Koch Discovers the Tuberculosis Bacillus (1882)

On 24 March 1882 the German physician Robert Koch told the Physiological Society in Berlin that he had found the cause of tuberculosis: a slender rod-shaped bacterium now called Mycobacterium tuberculosis. Tuberculosis was then the deadliest disease in Europe, and Koch’s careful proof that a specific germ caused it was a turning point for medicine. The discovery gave powerful support to germ theory and made bacteriology a leading science.

Background: the white plague

Tuberculosis, also called consumption or phthisis, attacks mainly the lungs, causing coughing, wasting, fever and often death after years of illness. In nineteenth-century Europe and North America it was a leading cause of death, especially in crowded industrial cities. In his 1882 lecture Koch estimated that it killed about one in seven people.

Many doctors believed tuberculosis was hereditary or caused by a weak constitution and bad air. In 1865 the French army doctor Jean-Antoine Villemin showed that the disease could be passed to rabbits by injecting tuberculous material, proving it was transmissible, but nobody had identified an agent. Meanwhile Louis Pasteur’s work on fermentation and the success of Joseph Lister’s antiseptic surgery made the idea that microbes caused disease increasingly persuasive.

Robert Koch

Robert Koch (1843–1910) began as a country doctor in Wollstein, in the Prussian province of Posen (today Wolsztyn, Poland). Working in a home laboratory, he worked out the life cycle of the anthrax bacterium in 1876, showing that it formed hardy spores. In 1880 he was appointed to the Imperial Health Office in Berlin, where he had proper facilities and assistants, including Friedrich Loeffler and Georg Gaffky.

Koch transformed laboratory methods. He developed ways to fix and stain bacteria on glass slides, used the new oil-immersion lenses and condensers for microscopes, took photographs through the microscope and grew bacteria on solid media, first on slices of potato and then on gelatin. The use of agar, suggested by Fanny Hesse, the wife of a colleague, followed in 1881. Solid media allowed separate colonies, and therefore pure cultures, to be grown.

How Koch found the bacillus

Koch began work on tuberculosis in August 1881. The bacterium was difficult to see because it resists ordinary stains. Koch found that staining with methylene blue in an alkaline solution, followed by a brown counterstain called vesuvin, made the thin rods visible in diseased tissue from humans and animals.

Seeing the germ was not enough. The bacterium grows extremely slowly, and many of Koch’s cultures seemed to fail until he waited longer than usual. Using heat-solidified blood serum, he obtained colonies after about two weeks. He then injected the cultured bacteria into guinea pigs and other animals, which developed tuberculosis, and recovered the same organism from them. This chain of evidence was far stronger than the observation of microbes alone.

The lecture of 24 March 1882

Koch presented his findings on the evening of 24 March 1882 at the Physiological Society in Berlin, displaying his slides and preparations for the audience. Among those present was Paul Ehrlich, who later recalled it as the greatest scientific experience of his life. Ehrlich soon improved the staining method, and his work led to the Ziehl-Neelsen acid-fast stain, still used to detect tuberculosis today.

Koch’s paper, “Die Aetiologie der Tuberculose” (The Aetiology of Tuberculosis), appeared in the Berliner Klinische Wochenschrift on 10 April 1882. News spread rapidly through the medical press of Europe and North America.

Koch’s postulates

Koch’s tuberculosis study is often taken as the model of what came to be called Koch’s postulates, building on ideas of his teacher Jakob Henle. Loeffler stated them formally in 1884. In simplified form:

• The microbe must be present in every case of the disease.
• It must be isolated from the host and grown in pure culture.
• The cultured microbe must cause the disease in a healthy experimental animal.
• The same microbe must be recovered from the infected animal.

The postulates are not perfect. Some microbes cannot be grown in culture, and some people carry germs without falling ill. Koch himself recognised such limits, but the postulates remain a classic framework for linking a microbe to a disease.

Consequences

The discovery made Koch famous and launched a golden age of bacteriology. In 1883–1884 he led an expedition to Egypt and India that identified the cholera bacterium, explaining outbreaks like the Hamburg cholera epidemic of 1892. Within two decades researchers in Germany, France, Japan and elsewhere identified the agents of diphtheria, typhoid, tetanus, plague and other diseases. Many of them had trained in Koch’s laboratory or used his techniques of staining, solid culture media and animal experiment, and German bacteriology became a model copied by medical schools around the world.

Koch’s attempt at a cure was less successful. In 1890 he announced tuberculin, an extract of the bacteria, as a remedy, and thousands of patients rushed to Berlin. It did not cure the disease, and the episode damaged his reputation, though tuberculin became the basis of the tuberculin skin test. In 1891 he became director of the new Institute for Infectious Diseases in Berlin, today the Robert Koch Institute. He won the Nobel Prize in Physiology or Medicine in 1905 and died in 1910.

Knowing that tuberculosis was infectious changed public health. Campaigns against spitting, sanatoriums that isolated patients, better housing and milk inspection followed. The BCG vaccine was introduced in 1921, and streptomycin, the first effective antibiotic against tuberculosis, was discovered in 1943.

Myths and facts

Myth: Koch discovered that germs cause disease. Fact: The idea was older, and Pasteur, Lister, Villemin and others had already gathered strong evidence. Koch’s contribution was to prove, with rigorous methods, that one specific bacterium caused one specific major disease.

Myth: Koch’s postulates were a fixed list written by Koch in 1882. Fact: Koch applied the reasoning in his tuberculosis paper, but the familiar formal list was set out by Loeffler in 1884 and by Koch himself in a later lecture in 1890, and it has been revised many times since.

Myth: Tuberculosis is a disease of the past. Fact: Despite vaccines and antibiotics, it remains one of the world’s leading infectious killers, and drug-resistant forms are a growing concern.

Koch and Pasteur

Koch’s achievements made him the great rival of Louis Pasteur. The two men disputed methods and priority, and their rivalry was sharpened by French and German nationalism after the Franco-Prussian War of 1870–1871. Together, however, their schools established germ theory as the foundation of modern medicine.

Timeline

• 1865: Jean-Antoine Villemin shows tuberculosis can be transmitted to animals.
• 1876: Koch describes the life cycle of the anthrax bacterium.
• 1880: Koch joins the Imperial Health Office in Berlin.
• 1881, August: Koch begins his research on tuberculosis.
• 1882, 24 March: Koch announces the tuberculosis bacillus to the Physiological Society in Berlin.
• 1882, 10 April: His paper is published in the Berliner Klinische Wochenschrift.
• 1884: Loeffler formally states the postulates; Koch reports the cholera bacterium.
• 1890: Koch announces tuberculin, which fails as a cure.
• 1905: Koch receives the Nobel Prize in Physiology or Medicine.

Why it matters

Koch’s discovery proved that one of humanity’s oldest and deadliest diseases had a specific, identifiable cause, and it showed how such causes could be demonstrated scientifically. His methods remain the basis of microbiology. Tuberculosis is still a major killer, causing more than a million deaths a year worldwide, which is why 24 March is marked as World Tuberculosis Day.