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.