Pasteur, the son of a tanner, was born at Dôle in France and studied chemistry at the Ecole Normale Supérieure in Paris where he obtained his doctorate for crystallographic studies in 1847. His first appointments were as professor of chemistry firstly at Strasbourg (1849) and then at Lille (1854). In 1857 Pasteur returned to Paris as director of scientific studies at the Ecole Normale but moved to the Sorbonne in 1867 as professor of chemistry. He returned once more to the Ecole Normale in 1874 to direct the physiological chemistry laboratory but spent his last years, from 1888 to 1895, as director of the specially created Pasteur Institute.

Although not a physician, Pasteur was undoubtedly the most important medical scientist working in the 19th century. His work possessed an originality, depth, and precision that was apparent even in his early work, which led to the discovery of molecular asymmetry. Tartaric and racemic acid were known to have the same formula, but their crystalline salts possessed different optical properties in solution: tartrate rotates a ray of polarized light to the right, and was accordingly described as dextrorotatory or d-tartrate, while the racemic salts were optically inactive. How, it was asked, could the same compound have such contrasting properties?

Pasteur, in 1848, examined d-tartrate crystals under a hand lens and noticed that they all possessed an identical asymmetry, which he assumed to be sufficient to twist a ray of light to the right. The racemate however, while also asymmetrical, appeared to contain crystals divided equally between the d-tartrate form and its mirror image. Pasteur painstakingly separated the crystals into two piles and found that, in solution, one pile behaved exactly as d-tartrate while the other rotated polarized light to the left. Racemic acid was therefore, he concluded, an equal mixture of d-tartrate and its mirror image, the levorotatory l-tartrate, each of which was optically active on its own but together neutralized each other.

Such facts would later have profound consequences for structural chemistry and Pasteur, in 1860, suggested that the effect was a result of the internal arrangement of atoms. Thus he asked “Are the atoms of the dextro acid arranged on a right-hand helix, or positioned at the corners of an irregular tetrahedron, or have they some other asymmetric grouping? We cannot answer these questions. But there is no doubt that an asymmetric arrangement exists that has a non superposable image. It is not less certain that the atoms of the levo acid have exactly the inverse asymmetric arrangement.” The idea of an asymmetric tetrahedral carbon atom was put forward in 1874 by Jacobus van't Hoff and Joseph Le Bel. In 1857 Pasteur also noted that a mold accidentally growing on a tartrate solution selected just one of the two racemic forms, the d-form. More generally, he realized that only living organisms could distinguish between such asymmetric forms and even went so far as to argue that this ability marks the only sharply defined difference between the chemistry of dead and living matter.

By 1856 Pasteur had also begun to work on fermentation, beginning with the fermentation of milk into lactic acid. He reported the presence of microorganisms, which continued to bud and multiply; if excluded, fermentation failed to occur but they could be transferred from one ferment to produce another in uncontaminated milk. Further, such organisms were quite specific, as the yeast used to produce beer was incapable of producing lactic acid from milk. With these and many other observations and experiments behind him Pasteur was ready to dispute the chemical theory of Justus von Liebig. He declared that all true fermentations are caused by the presence and multiplication of organisms, and are not, as Liebig insisted, purely chemical phenomena. With his germ theory of fermentation Pasteur anticipated much of his later work.

He next turned to the origin of such ‘organisms’ and ‘ferments’ and investigated spontaneous generation. In 1862 Pasteur published his famous paper, Mémoire sur les corpuscles organisés qui existent dans l'atmosphère (Note on Organized Corpuscles that Exist in the Atmosphere), which finally brought to an end centuries of earlier debate. Pasteur demonstrated that if sterilized fermentable fluid was placed in a swan-neck flask (a flask with a long curved thin neck that allows air to enter but prevents dust and microorganisms entering) then the fluid remained clear. However if the neck of the flask was then broken off, allowing dust to enter, contamination soon resulted.

In 1865 Pasteur was asked to investigate a new disease devastating the silkworms of southern France. Despite his protest that “I have never even seen a silkworm,” despite considerable confusion caused by the presence of two quite independent infections, and despite a stroke in 1868, which partially paralyzed his left side, Pasteur still managed to provide a comprehensive analysis of the disease and its prevention.

It was not until 1877 that Pasteur finally turned to human disease and pioneered effective methods of treatment against virulent infections such as anthrax. The breakthrough came in 1880 as a result of an oversight by his assistant, who had inadvertently left a batch of chicken cholera bacilli standing in the laboratory over a long hot summer. On injection into some healthy chickens the culture produced only mild and transient signs of disease. Pasteur then instructed his assistant to prepare a fresh batch of the bacillus and once more inject it into the chickens. They survived unscathed, whereas chickens fresh from the market succumbed rapidly to a similar injection. Pasteur had accidentally discovered an attenuated vaccine for chicken cholera. By May 1882 he had succeeded in deliberately producing a comparable vaccine against anthrax and was ready to test it publicly at Pouilly-le-Fort. Here his success was total with all 24 unvaccinated sheep dying of anthrax while those receiving his vaccine survived.

Events even more dramatic followed in 1885 when Pasteur used a rabies vaccine recently developed by him on a badly bitten nine-year-old boy, Joseph Meister. Against the advice of such colleagues as Emile Roux he began the course of 14 injections using virus attenuated in the spine of rabbits. Meister survived. He committed suicide 55 years later in 1940 when, as a caretaker at the Pasteur Institute, he preferred to die rather than open the tomb of Pasteur to the invading Nazi forces.

Thus nearly a century after Edward Jenner, Pasteur had introduced only the second vaccine effective against a serious human disease. Others would rapidly follow him so that by the turn of the century several would be in use. Shortly after his triumph Pasteur suffered a second stroke in 1887, one which affected his speech. Although he lived a further seven years his long creative period was at an end.