1. Robert Koch announces the cause of tuberculosis in 1882
On March 24, 1882, Robert Koch announced at the Berlin Physiological Society that he had found the cause of tuberculosis [1]. His paper, "The Aetiology of Tuberculosis," was published on April 10. It is said that the doctors in the room were too moved to clap [4]. Paul Ehrlich, a 28-year-old dermatologist, later recalled this as the most moving experience of his scientific life. According to the story, he rushed back to his simple lab and spent the night creating a new way to stain the tuberculosis germ [4]. So, what exactly did Koch show them?
2. How staining with color made the tuberculosis germ visible
The tuberculosis germ is a thin rod. Its length is about one-quarter to one-half the diameter of a red blood cell, though some grow as long as a red blood cell [2]. Because they are so small and mixed in with body tissue, they are hard to find. Koch’s trick was color contrast. By changing the staining method, animal tissue turns brown, while the tuberculosis germ appears blue. Koch wrote in his paper that this difference is so clear that even a few germs can be identified with certainty [2]. The process used strong alkaline solutions, which was unusual for the time [3]. However, this does not mean all other germs turn brown. The paper notes that the leprosy germ has a similar shape and stains similarly [2]. Seeing a germ with color is different from proving it causes the disease.
3. How Koch used a pure culture and animal injections to prove the cause
Just because a germ is visible does not mean it causes the disease. It might just be there by chance. Koch chose to isolate the germ, grow it, and inject it into healthy animals to see if they got sick. The tuberculosis germ did not grow on common media. It grew when Koch used a medium made of serum hardened by heat. It took weeks to see visible colonies [3]. It is said that Koch spent about six months in his lab for this work [4]. He obtained a pure culture with no other germs [5]. When he injected this into healthy animals, tuberculosis developed [5]. This moved the proof from "seeing" to "causing." Koch had actually tried this order before, during his time as a country doctor, with anthrax.
4. How Koch worked in a rural doctor's home instead of a big institute
Koch did not start in a large institute. He was a district doctor in Wollstein, a rural town in what is now Poland. In 1876, he proved that the anthrax germ causes anthrax. His lab was his home, which had four rooms. He gathered most of the tools himself. The microscope was a gift from his wife [6]. This experience built the foundation for his tuberculosis work. In his 1882 paper, Koch stated that this was the first complete proof of a parasitic cause for a human infectious disease. Before this, only anthrax had been proven [2]. What changed in the world once the germ was found?
5. How finding the germ changed tuberculosis from a sign of poverty into a specific enemy
Before the cause was found, tuberculosis was seen as a sign of poverty. It was hoped that it would decline as living conditions improved. Koch wrote that specific preventive measures for tuberculosis were not yet known [2]. In his lecture, Koch cited statistics from that time. He said one in seven people died of tuberculosis, and that the share was more than one-third among people in their working years. He argued it was a more important disease than plague or cholera. These are figures from 1882, not today [2]. Koch wrote that with the germ found, the fight was no longer against "something vague" but against a "tangible parasite." Since the conditions for its growth were mostly known, it could be studied and prepared for [2].
6. Kitasato Shibasaburo, Koch's student, and World TB Day
In 1905, Koch received the Nobel Prize in Physiology or Medicine for his work on tuberculosis [7]. Kitasato Shibasaburo from Japan studied under Koch in Germany from 1885 to 1892. In 1889, Kitasato achieved the first pure culture of the tetanus germ in the world in Berlin. The next year, he discovered the antitoxin. In 1892, he helped found the Institute for Infectious Diseases in Japan [9]. Today, March 24 is World TB Day. The WHO marks this date because it was when Koch announced the discovery of the tuberculosis germ in 1882, opening the path to diagnosis and treatment [8].
7. A color contrast activity: counting brown and blue paper scraps
Place 10 brown paper scraps and 10 blue paper scraps randomly on a piece of brown cardboard. Stand about 1 meter away and see which color is easier to count. You should find the opposite color easier to spot than those blending in. This is the idea of color contrast used to see the blue germ in brown tissue. Note that real staining uses chemicals and is not the same as paper. For information on World TB Day, see the WHO website [8].