk itself becomes less hospitable to other microbes, allowing the lactic acid bacteria to take over. But if you’ve ever added lemon juice to milk, you probably are familiar with the other effect the lactic acid has: it curdles the milk, the result of proteins breaking apart.

But Lactobacillus bulgaricus and Streptococcus thermophilus are not simply two bacteria who happen to co-exist within the same yogurt. These two organisms work together. They could survive just fine on their own.

But when cultured together, they grow faster thanks to a molecular exchange between the two. Lactobacillus bulgaricus offers amino acids made by breaking down the proteins in milk to Streptococcus thermophilus, which doesn’t have the tools it needs to be able to do the same. And in return, Streptococcus thermophilus produces various fatty acids and other chemicals that help Lactobacillus bulgaricus grow.

We don’t know how these two organisms originally found each other. We’re not even entirely sure how they found their way into milk. Our best theory is that they were both found on the roots of plants that were boiled with sheep’s milk by Bulgarian shepherds who were making yogurt.

And their existence has inspired other scientists as well. Shortly after Grigorov revealed the presence of Lactobacillus bulgaricus in yogurt, an immunologist named Elie Metchnikoff became interested as well. Metchnikoff’s work in immunology was influential and would go on to win him the Nobel prize.

But he was also fascinated with aging, and particularly the idea that microbes living within us would help us live longer, microbe’s like Grigorov’s recently discovered Lactobacillus bulgaricus. In his book The Prolongation of

Life: Optimistic Studies, Metchnikoff described his daily probiotic regiment. He would either consume a pure culture of the bacteria followed by milk, or he would go for the simpler option: yogurt. Metchnikoff didn’t specify exactly how he measured his satisfaction with his experiment, just that he was happy with the results. But the pathways he thought Lactobacillus bulgaricus would follow in the body don’t seem to have been borne out in later experiments, making his specific theories about their use as tools to help us live longer unlikely.

Of course, that is not to say that yogurt isn’t good for you, just that its benefits are different from what Metchnikoff envisioned. And moreover, his work got scientists thinking about using microbes as a sort of medicine decades before the term “probiotics” was even created. And it is fitting that Lactobacillus bulgaricus would catalyze this way of thinking, a microbe in partnership with Streptococcus thermophilus for thousands of years— and really, in partnership with us.

Even now, separated from the plants they once lived on, these bacteria cultivate our yogurt and feed us, just as they feed each other. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Babbel for sponsoring this episode of Journey to the Microcosmos.

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As a Microcosmos viewer, you’ll get up to 60% off when you sign up using our link. Plus Babbel comes with a 20 day money back guarantee, so you can see where Babbel takes you on our language learning journey. Before we go, we’d also like to say thank you to each and every one of our Patrons.

Some of their names are on the screen right now, and these are the people that make this channel, and videos like this possible and we are so so grateful. If you’d like to become one of them, you can go to patreon.com/journeytomicro. If you’d like to see more from our Master of Microscopes, James Weiss, you can check out Jam & Germs on Instagram, and if you’d like to see more from us, there’s probably a subscribe button somewhere nearby.