Louis Pasteur

Louis Pasteur was a French chemist and microbiologist whose work in the second half of the nineteenth century helped establish that microscopic organisms cause fermentation, spoilage and many diseases. He disproved the old belief in spontaneous generation, gave his name to pasteurisation, and developed vaccines against anthrax and rabies. His laboratory notebooks, opened to historians only in the late twentieth century, show that he was also a fierce competitor who sometimes presented his methods misleadingly.

Pasteur was born on 27 December 1822 in Dole, in the Jura region of eastern France, the son of a tanner who had served in the army of Napoleon. He grew up in Arbois, was an average pupil with a talent for drawing, and studied at Besançon before entering the École Normale Supérieure in Paris in 1843. He took his doctorate in 1847.

His first major discovery came in 1848 and concerned tartaric acid, a by-product of winemaking. The natural acid twisted the plane of polarised light, while a chemically identical form, called racemic acid, did not. Examining crystals of a salt of the racemic acid under a microscope, Pasteur noticed that they came in two mirror-image shapes, like left and right hands. He separated them by hand with tweezers and found that solutions of each turned light in opposite directions, while a mixture of equal amounts did nothing. He concluded that the molecules themselves must exist in left- and right-handed forms. This was the first demonstration of what chemists now call molecular chirality, and it founded the field of stereochemistry.

In 1848 he became professor of chemistry at Strasbourg, where in 1849 he married Marie Laurent, the daughter of the university's rector. She became his closest assistant and secretary. They had five children, three of whom died in childhood of typhoid.

In 1854 Pasteur was made dean of the new science faculty at Lille, in an industrial region full of breweries and distilleries. Asked by a local manufacturer why his beetroot alcohol was souring, he found that healthy fermentation was carried out by living yeast cells, while souring was caused by other microorganisms. The leading chemists of the day, such as Justus von Liebig, saw fermentation as a purely chemical breakdown, so Pasteur's claim that it was a process of life was controversial. He published papers on lactic and alcoholic fermentation in 1857 and 1858, by which time he had moved to Paris to direct scientific studies at the École Normale.

The work led him into the old question of spontaneous generation, the idea that living things could arise from non-living matter, as maggots seemed to appear from rotting meat. In experiments around 1859 to 1862, in a public dispute with the naturalist Félix Pouchet, he used flasks with long, curved necks shaped like a swan's. He boiled a nutrient broth in them and left them open to the air. Dust and microbes settled in the bend of the neck, and the broth stayed clear indefinitely. When he tilted a flask so the broth touched the contaminated neck, it soon teemed with life. The microbes, he concluded, came from the air, not from the broth itself. The French Academy of Sciences awarded him a prize in 1862.

His practical work for French industry followed. He showed that wine and beer were spoiled by unwanted microbes and that gently heating them to around 50 to 60 °C killed most of these without ruining the taste. He patented the method for wine in the mid-1860s, and the process, pasteurisation, was later applied above all to milk, where it has saved countless lives. From 1865 he spent years in the south of France studying diseases that were destroying silkworms, eventually showing silk farmers how to select healthy eggs under the microscope.

In 1868, aged 45, he suffered a stroke that left him partly paralysed on his left side. From then on he depended on assistants such as Émile Roux and Charles Chamberland to carry out experiments. After the Franco-Prussian War of 1870 to 1871, angered by the German bombardment of Paris, he returned an honorary degree the University of Bonn had given him.

Pasteur was not the first to suggest that microbes cause disease, but his work on fermentation made the idea far more convincing. In Glasgow, in the United Kingdom, the surgeon Joseph Lister read Pasteur's papers and from 1865 began using carbolic acid to kill germs in wounds, sharply reducing deaths after surgery. In Germany, Robert Koch identified the anthrax bacterium in 1876 and later the bacteria of tuberculosis and cholera, developing methods to grow pure cultures. The two men became bitter rivals, their quarrel sharpened by national feeling after the war.

Pasteur's great step was to make vaccines against diseases other than smallpox. Around 1879, working on chicken cholera, he found that an old culture of the bacteria, left standing for weeks, made chickens only mildly ill and then protected them against fresh, deadly bacteria. He called this weakening attenuation, and proposed that diseases in general might be prevented with attenuated germs.

He then turned to anthrax, which killed large numbers of sheep and cattle. In May and June 1881, at a farm at Pouilly-le-Fort near Melun, before an audience of farmers, vets and journalists, he vaccinated a group of sheep, cows and goats and left another group unvaccinated, then infected both with anthrax. The vaccinated animals survived and the others died. The trial made him a national hero. His notebooks later revealed that the vaccine used had been prepared with potassium dichromate, a chemical method close to that of his rival Henry Toussaint, a veterinary surgeon, and not by the oxygen-based method Pasteur publicly described.

His most famous triumph was rabies, an always fatal disease caused, as is now known, by a virus too small for his microscopes. His team grew it in the nervous tissue of rabbits and weakened it by drying the spinal cords for different periods. On 6 July 1885 a nine-year-old boy from Alsace, Joseph Meister, was brought to Paris after being badly bitten by a rabid dog. Because Pasteur was not a licensed doctor, physicians gave the boy a course of thirteen injections of increasingly virulent material over eleven days. Meister did not develop rabies. Within months people bitten by rabid animals were arriving from across Europe and beyond. Pasteur's statements suggested the method had been well tested on dogs beforehand; his notebooks indicate the testing was much more limited, and some colleagues, including Roux at first, thought treating a human was premature.

Public donations from France and abroad funded the Institut Pasteur in Paris, which opened in 1888 with Pasteur as director. Its scientists went on to major work on diphtheria, plague, tuberculosis and later HIV, and it now leads an international network of research institutes. Pasteur died on 28 September 1895 at Marnes-la-Coquette, near Paris, after a series of strokes. He was given a state funeral and was later buried in a crypt in the institute.

For a century he was celebrated in France as a nearly flawless hero of science. After his notebooks became available, the historian Gerald Geison argued in 1995 that he had misled the public about key experiments and claimed more credit than he deserved. Other scientists replied that the core discoveries remain sound. Most historians now see both sides: a brilliant, intensely ambitious experimenter whose findings transformed chemistry, medicine, public health and the food industry, and who also guarded his reputation by shaping the record.