The Germ Theory Revolution

Fifty sheep in a field

On 5 May 1881 Louis Pasteur vaccinated 25 sheep at a farm at Pouilly-le-Fort outside Paris with a weakened culture of anthrax, in front of farmers, vets, journalists and doctors who had come to watch him fail. On 31 May he injected all 50 sheep, the 25 vaccinated and 25 that were not, with a deadly strain. By 2 June every unvaccinated sheep was dead or dying and every vaccinated one was alive. The Times correspondent sent the result to London. It was the first public proof that a disease could be prevented by a method based on knowing what caused it. That knowledge was twenty years old. In 1861 Pasteur, a chemist asked by the wine industry why its vats went sour, had shown with flasks whose long swan necks kept dust out that broth stayed sweet until air-borne microbes reached it: decay was caused by living things, not produced by them. In 1876 Robert Koch, a German country doctor, isolated the microbe of anthrax and photographed it, and in 1882 he announced the bacterium that caused tuberculosis, a disease killing one European in seven. Each germ found was a target: Pasteur made vaccines for chicken cholera in 1879 and rabies in 1885, Behring an antitoxin for diphtheria in 1890, and in 1909 Paul Ehrlich found in his 606th compound a chemical that killed the germ of syphilis inside the body, the first magic bullet. And yet the Briton with a cough in 1900 still bought Beechams pills, and tuberculosis was still killing until the 1940s. AQA asks what the germ theory changed and how quickly, why the theory came so far ahead of the cure, and which factor drove the revolution. This module gives you the chain with dates and the four question shapes.

Five words the examiners use

Each definition ends with the moment it mattered, because a bare definition earns nothing in an 8-mark answer.

The chain of discovery, in order

Put these six developments in the order they happened.

• Pasteur uses swan-necked flasks to show that microbes in the air cause decay
• Koch isolates and photographs the anthrax bacterium
• A forgotten chicken cholera culture shows Pasteur that a weakened germ protects
• Koch announces the bacterium that causes tuberculosis
• Pasteur saves Joseph Meister with the rabies vaccine
• Ehrlich and Hata find Salvarsan 606, the first magic bullet

Why did the germ theory arrive when it did?

Microbes had been seen through microscopes since the 1670s. Which explanation best accounts for the theory that they cause disease arriving only in the 1860s and 1870s?

• Technology supplied the means: better microscopes from the 1830s, the dyes of the German chemical industry that let Koch stain a germ, the agar plate and the photograph that let him show it; industry supplied the questions, since Pasteur was paid by brewers and silk farmers to find out why their products died; individuals supplied the method, Pasteur with the flasks of 1861 and Koch with his rules for proving a germ caused a disease; and the rivalry between France and Germany after the war of 1870 turned each discovery into a national contest that funded the next.
• The Crimean War forced doctors to find the cause of disease.
• The British government funded a search for the cause of cholera.
• The churches lifted their ban on studying microbes.

Each discovery and what it opened

The grade-9 habit AQA rewards on this topic: read the discoveries as a chain in which each one made the next possible, and notice how long the chain took to reach a patient.

Question 1: how useful is a source?

Source A: a description of the report sent by the correspondent of The Times from Pouilly-le-Fort in June 1881. It describes the crowd of farmers, veterinary surgeons, doctors and journalists gathered at the farm, the two groups of 25 sheep, the injection of the deadly culture on 31 May, and the scene two days later: the unvaccinated animals lying dead in the pen while the vaccinated ones grazed, and the sceptics who had come to jeer shaking Pasteur by the hand. AQA Question 1 asks how useful the source is to a historian studying the impact of germ theory. Tap the TWO details a top-level answer would use.

• The two groups of 25 sheep and the two outcomes give the historian the controlled experiment itself, which shows that germ theory could now be tested in public and proved rather than argued.
• The report mentions farmers among the crowd.
• The sceptics who came to jeer and stayed to shake his hand show how fast opinion turned once a prediction based on the theory came true, which is the impact the enquiry asks about.
• The report was printed in The Times.
• The report gives the dates of the injections.

Microbe hunting and magic bullets

Select the THREE statements that correctly describe what followed the germ theory.

• Koch gave the hunters a method: stain the germ with industrial dyes so it could be seen, grow it pure on agar, prove it caused the disease by infecting an animal, and photograph the result; his own finds were anthrax in 1876, tuberculosis in 1882 and cholera in 1883, and he won the Nobel Prize in 1905.
• The hunt became a national race after the French defeat of 1870: French and German laboratories competed for each disease, governments funded institutes in Paris in 1888 and Berlin in 1891, and by 1900 the germs of typhoid, diphtheria, tetanus, plague and pneumonia had been identified.
• Ehrlich reasoned that if dyes could stain a germ a chemical could kill it, tested hundreds of arsenic compounds with his assistant Hata, and in 1909 found that the 606th cured syphilis in rabbits and then in patients; Salvarsan went on sale in 1910 and Domagk found the next magic bullet, Prontosil, in 1935.
• Koch discovered penicillin in 1882.
• Once the tuberculosis germ was found in 1882 the disease was quickly cured.
• Britain led the microbe hunt from the start.

Which factor mattered most, c1850 to c1910?

The 16-mark question names one factor and asks whether it was the main one across a period. For the germ theory the strong answer weighs the factors with dated evidence and notices which factor supplied the tools and which supplied the drive. Science and technology supplied every tool: the improved microscope, the chemical dyes that let Koch see a bacterium in 1876, the agar plate, the photograph, the chemistry that let Ehrlich test 606 compounds. The individual used them: Pasteur, who was not a doctor, thought of the swan-necked flask; Koch, who was, thought of the rules of proof. Chance gave Pasteur the forgotten chicken cholera culture of 1879 that revealed attenuation. War gave the rivalry of 1870 that funded two national institutes. Communication carried each result across Europe within months and let the sceptics watch at Pouilly-le-Fort. Government paid late, for the institutes of 1888 and 1891. Religion was silent. And the everyday patient noticed almost none of it before 1900, which is the limit the examiner rewards you for stating. This meant that the revolution in understanding was driven by science and technology in the hands of individuals, and a judgement that says which of the two the other depended on, with dates, and then explains why the revolution in treatment waited for the magic bullet, reaches the top level.

In the shoes of Pasteur and Koch

Four decisions between 1861 and 1909. Choose what was actually done, and notice what each choice opened.

• 1861. You are Pasteur. The Academy of Sciences has offered a prize to settle whether microbes arise spontaneously in decaying matter or arrive from outside. What experiment do you design?
• 1876. You are Koch, a district doctor in Prussia with a microscope your wife gave you. Sheep in your district are dying of anthrax and you have found rod-shaped bodies in their blood. What do you do?
• Summer 1879. You are Pasteur, back from holiday. Your assistant injected chickens with a cholera culture left standing all summer and they did not die; then he injected them with a fresh culture and they still did not die. What do you conclude?
• 1905. You are Ehrlich. Dyes stain some germs and not others, which means a chemical can pick out a germ. Syphilis has just been found to be caused by a spirochaete. What do you do?

From cause to consequence

Because Pasteur showed in 1861 that decay comes from living things in the air, medicine had for the first time a cause of disease that could be looked for. Because _____ devised in 1876 a method of staining, growing and proving a germ, his followers could hunt one disease at a time and found twenty-one in twenty years. Because a chicken cholera culture forgotten over the summer of _____ protected the birds it should have killed, Pasteur discovered attenuation and could make vaccines for anthrax and rabies. Because a vaccine prevents but does not cure, the sick still depended on patent medicines until _____ found in compound 606 a chemical that killed a germ inside the body. And because the theory came a generation ahead of the cure, tuberculosis, found in _____, went on killing until the antibiotics of the _____.

The white plague

When Koch announced the tuberculosis bacillus in 1882, the disease was causing one death in every seven in Europe. In a city recording 7,000 deaths a year, how many would tuberculosis account for at that rate?

Question 3: in what ways were they similar?

AQA Question 3 might read: Compare the work of Pasteur and the work of Koch. In what ways were they similar? Explain your answer with reference to both. It is marked on similarities explained with support from both, for 8 marks. Which answer scores in the top level?

• Both proved their claims by experiment rather than argument: Pasteur with the swan-necked flasks of 1861 and the public test on 50 sheep at Pouilly-le-Fort in 1881, and Koch with his rules for isolating a germ, infecting an animal and recovering the same germ, applied to anthrax in 1876 and tuberculosis in 1882. Both also built on the other and were driven by the same rivalry: Koch took the germ theory Pasteur had published and made it a method for finding particular germs, Pasteur used the germs Koch found to make his anthrax vaccine, and after the war of 1870 each worked in a national institute funded to beat the other.
• Both were famous scientists who studied germs.
• Pasteur disproved spontaneous generation in 1861 and made the rabies vaccine in 1885.
• They were different because Pasteur was French and Koch was German.

Question 2: significance, marked

Question 2: Explain the significance of the work of Louis Pasteur in the development of medicine. The significance of Pasteur at the time was that he gave medicine a cause. Before 1861 disease was blamed on bad air or on decay that generated its own microbes; his swan-necked flasks showed that living things in the air caused decay, and his work for the wine and silk industries showed that particular microbes caused particular spoilage, which he then argued was true of disease. This meant that within four years Lister had read his papers and applied carbolic acid to a wound in Glasgow, and within fifteen Koch had turned the theory into a method for finding the germ of any disease, so that the surgery revolution and the microbe hunt both begin with Pasteur. The longer significance is that he then showed how the theory could prevent disease: the chance discovery of attenuation in 1879 explained why Jenner had succeeded eighty years earlier and turned one cure into a technique, proved on 50 sheep in public at Pouilly-le-Fort in 1881 and on the boy Joseph Meister against rabies on 6 July 1885, which is the line that runs to every vaccine of the twentieth century. His limit is also significant: he prevented and did not cure, so that the patient already ill gained nothing from him, and it took Ehrlich in 1909 and the antibiotics of the 1940s to close the gap between knowing the cause and treating it. What earned the marks: significance explained at the time with dated support; the link phrase this meant that; significance over the longer term in two directions, the method and the vaccine; and the limit stated as part of the significance. Both timescales, tied together, are what reach the top level of the 8-mark significance question.

Question 4: the 16-mark factors judgement

Has science and technology been the main factor in the understanding of disease in Britain since c1750? Explain your answer with reference to science and technology and other factors.

• One paragraph on science and technology with at least three dated examples: the microscope and the swan-necked flasks of 1861, the dyes, agar and photographs Koch used from 1876, and the chemistry behind Salvarsan in 1909
• At least two paragraphs on other factors from the AQA list, each with dated support: the role of the individual (Pasteur, Koch, Ehrlich), chance (1879), war and national rivalry after 1870, communication (Pouilly-le-Fort in the press), and government funding the institutes of 1888 and 1891

Question 4: the judgement, tested

A student has argued that science and technology was the main factor in the germ theory revolution. Which is the strongest point to set AGAINST that judgement, in the way the top level of Question 4 requires?

• The tools were available to everyone and used by two men: microscopes had shown microbes since the 1670s and dyes were in every German factory, yet it took Pasteur to think of a flask with a bent neck in 1861 and Koch to think of proving cause by infecting a mouse in 1876, and it took a forgotten culture in 1879 and a national rivalry after 1870 to turn theory into vaccines; so science and technology was the main factor in what could be seen, and the individual, with chance and rivalry behind him, the main factor in what was understood, and a judgement should give each its share rather than one the whole.
• It was all chance because of the chicken cholera.
• Nobody was cured before 1909, so no factor mattered.
• Koch could not have seen the bacillus without a microscope, so technology was the main factor.