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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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