Above: Photograph of Robert Koch by Wilhelm Fechner, taken ca. 1900 (Wikimedia Commons)
Born in 1843, Robert Koch was a German doctor whose groundbreaking discoveries and contributions revolutionized microbiology. In fact, he established an entirely new branch of microbiology focused on the study of bacteria. His first major contribution to this new field of bacteriology was identifying Bacillus anthracis as the bacterium that causes anthrax.
Koch also identified the bacteria that cause other deadly diseases, including tuberculosis and cholera. He developed a new lab technique that made it possible to study pure cultures that contain only a single type of bacteria. And he even produced the very first photo-micrographs of bacteria, which are digital images taken with a microscope.
Did you know? One third of the world’s population is infected with tuberculosis.
Postulates on germ theory
But Koch’s postulates on germ theory may be his most significant contribution of all. Germ theory is the idea that microorganisms, such as bacteria and viruses, are the main cause of disease. This may not be very surprising to you, but back in the 1800s no one knew for sure why people got sick. Before germs were fully understood, many people, including scientists, believed in the miasma theory. It argued that diseases like tuberculosis and cholera were caused by bad air that came from decomposed organic matter.
Germ theory had already been proposed by fellow researchers like John Snow and Louis Pasteur. Koch expanded on the theory and his four postulates (criteria) made it possible to demonstrate that a particular disease is caused by certain microorganism. The four postulates are:
- The microorganism is found in every person, animal, or plant with the disease; but not in those that don’t have the illness.
- A pure culture of the microorganism can be grown on a medium.
- Injecting the microorganism into a healthy host (person, animal, or plant) causes the same illness.
- The microorganism can be recovered from the host that received the injection.
Koch is particularly famous for applying his four postulates to tuberculosis. He was able to demonstrate that the disease was caused by a single bacterium called Mycobacterium tuberculosis . In the 1800s, tuberculosis was the leading cause of death in Canada. Between 70% and 90% of people living in European and North American cities were infected with the disease and approximately 80% of those who developed active tuberculosis died from it. (Not everyone who contracts tuberculosis develops an active form. People infected with latent tuberculosis don’t show any symptoms of the disease.)
Did you know? Seventy-five percent of people infected with cholera do not exhibit any symptoms.
Isolating pure cultures
But before Koch could apply his postulates and identify M. tuberculosis as the cause of of the tuberculosis, he needed a way to isolate pure cultures of the bacterium. In 1880, he collected a culture from the sputum (saliva and mucus) of tuberculosis patients. But he needed a medium on which he could grow and isolate the microorganism.
Koch started out using a potato, but he noticed that some microorganisms were eating the potato. Next, he tried using gelatin. But gelatin is not solid at 37˚C, the ideal temperature for growing bacteria. Finally, he tried agar, which fellow microbiologist Walther Hesse had begun using as a medium on the suggestion of his wife and collaborator Angelina Hesse. Agar, extracted from red algae, is not eaten by bacteria and is solid at 37 ˚C. That makes it the perfect choice for growing microorganisms!
Infecting a healthy host
After successfully isolating and growing M. tuberculosis , Koch tried injecting it into a healthy host. He used guinea pigs, an animal prone to tuberculosis. The injected guinea pigs died within eight weeks, while the others showed no symptoms of tuberculosis. Finally, the bacterium responsible for the disease was isolated using samples taken from the dead guinea pigs.
In this way, Koch was able to use his postulates to demonstrate that tuberculosis is transmitted by an infectious agent (the bacterium M. tuberculosis ), not by miasmas. This discovery was key to developing tests and treatments for the disease. Scientists ultimately developed vaccines to immunize children and adults against tuberculosis.
During his research, Koch developed methods that make it easier to observe and identify microorganisms. At the time, looking at bacteria using a microscope was a big challenge. Not only are they transparent, but they are also constantly moving around. Koch’s solution was to use dyes to stain the bacterial cells and distinguish them from the background. He was also the first person to use oil immersion, a technique where you immerse the lens of a microscope in oil for better resolution. Even today, oil immersion and staining are regularly used by scientists to get higher-quality images of bacteria.
Did you know? Between 1880 and 1900, Robert Koch and his students identified 21 different disease-causing microorganisms.
Today, many of the techniques and theories developed by Koch are routinely used in the fields of microbiology and medicine. His discoveries continue to save lives. Decades after his death, he is still remembered for his transformational ideas.
Scientific journal articles on Robert Koch and his discoveries: