The scientific discoveries of Louis Pasteur in the 19th century revolutionized medicine and continue to save millions of lives today

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Rodney E. Rudd, Texas State University

(Conversation) Some of the greatest scientific discoveries did not result in Nobel Prizes.

Louis Pasteur, who lived from 1822 to 1895, is arguably the world’s most famous microbiologist. He is widely credited with the germ theory of disease and for inventing the process of pasteurization – which is named after him – to preserve foods. Remarkably, he also developed rabies and anthrax vaccines and made significant contributions to the fight against cholera.

But because he died in 1895, six years before the first Nobel Prize was awarded, that award is not on his biography. Had he lived in the age of Nobel Prizes, he would undoubtedly have deserved one for his work. The Nobel Prizes, awarded in various fields, including physiology and medicine, are not awarded posthumously.

During the current time of constant threats from the emergence or re-emergence of infectious diseases, from COVID-19 and polio to monkeypox and rabies, it is fascinating to look back at Pasteur’s legacy. His efforts radically changed people’s view of infectious diseases and how they were combated with vaccines.

I have worked in public health and medical laboratories specializing in viruses and other microbes, while training future medical laboratory scientists. My virology career began with a front row seat in rabies detection and surveillance and zoonoses, and it builds in large part on Pasteur’s pioneering work in microbiology, immunology, and vaccines.

First, chemist

In my estimation, Pasteur’s strongest contributions to science are his remarkable achievements in the field of medical microbiology and immunology. However, his story begins with chemistry.

Pasteur studied under the French chemist Jean-Baptiste André Dumas. During that time, Pasteur became interested in the origins of life and worked in the field of polarized light and crystallography.

In 1848, a few months after receiving his Ph.D., Pasteur was studying the properties of crystals formed in the process of making wine when he discovered that crystals occur in mirror image shapes, a property known as chirality. This discovery became the basis for a sub-field of chemistry known as stereochemistry, which is the study of the spatial arrangement of atoms within molecules. This asymmetry in molecules was a “revolutionary hypothesis” at the time.

These findings led Pasteur to suspect what would later be proven by molecular biology: all life processes ultimately stem from the precise arrangement of atoms within biological molecules.

Wine and beer – from fermentation to germ theory

Beer and wine were crucial to the economies of France and Italy in the nineteenth century. It was not uncommon during Pasteur’s life for products to spoil and become bitter or dangerous to drink. At the time, the scientific concept of “spontaneous generation” held that life could arise from inanimate matter, which was thought to be the cause of wine spoiling.

While many scientists have attempted to refute the theory of spontaneous generation, in 1745 the English biologist John Torberville Needham thought he had devised a perfect experiment that favored spontaneous generation. Most scientists believe that heat kills life, so Needham devised an experiment to show that microorganisms can grow on food, even after boiling. After the chicken broth boiled, he put it in a beaker, heated it, sealed it and waited, not realizing that air could make its way back into the flask before sealing. After some time, the microorganisms grew, and Needham claimed victory.

However, his experience has two main drawbacks. First, the boiling time was not enough to kill all the microbes. More importantly, his flasks allowed air to flow again, allowing bacterial contamination.

To settle the scientific battle, the French Academy of Sciences sponsored a competition for the best experiment to prove or disprove spontaneous generation. Pasteur’s response to the competition was a series of experiments, including an 1861 award-winning essay.

Pasteur considered one of these experiments to be “decisive and decisive” because, unlike Needham, having sterilized his cultures, he kept them free of contamination. Using the now popular swan-necked flasks, which have a long S-shaped neck, allowed air to flow while at the same time preventing dripping particles from getting into the broth while it was heating. As a result, the flask remained growth-free for a long time. This showed that if air was not allowed directly into his boiling batches, no “living microorganisms” would appear, even after months of observation. However, most importantly, if dust was introduced, live microbes appeared.

Through this process, Pasteur not only disproved the spontaneous generation theory, but also showed that microorganisms are ubiquitous. When he showed that food and wine spoiled due to contamination from invisible bacteria and not from spontaneous reproduction, the modern germ theory of disease was born.

The genesis of vaccination in the nineteenth century

In the 1860s, when the silk industry was devastated by two silkworm diseases, Pasteur developed a clever process for examining silkworm eggs under a microscope and preserving those that were otherwise healthy. Just like his efforts with wine, he was able to apply his observations to the methods of industry, and became something of a French hero.

Even as his health deteriorated from a severe stroke that partially paralyzed him, Pasteur continued his work. In 1878, he succeeded in identifying and culturing the bacteria that cause avian cholera. He realized that ancient bacterial cultures were no longer harmful and that chickens vaccinated with ancient cultures could survive exposure to wild strains of bacteria. His observation that surviving chickens secrete harmful bacteria helped establish an important concept that is now so familiar in the era of COVID-19 – asymptomatic “health carriers” can still spread germs during outbreaks.

After bird cholera, Pasteur turned to preventing anthrax, a widespread plague of livestock and other animals caused by the bacteria Bacillus anthracis. Building on his own work and that of German physician Robert Koch, Pasteur developed the concept of attenuated or weakened versions of microbes for use in vaccines.

In the late 1880s, he demonstrated beyond doubt that exposing cattle to a weakened form of the anthrax vaccine could lead to what is now known as immunity, dramatically reducing livestock mortality.

Rabies Vaccine Breakthrough

In my professional assessment of Louis Pasteur, the discovery of the rabies vaccination is his most important achievement.

Rabies has been called the “world’s most diabolical virus”, and is transmitted from animal to human through a bite.

Working with the rabies virus is extremely dangerous, with mortality close to 100% once symptoms appear and without vaccination. Through clever observation, Pasteur discovered that drying the spinal cords of dead rabid rabbits and monkeys resulted in a weakened form of the rabies virus. Using this weakened version as a vaccine to gradually expose dogs to the rabies virus, Pasteur showed that he could effectively immunize dogs against rabies.

Then, in July 1885, Joseph Maestre, a 9-year-old French boy, was badly bitten by a rabid dog. With Joseph facing almost certain death, his mother took him to Paris to see Pasteur because she had heard that he was working on developing a cure for rabies.

Pasteur took over the case and, with two doctors, gave the boy a series of injections over several weeks. Joseph survived and Pasteur shocked the world with a cure for a globally fatal disease. This discovery opened the door to the widespread use of the rabies vaccine invented by Pasteur around 1885, which greatly reduced rabies deaths in humans and animals.

A life worthy of a Nobel Prize

Pasteur once said in one of the famous lectures: “In the fields of observation, chance prefers only the ready mind.”

Pasteur was gifted in applying his brilliant – and prepared – scientific mind to the most practical dilemmas facing humanity.

While Louis Pasteur died before the Nobel Prize began, I would argue that his incredible life of discovery and contribution to science in medicine, infectious disease, immunization, medical microbiology and immunology places him among the greatest scientists of all time.

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