Discovery of Penicillin Was Made Possible By Germ Theory

FROM THE LECTURE SERIES: Turning Points in Modern History

By Vejas Gabriel Liulevicius, Ph.D., University of Tennessee

The discovery of penicillin was a turning point within a turning point, a revolution within a larger revolution. The advance of antibiotics took place in the larger context of the development of germ theory, which revealed how various diseases worked. So, how was germ theory developed, and who were the key scientists and pathologists responsible for this? Also, what was the human biological perspective prior to germ theory?

A petri dish in a laboratory with bacterial colonies in it, and a scientist is delicately picking up bacteria from the petri dish with gloved hand, and test tubes with a liquid substance can be seen beside the petri dish.
Even though the microscopic investigations of Antonie van Leeuwenhoek had revealed microorganisms, it was the development of germ theory that revealed how diseases actually worked, and that germs caused infectious diseases. (Image: Anyaivanova/Shutterstock)

Before effective therapies could be found, mechanisms of infection and disease needed to be understood. In just a few decades before 1900, germ theory overturned how we understood health and disease, and how we envisioned human beings in relation to their environment.

Learn more about Van Leeuwenhoek’s microscope.

The Theory of Humors

The microscopic investigations of Antonie van Leeuwenhoek had revealed microorganisms. What was not understood was the importance of those bacteria, for good or for ill, for human beings.

Over the previous centuries, indeed from the time of the ancient Greeks, health and disease were most often understood in terms of the theory of humors. The word ‘humor’ comes from the Latin word for liquid or fluid.

This theory of humors postulated that all human beings were creatures in whom four fluids mixed and operated. These fluids were blood, phlegm, yellow bile, and black bile. Having more of one of these determined a person’s temperament: sanguine, phlegmatic, choleric, or melancholic.

Even in our own times, the terminology of this older theory is still with us. We speak of someone as having a sense of humor, and we still speak of people being marked by a temperament. Sanguine, which is marked by blood, for someone who is lively and sociable, or phlegmatic for the understated and quiet among us.

Illness thus was understood as a loss of internal balance of these fluids. If a high fever occurred, for instance, this would be understood as an excess of blood, and bloodletting was called for. Such a therapy is familiar from novels and movies about the 18th and 19th centuries, and could involve the use of leeches to deplete an alleged excess of blood. These aquatic worms were fastened onto the skin to draw off blood and restore balance.

In addition, contemporaries also believed that internal imbalances could be caused by bad air, miasmas as they were called, as a source of infection. When we think of the theory of the humors, this older conception of the human body, it has sometimes been compared to a hydraulic system, the body as a finely tuned machine.

Learn more about turning points in modern history.

Germ Theory Revolutionized Pathology and Surgery

In the decades before 1900, the authority of the theory of humors was overturned by medical pioneers who instead advanced germ theory, which provided the understanding that important diseases were caused by infection with microorganisms. This revolutionized pathology and surgery. The pioneers of germ theory included a series of great scientists.

By the 1840s, a number of physicians were noticing evidence for the contagious nature of disease being spread from person to person, not being caused by miasmas or clouds of bad air. One discovery was made by Dr. Ignaz Semmelweis, and it was a horrifying discovery for doctors. He found that it had been doctors who had been spreading a deadly disease.

Semmelweis was a German-Hungarian doctor at work in Vienna, who noticed the high incidence of so-called childbed fever, or puerperal infection, among new mothers in maternity wards. He was astonished at two wards that he visited, which had mortality rates respectively of 29 percent and 3 percent.

Seeking the reason for the difference, he discovered that in the ward with soaring mortality, medical students who did autopsies went right on in to deliver babies, and thus infected their patients. When he ordered all staff to wash their hands in chlorinated water, deaths from childbed fever simply plummeted.

Another doctor, John Snow, traced deaths in London’s cholera epidemic to one neighborhood water pump, and showed that cholera was a waterborne disease. In 1865, Joseph Lister, a professor of surgery at Glasgow University, revolutionized antiseptic procedures in surgery, and infections were reduced markedly as a result of these sterilization procedures. Indeed, Listerine mouthwash was named after him, in tribute.

The 19th century saw worldwide reform movements to create more hygienic urban environments, to inspect food production, and to instill quite ordinary means of ensuring public health. Something as basic as washing one’s hands with soap turned into a revolution of its own.

Learn more about Darwin and the origin of species.

This is a transcript from the video series Turning Points in Modern History. Watch it now, on The Great Courses Plus.

Koch’s Postulates Added Scientific Method to the Hunt for Microbes

A digitally sketched portrait of Louis Pasteur, in which he is wearing a high-collared shirt and suit, he has a full beard and intense eyes.
French microbiologist Louis Pasteur was one of the first to conclusively show that germs caused infectious disease and, along with German physician Robert Koch, developed the germ theory of disease. (Image: Marusya Chaika/Shutterstock)

French microbiologist Louis Pasteur’s experiments proved that fermentation of wine and milk going bad were caused by microbes. Pasteurization was the process of heating foodstuffs to delay the action of microbes.

Pasteur arrived at the conclusion that germs caused infectious disease. This was confirmed in his investigations of an anthrax epidemic affecting sheep and humans in 1879. Pasteur developed a vaccine.

At the same time, in Germany, the German doctor Robert Koch was also studying anthrax and came to the same conclusions. He also identified the bacteria that caused tuberculosis in 1882 and cholera in 1883. As a result, Koch won the Nobel Prize for Medicine in 1905.

Along with his rival Pasteur, Koch was a founder of microbiology, and developed rules for identifying specific microbes that caused disease. These are called Koch’s postulates, and they’re still used as a guide today.

They stated that you had to first identify the microorganism that was always linked to a particular disease, then isolate that microbe and grow it in the laboratory, then verify it caused disease in a healthy animal, and then be able to isolate it again in that new subject.

Here was the rigor of the scientific method applied to the new exciting hunt for microbes. In his lab, by the way, Koch also chose an especially successful material on which bacteria colonies could grow. This was a slice of the ordinary potato.

In the hands of Robert Koch, the potato contributed to the triumph of germ theory by around 1900. The message spread to ordinary people, who became newly attuned to the dangers of microorganisms and contagion, as well as to the reality Leeuwenhoek had long ago discovered: Germs are everywhere, not least on and in each human being.

Learn more about the discovery of penicillin.

Germ Theory Enabled Rapid Advance in Medical Research 

From 1900, the hunt for bacteria and disease agents became an international and collective scientific enterprise. Big advances in medical knowledge were made due to international cooperation through medical journals and worldwide conferences. The microbes that caused typhoid, diphtheria, tetanus, plague, and rabies were identified.

Once the vectors of disease were discovered, it became possible to work methodically toward developing vaccines to prevent them or to cure them.

It was at this point that the discovery of penicillin was made. There’s no way to determine if penicillin would have been discovered even without the existence of germ theory, but it seems unlikely. The decades of medical research that Dr. Alexander Fleming had been able to study before he made the discovery of penicillin cannot be ignored.

Common Questions about Germ Theory

Q: What was the theory of the four humors?

Ancient Greeks understood health and disease in terms of the theory of humors. The word ‘humor’ comes from the Latin word for liquid or fluid. This theory of humors postulated that all human beings had four fluids that mixed and operated. These fluids were blood, phlegm, yellow bile, and black bile. A person’s temperament – sanguine, phlegmatic, choleric, or melancholic – was dependent on which fluid was most prevalent in the person.

Q: Why is the germ theory of disease important?

Germ theory provided the understanding that important diseases were caused by infection with microorganisms, which revolutionized pathology and surgery.

Q: What are the four Koch postulates?

Koch’s postulates stated that you had to first identify the microorganism that was always linked to a particular disease, then isolate that microbe and grow it in the laboratory, then verify it caused disease in a healthy animal, and then be able to isolate it again in that new subject.

Q: What did Ignaz Semmelweis do?

German-Hungarian physician Ignaz Semmelweis noticed the high incidence of childbed fever, or puerperal infection, among new mothers in maternity wards. He discovered that medical students who did autopsies went right on in to deliver babies, and thus infected their patients. When he ordered all staff to wash their hands in chlorinated water, deaths from childbed fever simply plummeted.

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