Louis Pasteur

I

Introduction

Louis Pasteur (1822-1895), world-renowned French chemist and biologist, who founded the science of microbiology, proved the germ theory of disease, invented the process of pasteurization, and developed vaccines for several diseases, including rabies.

Pasteur was born in Dôle on December 27, 1822, the son of a tanner, and grew up in the small town of Arbois. In 1847 he earned a doctorate at the École Normale in Paris, with a focus on both physics and chemistry. Becoming an assistant to one of his teachers, he began research that led to a significant discovery. He found that a beam of polarized light (see Optics) was rotated to either the right or the left as it passed through a pure solution of naturally produced organic nutrients, whereas when polarized light was passed through a solution of artificially synthesized organic nutrients, no rotation took place. If, however, bacteria or other microorganisms were placed in the latter solution, after a while it would also rotate light to the right or left.

Pasteur concluded that organic molecules can exist in one of two forms, called isomers (that is, having the same structure and differing only in mirror images of each other), which he referred to as “left-handed” and “right-handed” forms. When chemists synthesize an organic compound, both of these forms are produced in equal proportions, canceling each other’s optical effects. Living systems, however, which have a high degree of chemical specificity, can discriminate between the two forms, metabolizing one and leaving the other untouched and free to rotate light.

II

Work on Fermentation

After spending several years of research and teaching at Dijon and Strasbourg, Pasteur moved in 1854 to the University of Lille, where he was named professor of chemistry and dean of the faculty of sciences. This faculty had been set up partly to serve as a means of applying science to the practical problems of the industries of the region, especially the manufacture of alcoholic beverages. Pasteur immediately devoted himself to research on the process of fermentation. Although his belief that yeast plays some kind of role in this process was not original, he was able to demonstrate, from his earlier work on chemical specificity, that the desired production of alcohol in fermentation is indeed due to yeast and that the undesired production of substances (such as lactic acid or acetic acid) that make wine sour is due to the presence of additional organisms such as bacteria. The souring of wine and beer had been a major economic problem in France; Pasteur contributed to solving the problem by showing that bacteria can be eliminated by heating the starting sugar solutions to a high temperature.

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Pasteur extended these studies to such other problems as the souring of milk, and he proposed a similar solution: heating the milk to a high temperature and pressure before bottling. This process is now called pasteurization.

III

Disproof of Spontaneous Generation

Fully aware of the presence of microorganisms in nature, Pasteur undertook several experiments designed to address the question of where these “germs” came from. Were they spontaneously produced in substances themselves, or were they introduced into substances from the environment? Pasteur concluded that the latter was always the case. His findings resulted in a fierce debate with the French biologist Félix Pouchet—and later with the noted English bacteriologist Henry Bastion—who maintained that under appropriate conditions instances of spontaneous generation could be found. These debates, which lasted well into the 1870s, although a commission of the Académie des Sciences officially accepted Pasteur’s results in 1864, gave great impetus to improving experimental techniques in microbiology.

IV

Silkworm Studies

In 1865, Pasteur was summoned from Paris, where he had become administrator and director of scientific studies at the École Normale, to come to the aid of the silk industry in southern France. The country’s enormous production of silk had suddenly been curtailed because of an epidemic illness among the silkworms. Despite having no prior experience with animal biology, Pasteur determined the silkworms were dying from two parasitic infections: pébrine and flacherie. Suspecting that certain microscopic objects found in the diseased silkworms (and in the moths and their eggs) were disease-producing organisms, Pasteur experimented with the separation of uninfected from infected silkworms, the cleanliness of silkworm colonies, and controlled breeding. He concluded that only in diseased and living eggs was the cause of the disease maintained; therefore, selection of disease-free eggs was the solution. Through his work with silkworms, Pasteur not only saved the French silk industry, but also made great progress in the studies of infectious disease and immunology.

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