As we have come to understand in this series,
the most devastating diseases plaguing mankind throughout all of history were caused by microorganisms. And yet, humans remained clueless as to their
existence for more than 99% of that history. Even when solutions were eventually found,
like Jenner’s immunization technique, these were reached only through trial and error,
without any understanding of the etiology of the disease, meaning that we didn’t actually
know what was causing the disease, or precisely why these techniques worked against them. There had been speculation for centuries about
the possible existence of living beings too small to be seen by the naked eye. After all, humans had noticed exceptionally
small insects, which are barely visible to the naked eye, and it does not require a lot
of imagination to conceive of the possibility that even smaller organisms might exist. However, the definitive proof that these invisible
beings existed could only be reached upon the invention of the microscope. And in fact, bacteria were first described
by an amateur scientist. In 1674, a Dutch draper, Antonie van Leeuwenhoek,
whose hobby was to grind powerful single lenses, built a rather simple microscope, and using
it himself, he described what he referred to as “animalcules” moving under the lens. Over the years, he documented in some detail
many examples of what we now understand to be bacteria, protozoa, and fungi. Surprisingly, this discovery, now hailed as
the birth of microbiology, did not make much of an impact, and the decisive steps toward
establishing this new discipline had to wait for almost two centuries. As it came to pass, the field of microbiology
truly arose not in the context of therapeutic needs, but rather in response to intense debates
focused on the phenomena of fermentation and spoilage. There were many scientists, including top
chemists of the time like Berzelius and Liebig, who claimed that these were spontaneous processes
deriving from chemical decomposition pathways. Others thought that invisible organisms could
arise spontaneously from grapes or hops and trigger the process. Still others proposed, and correctly as it
turns out, that fermentation arose because microorganisms carried through the air settled
down onto nutrients such as the sugar in grape juice, and multiplied. Their metabolic activity was the cause of
the fermentation that we observed, for example in beer or wine production. One of the greatest scientists of all time,
French chemist Louis Pasteur, is widely acknowledged as the father of microbiology. Pasteur had a doctorate in both physics and
chemistry, and his contributions to chemistry and crystallography were unparalleled. He turned to microbiology to address the wine
fermentation problem, because the father of one of his students, a wine producer, sought
his help. Pasteur devised remarkably simple but conclusive
experiments using swan‐necked flasks filled with fermentation broth. As a control, he also used broth contained
in normal flasks. When he left the contents of all the flasks
exposed to the air, fermentation rapidly occurred in the normal flasks, but not in the swan-necked
flasks. This experiment suggested that microorganisms
from the air, falling into an open flask, are responsible for the fermentation process. He also showed that rapid heating of a potential
fermentation medium, such as milk, all the way to its boiling point, followed by rapid
cooling, would prevent spoilage. This simple idea is at the basis of the process
called pasteurization, named in his honor. These results led Pasteur to the idea that
microorganisms may also be the cause of human diseases. He had lost three of his five children to
typhus, and he was particularly interested in the root cause of this and other diseases. He then postulated the germ theory of disease,
which identifies microorganisms as etiological agents of many infectious diseases. This theory was slowly accepted, and it had
a profound impact on society. For example, British surgeon Joseph Lister,
once exposed to Pasteur’s ideas, developed the technique of sterile surgery. He sprayed his surgical instrument with a
substance named carbolic acid, which is now known as phenol, a substance which Pasteur
had proposed as a bactericide. Furthermore, he used a clean environment for
his surgeries, and even applied pads soaked in carbolic acid directly to wounds. Prior to this, at the end of the 19th century,
surgeons had been operating under very unsanitary conditions, often wearing dirty operating
aprons, because they were going to be stained with blood anyway. Even successful surgeries frequently resulted
in the death of the patient due to infection. But with his new techniques, Lister observed
a significant decrease in post-operative infections, and he published his results in 1867 through
a series of articles. However, these ideas were greeted with much
skepticism, and even mockery. In 1873 the prestigious journal The Lancet
published an editorial, warning the medical profession against Lister’s ridiculous practices. Nevertheless, as Lister perfected his techniques
and the field of microbiology developed further, his approach was slowly accepted, and he is
now recognized as the father of modern surgery. An antiseptic solution named after him, marketed
in 1879 as Listerine, is still used today as mouthwash. In 1900, Lister was named Surgeon to Queen
Victoria, and he was instrumental in saving the life of her successor, Edward VII. The new king had a severe case of appendicitis
two days before his coronation, and the team of surgeons assembled for the operation was
supervised by Lister. This operation was often lethal in those days,
due to post-operative infections. But the king was back on his feet for the
coronation thanks to Lister’s antiseptic concepts. As far as Pasteur goes, he became a legend
in his own time. In his middle age, he was already a French
national hero. A special institute was created for him, the
Pasteur Institute, which he supervised until his death, and where he is now buried in a
special vault. Later in life, he focused on immunization. He proposed that an etiological agent can
be weakened by chemical means and then used to immunize healthy volunteers. This was important, because in this way, scientists
did not have to rely on a naturally attenuated version of the microorganism, like Jenner
had done with smallpox and cowpox. This discovery revolutionized the field, and
Pasteur was the first to refer to these artificially weakened microorganisms with the name “vaccines”,
in honor of Edward Jenner. In his old age and after his death, Pasteur
was widely criticized for being extremely secretive, presumably for financial reasons. Indeed, he instructed his family to keep his
legendary lab notebooks secret and never to reveal them to anyone. They finally became publicly available in
1971 and are now displayed in the French National Library. Pasteur also became embroiled in a bitter
rivalry with an eminent German physician and microbiologist, Robert Koch, who is credited
as a co-founder of the new discipline. Koch’s enduring legacy is the formal establishment
of microbiology as a science. He developed the so-called Koch’s postulates,
which can be summarized as follows. One. To be demonstrated as the etiological agent
of a disease, a microorganism has to be isolated from a sick animal or patient. Two. It has to be grown in a lab culture and observed
to multiply. Three. The microorganism obtained in the lab culture
needs to be injected into a healthy animal and the healthy animal has to develop the
disease. And four. The original microorganism has to be observed
in the sick animal and has to again be isolated and grown in the lab. These criteria are still generally applied
in microbiology, although with some modifications given our modern understanding of pathogens
and their behavior. Although Koch worked on several diseases,
his best-known research was done on tuberculosis, which is one of the greatest killers of all
time, having caused the death of several billion humans since the dawn of mankind. In Koch’s time, it was believed that tuberculosis
was a hereditary disease. Koch proposed instead that the disease was
caused by a bacterium, and he was able to isolate it. He called it Mycobacterium tuberculosis, and
found that this microorganism satisfied all four of his postulates. In collaboration with his colleague and friend
Paul Ehrlich, he developed a diagnostic test for the disease, called tuberculin, which
is still used to this day. Initially, this was meant to be a therapy
against the bacterium, but it failed to cure the disease. For his work, Koch received the Nobel Prize
in 1905, and today, the Robert Koch Institute in Berlin is the primary German center for
microbiology. The contributions of Pasteur and Koch to mankind
cannot be overstated. In the span of about 50 years, humans went
from complete ignorance of the etiological agents of disease to the emergence of two
new scientific disciplines, microbiology and immunology. The door to finally overcoming the great scourges
of mankind was now wide open. The next step, which was quite obvious but
difficult to achieve in practice, was to find a treatment for these diseases. We saw in the previous tutorial how quinine
was isolated from the bark of a tree and had become a major strategy to combat malaria
even without any knowledge of microbiology. However, this time scientists took another
approach. Instead of relying on nature, chemists would
make the medicine in the lab themselves, via chemical synthesis. Now bacteria could be grown in vitro, and
therefore chemicals could be tested directly on the bacteria, and scientists could then
determine how effective any chemical might be in killing these bacteria. The most promising agents could then be tested
in infected animals and finally in human patients to see whether they could cure the disease. Though with far greater sophistication, this
is essentially how the pharmaceutical industry still operates. The father of chemotherapy is the German physician
we mentioned a moment ago, Paul Ehrlich, who worked in Berlin and Frankfurt at the institute
which now bears his name. As a first disease target he chose syphilis,
a bacterial infection caused by Treponema pallidum, and transmitted sexually. There is still no vaccine against it, and
mortality is around 58%, although the progression can last over thirty years, and the symptoms
are devastating. It is still a significant disease today, with
35 million new cases and 100,000 deaths per year, mainly in the developing world, though
it can now be treated quite easily with antibiotics. At the beginning of the 20th century, however,
it was still treated with mercury salts, which were ineffective and toxic. Interested in finding a cure, Ehrlich tested
hundreds of synthetic compounds, and he hit the jackpot with compound 606, named Arsphenamine,
then marketed as Salvarsan, an arsenic-containing molecule with a very unusual structure. He proceeded to test the substance in vivo,
and was successful in curing patients. In spite of his stunning success, and in spite
of having received the Nobel Prize for Medicine in 1909, Ehrlich suddenly became a controversial
scientist. He was Jewish, and much of the animosity displayed
against him may have been motivated by the anti-Semitism of the times. He was accused of fueling the moral destruction
of the German people by encouraging the sexually promiscuous behavior which had been kept in
check by the fear of the disease. At the same time, he was accused of unethical
behavior, as some of his initial patients had died from the side effects of the drug,
which was indeed quite toxic. In his famous Frankfurt Lecture of 1910, Paul
stated what is still one of the basic concepts of chemotherapy: In effect, the hope of finding
the ideal of modern therapy, i.e. entirely safe substances for curing bodily ills, seems
to me virtually incapable of being realized. We shall always have to reckon with some degree
of toxicity. But the negative aspects must not constitute
a risk that is excessive in relation to the danger from the disease itself.” Paul was deeply affected by the many personal
attacks, and eventually slipped into a depression, leading to his premature death. In his later years, to address the problems
of drug side effects, he proposed the idea that we could develop a “vehicle” which
would hit only the desired target, and we could use this vehicle to deliver very toxic
drugs quite selectively, therefore avoiding damage to the host. He called this approach “Zauberkugel”,
German for “magic bullet”. Ehrlich failed to develop this magic bullet. His ideas were 100 years too early, and only
now are we beginning to put this concept successfully into practice. In any case, the doors to chemotherapy and
the treatment of all bacterial diseases were opened by Paul Ehrlich’s approach. We will see moving forward how this initial
success led to more exciting results against a number of other diseases.