Louis Pasteur

Chapter 1 · The Tanner's Son and the Crystals

An average student who found handedness in molecules

1822 – 1854 · Dole · Arbois · Paris · Strasbourg

Louis Pasteur was born on 27 December 1822 in Dole, in eastern France, the son of a tanner who had served as a sergeant-major under Napoleon and been decorated. The family was not wealthy; his father's pride in that decoration marked Louis's intense patriotism throughout his life.

At school he was considered an unremarkable student, and his real early talent was drawing. The pastels and portraits he made as a teenager are good enough that some who saw them thought he should have been an artist. He failed to distinguish himself at first at the École Normale Supérieure, and had to retake an entrance examination he was dissatisfied with.

His first major discovery came at twenty-six. Chemists had been puzzled by tartaric acid from wine, which rotated polarised light, and an apparently identical synthetic compound, paratartaric acid, which did not. Working under a microscope, Pasteur noticed that crystals of the second substance came in two shapes that were mirror images of each other. He separated them by hand with tweezers, dissolved each pile, and found that one rotated light to the right and the other to the left, cancelling out when mixed.

He had discovered molecular chirality — that molecules can exist in left- and right-handed forms — and founded stereochemistry. It also gave him a lifelong conviction that asymmetry is a signature of life, which shaped everything he did next.

In 1849 he married Marie Laurent, who became his secretary, collaborator and, by every account, an essential part of the laboratory's functioning.

Why this matters

Molecular handedness is fundamental to modern chemistry and pharmacology: many drugs have mirror-image forms with completely different effects, a fact that begins with Pasteur's tweezers.

You have met the young chemist. What would you ask him?

Ask Pasteur

• “How did you separate two mirror-image crystals by hand?”
• “Why did you think asymmetry was a sign of life?”
• “Were you really an unremarkable student?”
• “Why does molecular handedness matter for medicines?”
• “What did Marie Pasteur contribute to your work?”

Chapter 2 · Fermentation Is Alive

Lille's distillers, swan-necked flasks, and the end of spontaneous generation

1854 – 1864 · Lille · Paris

As dean of the science faculty at Lille in 1854, Pasteur was asked by a local distiller why some batches of beet alcohol turned sour. He looked at the failed vats under a microscope and found rod-shaped organisms rather than the round yeast cells present in successful ones.

His conclusion contradicted the leading chemists, including Justus von Liebig, who held that fermentation was a purely chemical decomposition. Pasteur argued that fermentation is caused by living microorganisms, that different organisms produce different products, and that souring is simply the wrong organism getting in.

The practical remedy was heating: warming wine and beer briefly to a temperature that killed spoilage organisms without ruining the flavour. Patented in 1865, pasteurisation was applied to milk decades later and has since prevented an incalculable amount of illness.

The theoretical stakes were higher. If microbes come from somewhere, where? Many scientists still held that life arises spontaneously from decaying matter.

Pasteur designed the experiment that settled it. He boiled broth in flasks with long S-shaped necks open to the air. Air could enter freely, but dust and microbes settled in the bend. The broth stayed clear indefinitely. Tilt the flask so the liquid touched the neck, and it clouded within days.

It was decisive because it removed the objection that boiling had destroyed some vital principle in the air. The Academy of Sciences awarded him its prize in 1862. Some of his original sealed flasks remain clear in Paris today.

Why this matters

The swan-necked flask is a model of experimental design: it answers the opponents' objection within the experiment itself rather than arguing about it afterwards.

You have the experiment that killed a theory. What would you ask him?

Ask Pasteur

• “Why does the shape of the flask neck matter?”
• “What was spontaneous generation, and why did people believe it?”
• “How did helping distillers lead to a scientific revolution?”
• “What does pasteurisation actually do?”
• “Why did Liebig and others resist your conclusion?”

Chapter 3 · Germ Theory and the Silkworms

Six years on a plague of worms, while his children died

1865 – 1877 · Alès · Paris

In 1865 the French government asked Pasteur to save the silk industry, which was collapsing from a disease killing silkworms. He knew nothing about silkworms and spent six years on it, eventually identifying two separate diseases and devising a microscopic screening method that let breeders discard infected eggs. It worked, and the industry recovered.

Those years were personal catastrophe. His father died, and three of his five children died of typhoid — Jeanne at nine in 1859, then Cécile and Camille during the silkworm work. He continued in the laboratory throughout, and his biographers link the losses directly to the intensity of his later pursuit of infectious disease.

He suffered a stroke in 1868 that left him with a permanently weakened left side, and worked for the rest of his life with limited use of one hand.

The silkworm work convinced him that the microbe theory applied to animals and, by extension, to people. He argued publicly that infections in hospitals were caused by organisms carried on hands, instruments and dressings. In Britain, Joseph Lister read Pasteur and applied it directly, introducing carbolic acid antisepsis in surgery and cutting mortality dramatically; Lister credited Pasteur openly.

Pasteur's advocacy was combative. He demonstrated microbes in the air of hospital wards, told surgeons to boil their instruments and wash their hands, and was often resented by physicians who noted that he had no medical qualification whatever.

Why this matters

Germ theory is the foundation of modern medicine and public health: it explains infection, justifies sterilisation, and makes surgery survivable.

You have germ theory arriving. What would you ask him?

Ask Pasteur

• “How did you solve a disease of silkworms knowing nothing about them?”
• “How did losing three children shape your work?”
• “What did you tell surgeons to change?”
• “Why did doctors resent advice from a chemist?”
• “How did you keep working after a stroke?”

Chapter 4 · Chicken Cholera and Pouilly-le-Fort

A forgotten culture, and a public trial with sheep

1879 – 1881 · Paris · Pouilly-le-Fort

In 1879 an assistant left a culture of chicken cholera bacteria on the bench during the summer holiday. On their return, the old culture was injected into chickens, which fell only mildly ill and recovered. When those same chickens were later injected with a fresh, virulent culture, they survived while new birds died.

Pasteur recognised the significance immediately: an organism weakened by age could protect against the full-strength disease. It was the first artificially attenuated vaccine, and it generalised Edward Jenner's smallpox inoculation into a method that could in principle be applied to any pathogen. Pasteur named the technique vaccination in Jenner's honour.

He applied it to anthrax, which was killing large numbers of sheep and cattle. Challenged by sceptics, he agreed to a public trial at Pouilly-le-Fort in May 1881: fifty sheep, half vaccinated, all then injected with virulent anthrax. Journalists and officials attended. On the appointed day, the vaccinated animals were healthy and almost all the unvaccinated were dead or dying. It was a spectacular public demonstration and made him a national figure.

There is a complication, and it should be stated. His laboratory notebooks, opened to scholars in the 1970s and analysed in detail by the historian Gerald Geison, show that the vaccine used at Pouilly-le-Fort was prepared by chemical treatment with potassium dichromate — a method developed by his rival Jean-Joseph Toussaint — rather than the oxygen-attenuation method Pasteur described publicly. He took public credit for a technique he had not used in that trial.

The science was real and the vaccine worked. The account he gave of it was not accurate.

“In the field of observation, chance favours only the prepared mind.”

— Louis Pasteur, lecture at the University of Lille, 7 December 1854

Why this matters

Attenuated vaccination is the principle behind most vaccines developed since, and Pasteur's notebooks are one of the best-documented cases of a famous scientist's public account differing from his records.

You have vaccination and its complication. What would you ask him?

Ask Pasteur

• “What did the forgotten chicken cholera culture teach you?”
• “Why did you name the method after Jenner?”
• “What actually happened at Pouilly-le-Fort?”
• “Why did your public account differ from your notebooks?”
• “What did you owe to Toussaint?”

Chapter 5 · Joseph Meister

A nine-year-old boy and an untested treatment

1885 – 1895 and after · Paris

Rabies is invariably fatal once symptoms appear, and its long incubation period offers a rare window in which a treatment might work. Pasteur could not see the organism — rabies is caused by a virus, too small for the microscopes of the day — so he worked with infected nervous tissue, drying spinal cords from rabid rabbits for varying periods to weaken the agent.

On 6 July 1885 a nine-year-old Alsatian boy, Joseph Meister, was brought to him having been bitten fourteen times by a rabid dog. He was expected to die.

Pasteur was not a physician and had no licence to treat patients. The vaccine had been given to dogs and never to a human. He consulted two medical colleagues, who agreed the boy's death was otherwise certain, and administered thirteen injections of progressively more virulent material over ten days.

Meister survived. So did a shepherd boy, Jean-Baptiste Jupille, treated later that year after being mauled while protecting younger children.

The news travelled worldwide. Patients arrived in Paris from Russia, America and across Europe, and public subscription funded the Pasteur Institute, which opened in 1888 and remains a leading research centre.

It was a real risk with a child's life, taken on the reasoning that the alternative was certain death. Geison's analysis of the notebooks shows Pasteur had given a version of the treatment to two other people before Meister — one of whom had probably not been infected and one who died — which he did not mention publicly.

Pasteur died on 28 September 1895 after further strokes. He is buried in a crypt at the Institute.

Why this matters

The Meister case is the founding example of the ethical problem in medical innovation: when is it justified to give an untested treatment to a patient who will otherwise certainly die?

You have the case that made him a hero. What would you ask him?

Ask Pasteur

• “How did you make a vaccine for something you could not see?”
• “Was it right to treat Joseph Meister?”
• “Why did you not mention the earlier human attempts?”
• “What did the Pasteur Institute set out to do?”
• “How should we judge a scientist whose results were real but whose accounts were not?”

What Pasteur changed

Pasteur established that microorganisms cause fermentation and disease, disproved spontaneous generation, created the first artificially attenuated vaccines, and gave the world pasteurisation. Germ theory transformed surgery, public health and food safety, and the Pasteur Institute has remained a centre of infectious disease research for well over a century.

A debate that continues

The opening of Pasteur's laboratory notebooks revealed discrepancies between his published accounts and his actual methods — particularly over the anthrax vaccine and the earliest rabies treatments — and historians disagree about how seriously this should qualify his reputation.

Keep exploring — ask Pasteur

• “How do you design an experiment your opponents cannot dismiss?”
• “What did losing your children change about what you worked on?”
• “When may a doctor try something that has never been tried?”

Related lives

• Alexander Fleming — Discoverer of Penicillin
• Florence Nightingale — The Lady with the Lamp
• Gregor Mendel — Father of Genetics

Related themes

Microbes and infectious disease · Vaccination and immunity · Scientific integrity