what did louis pasteur experiment prove

• Scientific Biographies

Louis Pasteur

During the mid- to late 19th century, Pasteur demonstrated that microorganisms cause disease and discovered how to make vaccines from weakened, or attenuated, microbes. He developed the earliest vaccines against fowl cholera, anthrax, and rabies.

Louis Pasteur (1822–1895) is revered by his successors in the life sciences as well as by the general public. In fact, his name provided the basis for a household word— pasteurized .

His research, which showed that microorganisms cause both fermentation and disease, supported the germ theory of disease at a time when its validity was still being questioned. In his ongoing quest for disease treatments he created the first vaccines for fowl cholera; anthrax, a major livestock disease that in recent times has been used against humans in germ warfare; and the dreaded rabies.

Early Life and Education

Pasteur was born in Dole, France, the middle child of five in a family that had for generations been leather tanners. Young Pasteur’s gifts seemed to be more artistic than academic until near the end of his years in secondary school. Spurred by his mentors’ encouragement, he undertook rigorous studies to compensate for his academic shortcomings in order to prepare for the École Normale Supérieure, the famous teachers’ college in Paris. He earned his master’s degree there in 1845 and his doctorate in 1847.

Study of Optical Activity

While waiting for an appropriate appointment, Pasteur continued to work as a laboratory assistant at the École Normale. There he made further progress on the research he had begun for his doctoral dissertation—investigating the ability of certain crystals or solutions to rotate plane-polarized light clockwise or counterclockwise, that is, to exhibit “optical activity.” He was able to show that in many cases this activity related to the shape of the crystals of a compound.

He also reasoned that there was some special internal arrangement to the molecules of such a compound that twisted the light—an “asymmetric” arrangement. This hypothesis holds an important place in the early history of structural chemistry—the field of chemistry that studies the three-dimensional characteristics of molecules.

Fermentation and Pasteurization

Pasteur secured his academic credentials with scientific papers on this and related research and was then appointed in 1848 to the faculty of sciences in Strasbourg and in 1854 to the faculty in Lille. There he launched his studies on fermentation. Pasteur sided with the minority view among his contemporaries that each type of fermentation is carried out by a living microorganism.

At the time the majority believed that fermentation was spontaneously generated by a series of chemical reactions in which enzymes—themselves not yet securely identified with life—played a critical role.

In 1857 Pasteur returned to the École Normale as director of scientific studies. In the modest laboratory that he was permitted to establish there, he continued his study of fermentation and fought long, hard battles against the theory of spontaneous generation. Figuring prominently in early rounds of these debates were various applications of his pasteurization process, which he originally invented and patented (in 1865) to fight the “diseases” of wine.

He realized that these were caused by unwanted microorganisms that could be destroyed by heating wine to a temperature between 60° and 100°C. The process was later extended to all sorts of other spoilable substances, such as milk.

Germ Theory

At the same time Pasteur began his fermentation studies, he adopted a related view on the cause of diseases. He and a minority of other scientists believed that diseases arose from the activities of microorganisms—germ theory. Opponents believed that diseases, particularly major killer diseases, arose in the first instance from a weakness or imbalance in the internal state and quality of the afflicted individual.

In an early foray into the causes of particular diseases, in the 1860s, Pasteur was able to determine the cause of the devastating blight that had befallen the silkworms that were the basis for France’s then-important silk industry. Surprisingly, he found that the guilty parties were two microorganisms rather than one.

A New Laboratory

Pasteur did not, however, fully engage in studies of disease until the late 1870s, after several cataclysmic changes had rocked his life and that of the French nation. In 1868, in the middle of his silkworm studies, he suffered a stroke that partially paralyzed his left side. Soon thereafter, in 1870, France suffered a humiliating defeat at the hands of the Prussians, and Emperor Louis-Napoléon was overthrown. Nevertheless, Pasteur successfully concluded with the new government negotiations he had begun with the emperor.

The government agreed to build a new laboratory for him, to relieve him of administrative and teaching duties, and to grant him a pension and a special recompense in order to free his energies for studies of diseases.

Attenuating Microbes for Vaccines: Fowl Cholera and Anthrax

In his research campaign against disease Pasteur first worked on expanding what was known about anthrax, but his attention was quickly drawn to fowl cholera. This investigation led to his discovery of how to make vaccines by attenuating, or weakening, the microbe involved. Pasteur usually “refreshed” the laboratory cultures he was studying—in this case, fowl cholera—every few days; that is, he returned them to virulence by reintroducing them into laboratory chickens with the resulting onslaught of disease and the birds’ death.

Months into the experiments, Pasteur let cultures of fowl cholera stand idle while he went on vacation. When he returned and the same procedure was attempted, the chickens did not become diseased as before. Pasteur could easily have deduced that the culture was dead and could not be revived, but instead he was inspired to inoculate the experimental chickens with a virulent culture. Amazingly, the chickens survived and did not become diseased; they were protected by a microbe attenuated over time.

Realizing he had discovered a technique that could be extended to other diseases, Pasteur returned to his study of anthrax. Pasteur produced vaccines from weakened anthrax bacilli that could indeed protect sheep and other animals. In public demonstrations at Pouilly-le-Fort before crowds of observers, twenty-four sheep, one goat, and six cows were subjected to a two-part course of inoculations with the new vaccine, on May 5, 1881, and again on May 17. Meanwhile a control group of twenty-four sheep, one goat, and four cows remained unvaccinated.

On May 31 all the animals were inoculated with virulent anthrax bacilli, and two days later, on June 2, the crowd reassembled. Pasteur and his collaborators arrived to great applause. The effects of the vaccine were undeniable: the vaccinated animals were all alive. Of the control animals all the sheep were dead except three wobbly individuals who died by the end of the day, and the four unprotected cows were swollen and feverish. The single goat had expired too.

Rabies and the Beginnings of the Institut Pasteur

Pasteur then wanted to move into the more difficult area of human disease, in which ethical concerns weighed more heavily. He looked for a disease that afflicts both animals and humans so that most of his experiments could be done on animals, although here too he had strong reservations. Rabies, the disease he chose, had long terrified the populace, even though it was in fact quite rare in humans. Up to the time of Pasteur’s vaccine, a common treatment for a bite by a rabid animal had been cauterization with a red-hot iron in hopes of destroying the unknown cause of the disease, which almost always developed anyway after a typically long incubation period.

Rabies presented new obstacles to the development of a successful vaccine, primarily because the microorganism causing the disease could not be specifically identified; nor could it be cultured in vitro (in the laboratory and not in an animal). As with other infectious diseases, rabies could be injected into other species and attenuated. Attenuation of rabies was first achieved in monkeys and later in rabbits.

Meeting with success in protecting dogs, even those already bitten by a rabid animal, on July 6, 1885, Pasteur agreed with some reluctance to treat his first human patient, Joseph Meister, a nine-year-old who was otherwise doomed to a near-certain death. Success in this case and thousands of others convinced a grateful public throughout the world to make contributions to the Institut Pasteur.

Historian Bert Hansen discusses his book,  Picturing Medical Progress from Pasteur to Polio.

It was officially opened in 1888 and continues as one of the premier institutions of biomedical research in the world. Its tradition of discovering and producing vaccines is carried on today by the pharmaceutical company Sanofi Pasteur.

A Great Experimenter and Innovative Theorist

Pasteur’s career shows him to have been a great experimenter, far less concerned with the theory of disease and immune response than with dealing directly with diseases by creating new vaccines. Still it is possible to discern his notions on the more abstract topics. Early on he linked the immune response to the biological, especially nutritional, requirements of the microorganisms involved; that is, the microbe or the attenuated microbe in the vaccine depleted its food source during its first invasion, making the next onslaught difficult for the microbe.

Later he speculated that microbes could produce chemical substances toxic to themselves that circulated throughout the body, thus pointing to the use of toxins and antitoxins in vaccines. He lent support to another view by welcoming to the Institut Pasteur Élie Metchnikoff and his theory that “phagocytes” in the blood—white corpuscles—clear the body of foreign matter and are the prime agents of immunity.

Featured image: Portrait of Louis Pasteur , photograph of Louis Pasteur taken in 1886, reproduced in the 1911 biography   The Life of Louis Pasteur . Science History Institute

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Science News

Louis pasteur’s devotion to truth transformed what we know about health and disease.

Louis Pasteur demonstrated, more dramatically than any other scientist, the benefit of science for humankind.

Sam Falconer

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By Tom Siegfried

November 18, 2022 at 7:00 am

Great scientists become immortalized in various ways.

Some through names for obscure units of measurement (à la Hertz, Faraday and Curie). Others in elements on the periodic table ( Mendeleev , Seaborg, Bohr , among many others). A few become household names symbolizing genius — like Newton in centuries past and nowadays, Einstein . But only one has been honored on millions and millions of cartons of milk: the French chemist, biologist and evangelist for experimental science Louis Pasteur.

Pasteur was born 200 years ago this December, the most significant scientist birthday bicentennial since Charles Darwin’s in 2009 . And Pasteur ranked behind only Darwin among the most exceptional biological scientists of the 19th century.

Pasteur not only made milk safe to drink, but also rescued the beer and wine industry. He established the germ theory of disease, saved the French silkworm population, confronted the scourges of anthrax and rabies, and transformed the curiosity of vaccination against smallpox into a general strategy for treating and preventing human diseases. He invented microbiology and established the foundations for immunology.

Had he been alive after 1901, when Nobel Prizes were first awarded, he would have deserved one every year for a decade. No other single scientist demonstrated more dramatically the benefit of science for humankind.

He was not, however, exactly a saint. A Pasteur biographer, Hilaire Cuny, called him “a mass of contradictions.” Pasteur was ambitious and opportunistic, sometimes arrogant and narrow-minded, immodest, undiplomatic and uncompromising. In the scientific controversies he engaged in (and there were many), he was pugnacious and belligerent. He did not suffer criticism silently and was often acerbic in his responses. To his laboratory assistants, he was demanding, dictatorial and aloof. Despite his revolutionary spirit in pursuing science, in political and social matters, he was conformist and deferential to authority.

And yet he was a tireless worker, motivated by service to humankind, faithful to his family and unwaveringly honest. He was devoted to truth, and therefore also to science.

How Pasteur developed pasteurization

In his youth, Pasteur did not especially excel as a student. His interests inclined toward art rather than science, and he did display exceptional skill at drawing and painting. But in light of career considerations (his father wanted him to be a scholar), Pasteur abandoned art for science and so applied to the prestigious École Normale Supérieure in Paris for advanced education. He finished 15th in the competitive entrance examination, good enough to secure admission. But not good enough for Pasteur. He spent another year on further studies emphasizing physical sciences and then took the École Normale exam again, finishing fourth. That was good enough, and he entered the school in 1843. There he earned his doctoral degree, in physics and chemistry, in 1847.

Among his special interests at the École Normale was crystallography. In particular he was drawn to investigate tartaric acid. It’s a chemical found in grapes responsible for tartar, a potassium compound that collects on the surfaces of wine vats. Scientists had recently discovered that tartaric acid possesses the intriguing power of twisting light — that is, rotating the orientation of light waves’ vibrations. In light that has been polarized (by passing it through certain crystals, filters or some sunglasses), the waves are all aligned in a single plane. Light passing through a tartaric acid solution along one plane emerges in a different plane.

Even more mysteriously, another acid (paratartaric acid, or racemic acid), with the exact same chemical composition as tartaric acid, did not twist light at all. Pasteur found that suspicious. He began a laborious study of the crystals of salts derived from the two acids. He discovered that racemic acid crystals could be sorted into two asymmetric mirror-image shapes, like pairs of right-handed and left-handed gloves. All the tartaric acid crystals, on the other hand, had shapes with identical asymmetry, analogous to gloves that were all right-handed.

Pasteur deduced that the asymmetry in the crystals reflected the asymmetric arrangement of atoms in their constituent molecules. Tartaric acid twisted light because of the asymmetry of its molecules, while in racemic acid, the two opposite shapes canceled out each other’s twisting effects.

Pasteur built the rest of his career on this discovery. His research on tartaric acid and wine led eventually to profound realizations about the relationship between microbes and human disease. Before Pasteur, most experts asserted that fermentation was a natural nonbiological chemical process. Yeast, a necessary ingredient in the fermenting fluid, was supposedly a lifeless chemical acting as a catalyst. Pasteur’s experiments showed yeast to be alive, a peculiar kind of “small plant” (now known to be a fungus) that caused fermentation by biological activity.

Pasteur demonstrated that, in the absence of air, yeast acquired oxygen from sugar, converting the sugar to alcohol in the process. “Fermentation by yeast,” he wrote, is “the direct consequence of the processes of nutrition,” a property of a “minute cellular plant … performing its respiratory functions.” Or more succinctly, he proclaimed that “fermentation … is life without air.” (Later scientists found that yeast accomplished fermentation by emitting enzymes that catalyzed the reaction.)

Pasteur also noticed that additional microorganisms present during fermentation could be responsible for the process going awry, a problem threatening the viability of French winemaking and beer brewing. He solved that problem by developing a method of heating that eliminated the bad microorganisms while preserving the quality of the beverages. This method, called “pasteurization,” was later applied to milk, eliminating the threat of illness from drinking milk contaminated by virulent microorganisms. Pasteurization became standard public health practice in the 20th century.

Incorporating additional insights from studies of other forms of fermentation, Pasteur summarized his work on microbial life in a famous paper published in 1857. “This paper can truly be regarded as the beginning of scientific microbiology,” wrote the distinguished microbiologist René Dubos, who called it “one of the most important landmarks of biochemical and biological sciences.”

The germ theory of disease is born

Pasteur’s investigations of the growth of microorganisms in fermentation collided with another prominent scientific issue: the possibility of spontaneous generation of life. Popular opinion even among many scientists held that microbial life self-generated under the proper conditions (spoiled meat, for example). Demonstrations by the 17th century Italian scientist Francesco Redi challenged that belief , but the case against spontaneous generation was not airtight.

In the early 1860s Pasteur undertook a series of experiments that should have left no doubt that spontaneous generation, under conditions encountered on Earth today, was an illusion. Yet he was nevertheless accosted by critics, such as the French biologist Charles-Philippe Robin, to whom he returned verbal fire. “We trust that the day will come when M. Robin will … acknowledge that he has been in error on the subject of the doctrine of spontaneous generation, which he continues to affirm, without adducing any direct proofs in support of it,” Pasteur remarked.

It was his work on spontaneous generation that led Pasteur directly to the development of the germ theory of disease.

For centuries people had suspected that some diseases must be transmitted from person to person by close contact. But determining exactly how that happened seemed beyond the scope of scientific capabilities. Pasteur, having discerned the role of germs in fermentation, saw instantly that something similar to what made wine go bad might also harm human health.

After disproving spontaneous generation, he realized that there must exist “transmissible, contagious, infectious diseases of which the cause lies essentially and solely in the presence of microscopic organisms.” For some diseases, at least, it was necessary to abandon “the idea of … an infectious element suddenly originating in the bodies of men or animals.” Opinions to the contrary, he wrote, gave rise “to the gratuitous hypothesis of spontaneous generation” and were “fatal to medical progress.”

His first foray into applying the germ theory of disease came during the late 1860s in response to a decline in French silk production because of diseases afflicting silkworms. After success in tackling the silkworms’ maladies, he turned to anthrax, a terrible illness for cattle and humans alike. Many medical experts had long suspected that some form of bacteria caused anthrax, but it was Pasteur’s series of experiments that isolated the responsible microorganism, verifying the germ theory beyond doubt. (Similar work by Robert Koch in Germany around the same time provided further confirmation.)

Understanding anthrax’s cause led to the search for a way to prevent it. In this case, a fortuitous delay in Pasteur’s experiments with cholera in chickens produced a fortunate surprise. In the spring of 1879 he had planned to inject chickens with cholera bacteria he had cultured, but he didn’t get around to it until after his summer vacation. When he injected his chickens in the fall, they unexpectedly failed to get sick. So Pasteur prepared a fresh bacterial culture and brought in a new batch of chickens.

When both the new chickens and the previous batch were given the fresh bacteria, the new ones all died, while nearly all of the original chickens still remained healthy. And so, Pasteur realized, the original culture had weakened in potency over the summer and was unable to cause disease, while the new, obviously potent culture did not harm the chickens previously exposed to the weaker culture. “These animals have been vaccinated,” he declared.

Vaccination, of course, had been invented eight decades earlier, when British physician Edward Jenner protected people from smallpox by first exposing them to cowpox, a similar disease acquired from cows. (Vaccination comes from cowpox’s medical name, vaccinia, from vacca , Latin for cow.) Pasteur realized that the chickens surprisingly displayed a similar instance of vaccination because he was aware of Jenner’s discovery. “Chance favors the prepared mind,” Pasteur was famous for saying.

Because of his work on the germ theory of disease, Pasteur’s mind was prepared to grasp the key role of microbes in the prevention of smallpox, something Jenner could not have known. And Pasteur instantly saw that the specific idea of vaccination for smallpox could be generalized to other diseases. “Instead of depending on the chance finding of naturally occurring immunizing agents, as cowpox was for smallpox,” Dubos observed, “it should be possible to produce vaccines at will in the laboratory.”

Pasteur cultured the anthrax microbe and weakened it for tests in farm animals. Success in such tests not only affirmed the correctness of the germ theory of disease, but also allowed it to gain a foothold in devising new medical practices.

Later Pasteur confronted an even more difficult microscopic foe, the virus that causes rabies. He had begun intense experiments on rabies, a horrifying disease that’s almost always fatal, caused usually by the bites of rabid dogs or other animals. His experiments failed to find any bacterial cause for rabies, leading him to realize that it must be the result of some agent too small to see with his microscope. He could not grow cultures in lab dishes of what he could not see. So instead he decided to grow the disease-causing agent in living tissue — the spinal cords of rabbits. He used dried-out strips of spinal cord from infected rabbits to vaccinate other animals that then survived rabies injections.

Pasteur hesitated to test his rabies treatment on humans. Still, in 1885 when a mother brought to his lab a 9-year-old boy who had been badly bitten by a rabid dog, Pasteur agreed to administer the new vaccine. After a series of injections, the boy recovered fully. Soon more requests came for the rabies vaccine, and by early the next year over 300 rabies patients had received the vaccine and survived, with only one death among them.

Popularly hailed as a hero, Pasteur was also vilified by some hostile doctors, who considered him an uneducated interloper in medicine. Vaccine opponents complained that his vaccine was an untested method that might itself cause death. But of course, critics had also rejected Pasteur’s view of fermentation, the germ theory of disease and his disproof of spontaneous generation.

Pasteur stood his ground and eventually prevailed (although he did not turn out to be right about everything). His attitude and legacy of accomplishments inspired 20th century scientists to develop vaccines for more than a dozen deadly diseases. Still more diseases succumbed to antibiotics, following the discovery of penicillin by Alexander Fleming — who declared, “Without Pasteur I would have been nothing.”

Even in Pasteur’s own lifetime, thanks to his defeat of rabies, his public reputation was that of a genius.

Pasteur’s scientific legacy

As geniuses go, Pasteur was the opposite of Einstein. To get inspiration for his theories, Einstein imagined riding aside a light beam or daydreamed about falling off a ladder. Pasteur stuck to experiments. He typically initiated his experiments with a suspected result in mind, but he was scrupulous in verifying the conclusions he drew from them. Preconceived ideas, he said, can guide the experimenter’s interrogation of nature but must be abandoned in light of contrary evidence. “The greatest derangement of the mind,” he declared, “is to believe in something because one wishes it to be so.”

So even when Pasteur was sure his view was correct, he insisted on absolute proof, conducting many experiments over and over with variations designed to rule out all but the true interpretation.

“If Pasteur was a genius, it was not through ethereal subtlety of mind,” wrote Pasteur scholar Gerald Geison. Rather, he exhibited “clear-headedness, extraordinary experimental skill and tenacity — almost obstinacy — of purpose.”

His tenacity, or obstinacy, helped him persevere through several personal tragedies, such as the deaths of three of his daughters, in 1859, 1865 and 1866. And then in 1868 he suffered a cerebral hemorrhage that left him paralyzed on his left side. But that did not slow his pace or impair continuing his investigations.

“Whatever the circumstances in which he had to work, he never submitted to them, but instead molded them to the demands of his imagination and his will,” Dubos wrote. “He was probably the most dedicated servant that science ever had.”

To the end of his life, Pasteur remained dedicated to science and the scientific method, stressing the importance of experimental science for the benefit of society. Laboratories are “sacred institutions,” he asserted. “Demand that they be multiplied and adorned; they are the temples of wealth and of the future.”

Three years before his death in 1895, Pasteur further extolled the value of science and asserted his optimism that the scientific spirit would prevail. In an address, delivered for him by his son, at a ceremony at the Sorbonne in Paris, he expressed his “invincible belief … that science and peace will triumph over ignorance and war, that nations will unite, not to destroy, but to build, and that the future will belong to those who will have done most for suffering humanity.”

Two hundred years after his birth , ignorance and war remain perniciously prominent, as ineradicable as the microbes that continue to threaten public health, with the virus causing COVID-19 the latest conspicuous example. Vaccines, though, have substantially reduced the risks from COVID-19, extending the record of successful vaccines that have already tamed not only smallpox and rabies, but also polio, measles and a host of other once deadly maladies .

Yet even though vaccines have saved countless millions of lives, some politicians and so-called scientists who deny or ignore overwhelming evidence continue to condemn vaccines as more dangerous than the diseases they prevent. True, some vaccines can induce bad reactions, even fatal in a few cases out of millions of vaccinations. But shunning vaccines today, as advocated in artificially amplified social media outrage, is like refusing to eat because some people choke to death on sandwiches.

Today, Pasteur would be vilified just as he was in his own time, probably by some people who don’t even realize that they can safely drink milk because of him. Nobody knows exactly what Pasteur would say to these people now. But it’s certain that he would stand up for truth and science, and would be damn sure to tell everybody to get vaccinated.

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