Louis Pasteur

Until the middle of the nineteenth century most scientists accepted that living organisms, at least the simplest ones, could arise spontaneously from non-living matter, a maggot from rotting meat, a microbe from broth left standing in the open air. In 1859 the French Academy of Sciences offered a prize for whoever could settle the question experimentally, and Louis Pasteur set out to win it, designing what became one of the most elegant experiments in the history of science.

Pasteur boiled nutrient broth in flasks with long, curved swan necks that let air reach the liquid while trapping dust and microorganisms in the bend of the glass. Broth prepared this way stayed sterile indefinitely, but the moment Pasteur tilted a flask so the broth touched the trapped dust in the neck, microorganisms grew. The experiment showed that air itself was not the source of life in the broth, dust-borne microbes were, and by 1864 it had effectively ended serious scientific belief in spontaneous generation.

Pasteur went on to argue that the same invisible microorganisms responsible for fermentation and spoilage were also responsible for disease, the germ theory of disease that transformed medicine. His applied work followed directly: the heat treatment now called pasteurization, developed first to stop wine and beer from spoiling, and vaccines against anthrax, demonstrated publicly at Pouilly-le-Fort in 1881, and rabies, first used on a human patient, nine year old Joseph Meister, in 1885. Meister survived a series of otherwise fatal bites from a rabid dog, and the treatment's success made Pasteur an international figure whose name still identifies the institute in Paris founded to continue his work.

Before germ theory made him famous, Pasteur was a young chemist working on a puzzle in crystallography. Tartaric acid, a byproduct of winemaking, rotates polarized light as it passes through a solution of it, a property called optical activity. A related compound, racemic acid, had the same chemical formula and behaved identically in every respect chemists could test, except that it had no effect on polarized light at all, and no one could explain why two substances that seemed chemically identical behaved so differently.

In 1848, working with crystals of a sodium ammonium salt of racemic acid, Pasteur noticed under a microscope that the crystals came in two subtly different shapes, mirror images of one another, like a left hand and a right hand. Working with tweezers, he sorted thousands of tiny crystals into two piles by hand, one shape in each pile. When he dissolved each pile separately and tested it, one solution rotated polarized light clockwise and the other rotated it counterclockwise by an equal amount, mixed back together, the two solutions canceled out and behaved exactly like racemic acid.

Pasteur had discovered molecular chirality, the fact that some molecules exist in mirror-image forms with identical composition but different three-dimensional structure, without yet having any way to picture what a molecule's three-dimensional shape actually was, the atomic-scale explanation came only decades later, from the chemists Jacobus van 't Hoff and Joseph Le Bel. The finding founded the field of stereochemistry, the study of how the spatial arrangement of atoms within a molecule affects its properties, a field that later proved essential to understanding why one mirror-image form of a drug can be therapeutic while the other is inert or even harmful.