Accelerating Change, 1875-1905: Ehrlich, Koch and Chemistry » Koch and the Hunt for Germs

From germ theory to germ hunting

In 1861 Louis Pasteur put forward the germ theory: tiny living things called microbes in the air and water cause decay and, many people came to believe, disease. But a theory was not enough. Nobody could yet say which microbe caused which illness. Doctors needed proof for each disease, one at a time. That search became known as the hunt for germs.

The man who did most to make the hunt work was Robert Koch, a German doctor. He started as a country doctor in a small town. He studied microbes in his spare time and, from 1880, worked in a government laboratory in Berlin. He had a clear aim: to show that one particular germ causes one particular disease.

Key terms:

• Bacteriology: the scientific study of bacteria.
• Bacterium (plural bacteria): a type of microbe, a single living cell. Many bacteria cause disease.
• Culture: a batch of bacteria grown on purpose in a laboratory.
• Stain: a dye used to colour bacteria so they show up under a microscope.
• Vaccine: a weakened form of a germ, given to a person or animal so the body learns to fight the real disease.

Koch's methods

Germs are very small and most are clear and almost invisible under a microscope. Koch solved two problems.

With these two methods Koch could do something new. He could take a germ from a sick animal, grow it on its own, give it to a healthy animal, and see if the same disease appeared. If it did, he had linked that germ to that disease.

Linking germs to diseases

• Anthrax (1876): anthrax killed farm animals and sometimes people. Koch showed that a particular bacterium caused it. He grew the bacterium outside the body and found that it can form tough spores (resting forms that survive for a long time). This was the first time a particular germ was proved to cause a particular disease.
• Tuberculosis (1882): TB was one of the biggest killers of the 19th century. Koch used stains to find the bacterium that causes it and announced his discovery in 1882. His work on TB later won him the Nobel Prize in 1905.
• Cholera (1883-84): working in Egypt and India, Koch found the bacterium that causes cholera. It spreads through dirty water, which fitted John Snow's idea, from 1854, that cholera in London was linked to a dirty water pump.

Other scientists in Koch's laboratory and elsewhere copied his methods. Within about twenty years germs had been found for many diseases, including typhoid, diphtheria and tetanus. Medicine now had a fast-growing new science, bacteriology.

Vaccines from Pasteur's team

Pasteur and his team in France went in a different direction. They worked on ways to protect people and animals from germs they now understood better.

• Chicken cholera (1879-80): Pasteur found that an old, weakened culture of the germ protected chickens from the real disease.
• Anthrax (1881): Pasteur's team made an anthrax vaccine. In a public test, vaccinated sheep survived and unvaccinated sheep died.
• Rabies (1885): a vaccine made by Pasteur's team was used on a boy, Joseph Meister, who had been bitten by a dog with rabies. He survived.

Koch showed which germs caused which diseases. Pasteur's team showed that this knowledge could be used to protect against them.

What changed and what did not

• Change: the cause of illness was now understood, disease by disease. Doctors could identify a germ in the laboratory. Vaccines meant that some diseases could be prevented.
• Continuity: finding a germ did not mean finding a cure. Koch's own TB treatment, tuberculin (1890), did not cure patients. Vaccines existed only for a few diseases, and most infections still had no effective treatment.

Exam-style question

Explain TWO ways in which the work of Robert Koch on germs in the 1870s and 1880s was similar to the work of Louis Pasteur on germs in the 1860s to the 1880s. [6 marks]