Pasteur, Louis (1822-1895), a French chemist, made major contributions to chemistry, medicine, and industry that greatly benefited humanity. His discovery that bacteria spread diseases saved countless lives. Pasteur was a great theoretical scientist who applied his basic discoveries to important practical problems in both industry and medicine.

Pasteur was born on Dec. 27, 1822, in Dole, France. His family moved to Arbois, where he received his early education. He earned a place at the École Normale Superieure, a teacher-training school in Paris, and received his doctorate in chemistry from there in 1847. By the age of 26, he had gained some fame for his work on the structure of crystals.

In 1849, Pasteur became a chemistry professor in Strasbourg, France, where he began studying fermentation, a chemical process that breaks down organic materials. Such microbes as bacteria, molds, and yeasts carry out this process, and humans long ago learned to use this chemical process—well before they understood it. For example, yeast breaks down sugar obtained from malted grain into ethyl alcohol and carbon dioxide gas for use in beer. Sugar from grape juice is broken down in the same way for use in wine. Fermentation also is essential in the production of bread, cheese, and yogurt. Fermentation can also be unhealthy, such as when it turns milk sour.

Pasteur observed reproduction and growth in microorganisms, specializing in bacteriology (the study of bacteria). In the late 1600's, Anton van Leeuwenhoek, a Dutch amateur scientist, had recorded the first clear descriptions of bacteria. Using his simple microscopes that were among the most powerful of his day, in 1674, Leeuwenhoek concluded that the moving objects he viewed through his microscopes were tiny animals, which he called animalcules. They included what are now known as bacteria, protozoa, and rotifers. Leeuwenhoek hoped that his discovery of the animalcules would disprove the widely believed theory of spontaneous generation. This theory holds that certain forms of life, such as flies, worms, and mice, can develop directly from nonliving things, such as mud and decaying flesh. Italian biologist Francesco Redi in 1668 demonstrated that maggots (the young of flies) did not appear in meat if he kept adult flies away from it. Before that, many people had believed that flies developed directly from decaying meat.

Pasteur became the first to show that living things come only from living things. He demonstrated that the microorganisms would grow in sterilized broth only if the broth was first exposed to air that contained their spores (reproductive cells). Pasteur's discoveries led to the development of the cell theory of the origin of living matter. The cell theory states that all life originates from preexisting living material. Pasteur also showed that although bacteria live almost everywhere, their spread can be controlled.

For 10 years, from 1857, Pasteur worked as director of scientific studies at the École Normale. He left this post in 1867 to focus on his research. In the early 1860's, he realized that wine turns bitter because of microbes (germs) that enter the wine while it is being made. He showed that applying a controlled amount of heat could kill the microbes without changing the flavor of the wine. This use of heat as a means to kill germs became known as pasteurization, after Pasteur. Pasteur also used this method to preserve milk, beer, and food.

In 1865, Pasteur set out to help the silk industry. A disease called pebrine was killing great numbers of silkworms. He worked several years to prove that a microbe that attacks silkworm eggs causes the disease. He showed that eliminating this germ in silkworm nurseries would wipe out the disease. His research into silkworm disease saved the silk industry in southern France.

Pasteur proved that many diseases are caused by germs that multiply in the body. He also proved that if microbes are weakened in a laboratory and then placed in an animal's body, the animal develops an immunity (resistance) to the microbe. He called this method of fighting off microbes vaccination. Pasteur proved the value of vaccination by vaccinating sheep against a severe infectious disease called anthrax. Pasteur borrowed ideas from the success of Edward Jenner, a British physician who in the 1790's had discovered a safe method of making people immune to smallpox. Jenner inoculated a young boy with matter from a cowpox sore. The boy developed cowpox, a relatively harmless disease related to smallpox. But when Jenner later injected the boy with matter from a smallpox sore, the boy did not come down with that disease. His bout with cowpox had helped his body build up an immunity to smallpox. Jenner's classic experiment was the first officially recorded vaccination.

Pasteur began in 1881 to study rabies, an infectious disease spread by the bite of rabies-infected animals. Rabies destroys the nerve cells of part of the brain and almost always causes death. Human beings and most other mammals can catch the disease. A virus known as a rhabdovirus that is carried in the salivary glands causes rabies. If the host bites another animal or a human being, or if some of its infected saliva enters an open wound, the victim may get rabies.

Pasteur spent endless hours in his laboratory seeking a vaccine to prevent rabies. In 1885, a rabid dog bit a boy named Joseph Meister. The boy's parents begged Pasteur to save their son. Pasteur hesitated to use his new vaccine on a human being, but he finally agreed. After several weeks of treatment, the vaccine proved successful. The boy did not become sick with rabies.

Standard preventive treatment for rabies today in the United States consists of one injection of antirabies globulin followed by five injections of rabies vaccine. Vaccinating all dogs and cats against rabies has become an important means of controlling the disease. Pasteur also showed that vaccination could be used to prevent chicken cholera and other animal diseases.

Pasteur and the German physician Robert Koch are credited with establishing what is called the germ theory of disease. While Pasteur proved that germs cause infectious diseases and that killing the germs responsible stopped the spread of such diseases, Koch developed a method for determining which bacteria cause particular diseases.

Pasteur wrote founding documents in the field of stereochemistry, which studies the three-dimensional arrangement of atoms in molecules and the properties that follow from such arrangements. The concepts of stereochemistry are important in the pharmaceutical industry to understand how a chemical will act in the body.

In 1888, the Pasteur Institute, an agency devoted to biological research, in Paris was founded in gratitude to Pasteur. He headed the institute until he died at a chateau near Paris on Sept. 28, 1895. The Pasteur Institute became a world center for the study, prevention, and treatment of disease, and Pasteur is buried in a tomb in the building.

Pasteur's work paved the way for many important achievements in health care, some in his lifetime. In 1865, Joseph Lister, a surgeon, realized that the formation of pus was also due to germs. He also realized that the surgeon's hands and instruments carried germs. He insisted on the use of antiseptics on hands, instruments, and dressings, as well as on the patient. Lister's application of antiseptics revolutionized surgery, and the use of his techniques eliminated nearly all postsurgical infections. Before antiseptic surgery, nearly half of all surgeries resulted in death.

Prior to pasteurization, people suffered many health problems related to food spoilage. As pasteurization became common, the storage time of milk increased and the incidents of food poisoning decreased. Nearly all milk sold in the United States today is pasteurized, in a rapid heat treatment that destroys harmful bacteria but causes little nutrient damage. Most is pasteurized by the high-temperature short-time, or HTST, method, which involves heating milk to 161 ºF (72 °C) for 15 seconds and then quickly cooling it.

Over a century after Pasteur's work, many vaccines have been developed and more are being investigated. Millions of lives are saved by vaccinations in childhood against the diseases of diphtheria, tetanus, whooping cough, measles, tuberculosis, and poliomyelitis. Many researchers worldwide are working on a vaccine for AIDS.