Pasteur, Louis (1822–1895)

Louis Pasteur, a French chemist, is often called the father of modern microbiology. Through the development of vaccines for cholera, anthrax, rabies, staphylococcus, and streptococcus, he discovered much about the nature of infection and laid the groundwork for the microbial theory of disease. Pasteur also contributed greatly to the field of infectious epidemiology by demonstrating how pathogens spread through animal and human populations.

[Page 791]Pasteur examined the role of microorganisms in the transformation of organic matter, which at the time was greatly misunderstood. Instructed by Napoleon to investigate diseases infecting wines, he determined that fermentation results from the action of a specific microorganism. To enable fermentation, the right microorganism must be introduced, and microorganisms that could alter the process must be kept out. With Claude Bernard, Pasteur developed a process, eventually known as ‘pasteurization,’ in which wine, beer, vinegar, and milk were heated to kill bacteria and molds present within them.

Pasteur discredited the theory of spontaneous generation by demonstrating that microorganisms in a presterilized medium could be explained by outside germs. His research also showed that juice will not ferment if environmental yeasts are prevented from being deposited on grapes. By analogy, Pasteur believed that infectious diseases are probably caused by germs and that just as grapes can be protected against yeast, it might be possible to protect human beings against germs.

In 1879, Pasteur discovered that fowl cholera is caused by a type of bacteria now known as ‘Pasteurella.’ Chickens inoculated with a few drops of these bacteria would die. However, chickens inoculated with an old, weakened culture of Pasteruella did not die and were protected against a later inoculation with a more virulent culture. Through this chance observation, Pasteur discovered the principle of vaccination with attenuated pathogens. Because of Edward Jenner's work on vaccination, scientists knew that a weakened form of a disease could provide immunity to a more virulent version. However, whereas Jenner's vaccines used cowpox, a naturally occurring infection similar to but much less severe than smallpox, Pasteur's cholera and anthrax vaccines used artificially generated, weakened forms of disease organisms.

Pasteur grew the rabies virus in rabbits and then weakened it by drying the affected nerve tissue. He gave these artificially weakened diseases the generic name of ‘vaccines’ to honor Jenner. In 1885, Pasteur conducted the first experimental rabies inoculations on a human. Joseph Meister, a 9-year-old boy who had been bitten multiple times by a rabid dog, was brought to Pasteur by his mother. Since the death of the child appeared inevitable, Pasteur attempted a method of inoculation that had proved consistently successful on dogs. Pasteur was not a licensed physician and could have faced prosecution for this. Ultimately, Meister's health improved after 12 inoculations, and Pasteur was hailed as a hero.