What Did Louis Pasteur Discover? Germs, Pasteurization and the First Rabies Vaccine

Louis Pasteur (1822–1895) was a French chemist and microbiologist whose discoveries laid much of the groundwork for hygiene, public health and modern medicine. In short, he discovered that tiny living organisms are responsible for fermentation and for the spoiling of food and drink, helped prove that germs, not "bad air" or spontaneous generation, are behind many diseases, invented the heat treatment we now call pasteurization, and developed vaccines against chicken cholera, anthrax and rabies. Every time you open a carton of milk with "pasteurized" on the label, you are reading his name.

From a tanner's house in Dole to the laboratory

Pasteur was born on 27 December 1822 in Dole, in eastern France. His family were tanners, and as a boy he was better known for drawing portraits than for his marks in science. He trained as a chemist, and his first major discovery had nothing to do with germs at all.

A first discovery: molecules that are mirror images

As a young researcher, Pasteur studied crystals of tartaric acid, a substance found in wine deposits. He noticed that some crystals were mirror images of each other, like a left hand and a right hand, and that this asymmetry affected how they interacted with polarised light. It was the first demonstration of what chemists now call molecular chirality, and it helped launch the study of optical isomers.

That may sound abstract, but chirality matters enormously today: two mirror-image versions of the same drug molecule can behave completely differently in the body.

Fermentation: yeast is alive

Wine, beer and vinegar makers had used fermentation for thousands of years without really knowing what it was. Pasteur showed that yeast, a living microorganism, drives alcoholic fermentation, and that other microorganisms are what make drinks go sour or spoil.

This led directly to a practical invention. If microbes spoil wine, then gently heating the wine should kill enough of them to stop the damage without ruining the taste. Pasteur developed this heat treatment for wine and later for milk, and he patented the process in 1865. It became known as pasteurization.

The swan-neck flask experiment

For centuries many people believed in "spontaneous generation", the idea that living things such as maggots or microbes could appear on their own from non-living matter. Pasteur tested it with an elegant piece of glassware: a flask with a long neck bent into an S-shape, like a swan's neck.

He boiled a nutrient broth in the flask. Air could still reach the broth, but dust and microbes got trapped in the curve of the neck. The broth stayed clear. When the neck was broken, or the broth was tipped into the curve, microbes soon grew. The conclusion was clear: microbes came from other microbes in the environment, not from nowhere. The work earned him the Alhumbert Prize in 1862.

Germ theory: from wine to disease

If microbes could spoil wine, could they also make people and animals ill? Pasteur is considered one of the founders of germ theory and of bacteriology, alongside rivals such as the German physician Robert Koch. Germ theory, the idea that specific microorganisms cause specific diseases, replaced older explanations such as "miasma", or bad air.

Pasteur also helped France's silk industry, which was being devastated by diseases of silkworms. He worked on controlling a disease called pébrine by screening silkworm eggs.

Vaccines by "weakening" a germ

Pasteur's next breakthrough came from a lucky accident that he was ready to notice. Working on chicken cholera, he found that an old, weakened culture of the bacteria did not kill chickens, and that those chickens were then protected against fresh, dangerous cultures. He called this method attenuation and used it to develop vaccines for chicken cholera and anthrax. He coined the general word "vaccine" in honour of Edward Jenner, whose smallpox vaccine came from cowpox (vacca is Latin for cow).

In 1881, at a farm at Pouilly-le-Fort, he staged a public trial of his anthrax vaccine on sheep in front of journalists and officials, and it was a public success. He liked to say that chance favours only the prepared mind, and his career is full of examples.

Joseph Meister and the rabies vaccine

Rabies was terrifying: once symptoms appeared, it was almost always fatal. Pasteur and his colleagues developed a rabies vaccine, and in 1885 it was first given to Joseph Meister, a boy who had been bitten by a rabid dog. The boy survived, and Pasteur became world-famous almost overnight. Donations poured in, and the Pasteur Institute was founded in Paris in 1887. Pasteur directed it until his death in 1895.

The less heroic side

Historians have looked closely at Pasteur's private notebooks, which were made public long after his death. They found that some of his published accounts were misleading. For example, he had vaccinated fewer dogs before treating Meister than he claimed, and his treatment of the boy raised ethical questions because he was not a licensed physician. He also had bitter rivalries, especially with Robert Koch and the chemist Antoine Béchamp.

None of this erases his achievements, but it is a useful reminder that great scientists are also competitive, ambitious human beings.

Learn with Pasteur: English for science and discovery

Phrase	Meaning	Example
to disprove a theory	to show a theory is false	The experiment disproved the old theory.
trial and error	learning by trying different ways	We found the answer by trial and error.
a breakthrough	an important new discovery	The vaccine was a medical breakthrough.
to go sour	to become spoiled (food) or go badly (plans)	The milk went sour; so did our plans.
chance favours the prepared mind	luck helps people who are ready for it	Keep practising: chance favours the prepared mind.

Try this: Describe a small "experiment" from your own life, like a new study routine, in four English sentences: what you expected, what you did, what happened, and what you concluded. Use "trial and error" and "breakthrough" at least once.