What makes beer? Single-celled organisms. What makes us cough and feel like sleeping
for twelve straight hours Straight? Single-celled organisms. AKA microbes, germs, the tiny machines
that allow us to digest—and can kill us. “What is life?” Turns out, life is relentless,
everywhere on earth!
[Intro Music Plays]
Microbes are invisible: they’re single cells.
So to understand microbes—that is, to do microbiology—scientists first needed to
study cells.
German botanist Matthias Schleiden and physiologist Theodor Schwann
proposed a modern cell theory around 1837: every living thing
is made up of one or more cells. So the cell was the fundamental unit of life, the thing
that gives structure to all forms of life. Amazingly, Schleiden—who studied plants—and
Schwann—who studied animals—both noticed tiny dots in the middles of the cells they
looked at under their microscopes. These nuclei must somehow be important to all life, they figured—correctly.
Darwin’s chief public defender, Thomas Henry
Huxley, also wrote a article called “The Cell–Theory” in 1853.
Where the Germans were cataloging different microscopic forms, Huxley proposed a mechanistic
model: cells are little factories with different parts that have different functions. Their
functions make the parts add up into an organic whole.
Not every scientist believed cell theory right away, but many came around, as microscope-enabled
evidence mounted.
And yet, being able to observe single-celled organisms didn’t immediately lead scientists
to connect them to disease. In fact, medicine in the early 1800s mostly
worked the way it had hundreds or even a thousand years earlier. It was still all about liquids
called humors. But some diseases were understood differently.
These were caused by a kind of invisible badness in the environment called a miasma.
According to miasma theory, diseases were caused by foul-smelling airs that came from
rotting meats, swamps, and other putrefying sources.
Before you knock it, think about it: this theory mostly works. Dangerous microbes grow
on smelly, dirty stuff. The smell is a byproduct of the microbes’ eating organic matter.
But even today, we can’t see these microbes without instruments. So following our unaided
senses is still smart: don’t eat gross trash!
Sniff the turkey before you eat it. And then, just eat it anyway like I did this morning.
By the mid-1800s, however, a few scientific rebels were pushing a germ theory of disease. This holds that germs, or pathogenic microorganisms,
cause infectious diseases. [Snow]
Around this time, a nasty germ called Vibrio cholerae provided
an opportunity to test the theory out.h Cholera had ravaged England since the germ
started to travel widely out of south Asia in the early 1800s, hitching rides on the
expanding armada of merchant ships. Empire had all kinds of unpredicted downsides.
In 1854, a bad cholera epidemic hit Broad Street in the Soho neighborhood of London,
killing over six hundred people. One doctor named John Snow had a hunch about what was
going on. Snow created a map of the outbreak and noted
that the cases seemed to cluster around one public well. He hypothesized that cholera
microbes were rapidly multiplying in its water. So he convinced the Soho council to remove
the well’s pump handle. People went elsewhere for water, and the cholera infections decreased.
John Snow became a hero! Saving Lives! Even though, it turned out, the outbreak was already in decline when
he removed the handle. Thus proving that some John Snows, do know
somethings.
Snow’s quick thinking didn’t really prove germ theory. For germs to replace miasmas
in their minds, scientists would require a better model of what germs are, and how they
behave. For that, we turn to one of the first experimental
microbiologists and a dude whose name you probably should know from milk labels: Louis Pasteur
Born in France in 1822—the same year as
Galton—Pasteur was a chemist, not a physician. But he is known for his work on germs and
health. Pasteur is credited as the co-founder of microbiology
and the founder of modern zymology, the science of fermentation. Fermentation
is the biological and chemical process needed to make beer, wine, yogurt, cheese, bread,
and many other foods. ThoughtBubble, show us what that means.
Chemically, fermentation is the conversion of sugar molecules into ethanol, or alcohol,
and carbon dioxide. In food, this usually happens thanks to bacteria
or yeast. Looking good, Saccharomyces! Pasteur first connected yeast to the fermentation
of wine in 1857. He observed that microscopic yeast on grape skins makes grape juice ferment
into wine. If you sterilize the skins, killing the yeast, fermentation won’t happen.
Pasteur even made the the analogy that the microbes growing in wine and beer were similar
to the microbes causing diseases in humans and animals. He hypothesized that microbes
are everywhere and must be responsible for all kinds of phenomena, like making food spoil.
So in 1865, he patented a process in which liquids such as milk were heated to a temperature
between sixty and one hundred degrees Celsius in order to kill microbes. This became known
as pasteurization. It may sound kinda simple, but this was revolutionary!
Earlier, Nicolas Appert had invented canning to make foods safe, so Napoleon could fight
wars for longer. But Pasteur’s method used a lower temperature, thus preserving tastes
and textures. Related to this idea of killing bad germs,
Pasteur also helped develop hygienic medical practices, along with British surgeon Joseph
Lister. No germs in the operating room! Most importantly for the history of biology,
Pasteur got tangled up in a debate with a fellow scientist named Félix Pouchet
who believed that living things spawn naturally from certain environments. This widely held
belief was called spontaneous generation.
Pasteur, based on his observations of grapes, reasoned that living things can only grow
from other living things. So he created an ingenious experiment to prove the theory of
biogenesis, or life from life. Pasteur showed that no germs would grow in
sealed, sterilized long-necked flasks full of beef broth. But in sterilized but open
flasks, microbes would eventually show up.
Thanks Thoughtbubble. After proving biogenesis, in the 1870s, Pasteur worked on immunology
and vaccination. Pasteur’s work with the rabies and anthrax microbes supported the
germ theory and its use in medicine: if you could identify a microorganism, you could
give someone a weakened version of it, or vaccine, and the patient taking the vaccine
would develop immunity to the real bug. And in 1879, Pasteur developed the first lab-manufactured
vaccine, for chicken cholera. It was around this time that germ theory was
finally widely accepted, in part due to Pasteur’s work.
But Pasteur’s ethics were questionable: when rabies vaccines for humans rolled out,
Pasteur kept his data on their effectiveness secret. He even kept secret mortality data,
or how many people his vaccines didn’t help. So unlike today, in Pasteur’s time, it wasn’t
necessarily a norm that useful medical research should be shared publicly.
But Pasteur was not the only notable germ
wrangler. Born in 1843, the German physician Robert Koch painstakingly worked
at a microscope to definitively establish the germ concept of disease.
He identified different microbes, becoming the leader of a large research team in Berlin.
They founded the discipline of bacteriology, or the study of bacteria, particularly in
relation to human health. In the late 1880s and ‘90s, Koch and his
team traveled into areas experiencing epidemics, including India and Egypt, to identify the
microbes causing diseases there. By ID-ing these microorganisms, they hoped to create
vaccines. First, Koch proved that the anthrax germ causes
anthrax-the-disease. Later, he identified the bacterium that causes tuberculosis or
TB, which was a very widespread disease. Koch also characterized cholera, vindicating
the work of John Snow by showing that the bacterium lives naturally in the small intestine
of humans. Snow wasn’t just high on ether: he’d correctly guessed how cholera spreads!
Eventually, Koch’s team contributed to the identification of the germs that causes diphtheria,
typhoid, pneumonia, gonorrhea, meningitis, whooping cough, tetanus, plague, leprosy,
and syphilis. …I would not want to borrow this dude’s
lucky microscope!
It's been through a lot!
Through this work, Koch developed the four postulates or steps for identifying infectious
diseases. One, the microorganism must always be present,
in every case of the disease.
Two, the germ must be able to be isolated from a host containing the disease and grown
in a lab, in a “culture.” Three, samples taken from the culture must
cause the same disease when introduced into a healthy model animal like a mouse.
Four, the germ must be isolated from that new animal, meaning it is the same organism
that caused the disease in the original host.
So structure matters: in all infections, germs grow out of hand, throwing the body out of
whack and causing inflammation. These bad germs, or pathogens, develop and live within
us, and can kill us. But with Pasteur’s vaccines and Koch’s
scientific method of ID-ing bacteria, humans could now develop medicines to fight jerk-sauce
germs. Go team us!
Back in the world of multicellular organisms, another scientist at the intersection of biology
and medicine was exploring the structure of living things.
In 1892, German-American physiologist Jacques Loeb began his experiments
on embryonic development, or how organisms grow from single-celled embryos into whatever
they’re supposed to look like. So he started dunking sea urchin embryos into
different salt solutions. And, in 1899, he got one to give birth… just from being salty!
Loeb’s artificial parthenogenesis, or birth from the self, was mega-astounding news. It
raised basic questions, such as what is sex? Could a human one day give birth by herself…
or him-self? Causing sea urchins to give virgin births made Loeb a celebrity.
But for the history of biology, it was revolutionary that Loeb’s sense of scientific understanding
of life was tied to his ability to control it. His work with sea urchins can be seen
as an early form of bioengineering, or treating cells and tissues as machine-parts to be used
constructively by a human designer. Loeb’s notable students included the behavioral
psychologists, B. F. Skinner and J. B. Watson, and a creator of birth control, Gregory Pincus.
Biology, like other scientific disciplines, shows a historical arc from observations to
experiments, and from less control to more. From the mid-1800s to the early 1900s, scientists
moved from mostly believing that living things spontaneously appear out of nowhere to controlling
fermentation on a massive scale. Cheers to that!
Next time—we’ll add the missing mechanism to this crazy Rube Goldberg machine we call
biology: it’s time for a monk named Mendel to accidentally figure out genetic variation—in
mice, men, and peas. Crash Course History of Science is filmed
in the Dr. Cheryl C. Kinney studio in Missoula, Montana and it’s made with the help of all
this nice people and our animation team is Thought Cafe.
Crash Course is a Complexly production. If you wanna keep imagining the world complexly
with us, you can check out some of our other channels like Scishow, Eons, and Sexplanations.
And, if you’d like to keep Crash Course free for everybody, forever, you can support
the series at Patreon; a crowdfunding platform that allows you to support the content you
love. Thank you to all of our patrons for making Crash Course possible with their continued
support.