Discovery and Development, 1860-75: Lister and Pasteur » Pasteur and the Germ Theory

Pasteur and the puzzle of decay

By 1860, medicine still did not know what caused disease. Most doctors believed in miasma (bad air), and many also believed in spontaneous generation, the idea that the tiny living things seen in rotting food appeared by themselves out of the decay. Under that idea, germs were a result of rot, not a cause of it.

Louis Pasteur (born in 1822) was a French chemist, not a doctor. He began by studying fermentation, the process that turns grape juice into wine and makes milk go sour. He showed that tiny living things, which he called microbes, were at work in each case. Some microbes made good wine. Others made it taste sour and spoil. Pasteur found that gently heating the liquid killed the harmful microbes. This idea was later called pasteurisation.

These results made Pasteur ask a bigger question. If microbes made liquids go bad, where did they come from? Were they born from the rot, or did they arrive from somewhere else?

Key terms:

• Microbe: a living thing too small to see without a microscope. Germs are microbes that cause disease.
• Fermentation: the change in a liquid caused by microbes, such as grape juice becoming wine.
• Germ theory: the idea that disease is caused by microbes (germs) that get into the body from outside.
• Pasteurisation: heating a liquid to kill harmful microbes in it.

The swan-neck flask experiment

Pasteur had a clear idea: microbes were already in the air, carried on dust, and they landed in food and drink. To test it, he did an experiment in the early 1860s. It is one of the most famous in the history of science.

The result was clear. The broth did not go bad by itself. It only went bad when microbes from outside got into it. Supporters of spontaneous generation, such as the French naturalist Félix Pouchet, had argued the opposite. Pasteur's experiments were so careful that most scientists came to accept that he had defeated the idea.

The germ theory of 1861

From this work Pasteur developed the germ theory. In 1861 he argued that microbes in the air cause decay, and that the same kind of outside microbes could cause disease in people and animals. This was a completely new way to explain illness. Disease did not come from bad smells, and germs did not appear from nowhere. Germs came from outside and attacked the body.

It is important to see what Pasteur had and had not shown. He had proved that microbes cause decay and that they do not appear by themselves. But in 1861 he had not yet proved which germ caused which disease. That step came later (see the lesson on Koch and the Hunt for Germs).

Effects of the germ theory

Notice that the effects were not instant. The first big change in medical practice came in surgery, and the others took years.

How slowly did Britain accept it?

Pasteur's experiments convinced many scientists, but many doctors in Britain stayed doubtful for years. Four reasons stand out:

• He was not a doctor. Many doctors did not trust a chemist telling them about disease.
• The link was missing. Pasteur had not shown which germ caused which disease. Doctors still saw germs in sick patients and thought the disease produced the germs, not the other way round.
• No new cures. The theory did not at first help doctors treat patients, so it seemed of little use.
• Old ideas were strong. Miasma and the belief in spontaneous generation fitted what doctors had always taught, and they had been trained to believe them.

Some doctors did take the theory seriously, and Lister was the most important. But widespread acceptance came only after other scientists showed that particular germs caused particular diseases.

Exam-style question

Explain TWO ways in which the reception of Pasteur's germ theory in the 1860s was similar to the reception of John Snow's ideas about cholera in the 1850s. (Snow argued in 1849 and 1854 that cholera spread through contaminated water. Most doctors continued to believe in miasma.) [6 marks]