Pasteur, Louis

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French chemist and microbiologist who discovered that fermentation is caused by micro-organisms and developed the germ theory of disease. He also created a vaccine for rabies, which led to the foundation of the Pasteur Institute in Paris in 1888.

Stereoisomers
Pasteur first gained recognition through his early work on the optical activity of stereo isomers. In 1848 he presented a paper to the Paris Academy of Sciences in which he reported that there are two molecular forms of tartaric acid, one that rotates plane polarized light to the right and another (a mirror image of the first) that rotates it to the left. In addition, he showed that one form can be assimilated by living micro-organisms whereas its optical antipode cannot.

Fermentation
A query from an industrialist about wine- and beer-making prompted Pasteur's research into fermentation. Using a microscope he found that properly aged wine contains small spherical globules of yeast cells whereas sour wine contains elongated yeast cells. He proved that fermentation does not require oxygen, yet it involves living micro-organisms, and that, to produce the correct type of fermentation (alcohol-producing rather than lactic acid-producing), it is necessary to use the correct type of yeast. Pasteur also realized that, after wine has formed, it must be gently heated to about 50°C/122°F – pasteurized – to kill the yeast and thereby prevent souring during the ageing process.

Spontaneous generation and the germ theory of disease
Pasteur then turned his attention to spontaneous generation, a problem that had once again become a matter of controversy. Pasteur showed that dust in the air contains spores of living organisms that reproduce when introduced into a nutrient broth. Then he boiled the broth in a container with a U-shaped tube that allowed air to reach the broth but trapped dust in the U-bend. He found that the broth remained free of living organisms, disproving the theory of spontaneous generation.

In the mid-1860s, the French silk industry was seriously threatened by a disease that killed silkworms and Pasteur was commissioned by the government to investigate the disease. He announced in 1868 that he had found a minute parasite that infects the silkworms, and recommended that all infected silkworms be destroyed. His advice was followed and the disease eliminated. This stimulated his interest in infectious diseases and, from the results of his previous work on fermentation, spontaneous generation, and the silkworm disease, Pasteur developed the germ theory of disease. This theory was probably the most important single medical discovery of all time, because it provided both a practical method of combating disease by disinfection and a theoretical foundation for further research.

The prevention of disease
Continuing his research into disease, in 1881 Pasteur developed a method for reducing the virulence of certain pathogenic micro-organisms. By heating a preparation of anthrax bacilli he attenuated their virulence but found that they still brought about the full immune response when injected into sheep. Using a similar method, Pasteur then inoculated fowl against chicken cholera, following the work of Edward Jenner (who first vaccinated against cowpox in 1796).

In 1882 Pasteur began what proved to be his most spectacular research: the prevention of rabies. He demonstrated that the causative micro-organism (actually a virus, although the existence of viruses was not known at that time) infects the nervous system and then, using the dried tissues of infected animals, he succeeded in obtaining an attenuated form of the virus suitable for the inoculation of human beings. The culmination of this work came on 6 July 1885, when Pasteur used his vaccine to save the life of a young boy who had been bitten by a rabid dog. The success of this experiment brought Pasteur even greater acclaim and led to the establishment of the Pasteur Institute in 1888.

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