Pasteur: the discovery of germs

Through his research on the fermentation of wine and beer, Louis Pasteur discovers the existence and behavior of bacteria, opening the road to modern microbiology.


Nico Zardo

“Peter has a bad cough and his throat is sore: maybe he’s caught some sort of virus”.

“Blow your nose with a paper handkerchief: coughs and sneezes spread colds!”

“Cover that food; I wouldn’t want flies to bring in germs”.

These phrases are familiar to us. They are part of hygienic behaviors that help us defend ourselves from disease transmitted by living organisms so small that... we can’t even see them: germs.

Now we know why and how to defend ourselves but five-hundred years ago we did not. At that time, cleanliness habits were scarce, overall hygiene was poor and the presence of lice was an integral part of daily life.

Scientists used microscopes, but they did not understand the nature of germs and of the other minute organisms they were able to see through them. It was only after the experiments by French scientist Louis Pasteur, founder of microbiology, that we were able to study the behavior of bacteria and viruses, yielding great progress in public health and hygiene.



in eastern France, in 1822. His father, a veteran of the Napoleonic campaigns, owned a tannery where leather was processed. Interested in drawing and painting since he was a child, in 1839 he obtained a diploma in both art and science from the Royal College in Besançon. A resolute, competitive young man, he dedicated himself to teaching and studying simultaneously. In 1847 he graduated in physics and chemistry. The following year, he was nominated professor of physics in a secondary school in Dijon and then went on to teach at the University of Strasbourg where, in 1852, he became tenured professor of chemistry.

During his studies, young Pasteur’s interest was captured by the work of several scientists whose example directed his research, among these, Claude Bernard (1813-1878) whose experiments in the field of newly born physiology allowed to better understand the interaction of organs within the human body; Jean-Baptiste Dumas (1800-1884), one of the founders of organic chemistry and scholar of chemical processes involving carbon, whose compounds create the bases for life on Earth; Justus von Liebig (1803-1873), German professor, who gave important contributions to chemistry in the realm of agriculture, designed new systems to analyze chemical substances and studied the use of artificial fertilizers.



saw him involved in the field of crystallography where he succeeded in demonstrating that substances having the same chemical composition (in this case, he was dealing with tartaric acid and racemic acid) once crystallized, could give rise to different forms identifiable through tests using polarized light. This discovery - very important for the time - gave birth to stereochemistry, the study of the spatial properties of molecules. In 1854 he was dean and professor in Lille, in the new science department, where he began to take interest in the production processes of alcoholic beverages, in particular wine and beer, often affected by contaminations that deteriorated the product, causing great damages to the French economy.

Through his research, Pasteur discovered that the transformation from sugar into alcohol that was up until then retained fruit of a mere chemical combination, actually took place due to the action of live microorganisms that aid in fermentation. Performing more in-depth studies during his experiments, he arrived at the conclusion that the important action of these germs can be controlled by bringing the temperature of the substances involved to 50-60°C for a few seconds to eliminate all the microorganisms once they have performed their action. This operation that initially met with skepticism but was later used to the great benefit of all, today bears the name of “pasteurization”: it keeps wine and beer (and later milk and other substances) from deteriorating and prolongs the product’s duration.



Since the days of Aristotle, it was retained that the simplest living organisms (worms, maggots, frogs and salamanders) could generate spontaneously from dust and mud. Some also sustained that rodents generated from wet wheat and that plant lice were born through condensation of dew drops. Half way through the 17th century, naturalist Francesco Redi (1626-1697), physician to the Medici family of Florence, performed a famous experiment to verify if spontaneous generation was indeed possible. He placed some rotting meat in a series of containers, some closed with a lid, others left open. In this way, he was able to demonstrate that the larvae were born only in those containers where flies had been able to deposit their eggs.



English Reverend John Needham (1713-1781) and Jesuit Abbot Lazzaro Spallanzani (1729-1799), tenured professor of Natural History in Pavia. In support of spontaneous generation, Needham published a book containing the results of his research. His experiment consisted in putting some meat stock in what he sustained was a hermetically sealed container and heating it by placing it on some hot cinders for a few minutes. In the days that followed, “small animals” were found in the containers left at room temperature. Biologist and science scholar George-Louis Leclerc de Buffon (1707-1788) was of his same opinion, retaining that the organic molecules deriving from the decomposition of bodies could come together spontaneously to give rise to new organisms.

Needham’s and Buffon’s theories received credit until the results of the experiments by the Italian abbot were published in France. Spallanzani retraced Needham’s experiments, initially obtaining the same results. But when he modified the heat exposure time bringing it to 45 minutes and perfected a hermetic sealing system for the containers, no signs of life could be detected in the stock. Since the issue of spontaneous generation involved not only scientific but also religious considerations that effected the ways through which human life could originally have generated, the debate - still alive today between Creationists and Evolutionists - was not appeased.



in reference to the results obtained in the study of germs. Puochet, a naturalist and professor at the University of Rouen and convinced supporter of spontaneous generation, performed an experiment in 1858: he filled a flask with boiled water, hermetically sealed it, heated it and then added oxygen and a small quantity of calcined hay. After a few days, forms of life appeared in the sealed flask through fermentation. Although admitting that he had not performed many similar tests, Pasteur retained that the appearance of ferments in Pouchet’s experiment could be caused by contaminated air introduced inside the flask. The controversy became heated and, since demonstrating that spontaneous generation was impossible entailed obtaining the consensus of the Church and of the Empire, Pasteur - fervent catholic and always looking for funds for his research - decided to focus his efforts on the issue.

The duel between the two scientists continued for six years and experiments flocked both in the laboratories and at high altitudes: Pasteur headed for the Alps and Pouchet to the Pyrenees where the containers treated with different substances were exposed to pure air at height.

The results remained on opposite sides and only after the Academy of Science directly addressed the issue and promised a reward of 2500 Francs “to whoever could shed new light on the question of so-called spontaneous generation” was an official definition of the dispute attained.



perfected after a long series of research, consisted in filling a special container (a swan-neck flask) with water, sugar and brewer’s yeast. First the container was brought to a boil and atmospheric air was allowed to enter inside it. Then, everything was calcined and the sealed container was put in an oven at 30°C. Following these operations, the liquid remained stably sterile. To demonstrate, then, that germs are present in the atmosphere, Pasteur perfected a second experiment. He filtered the atmospheric air with a wad of cotton placed on a tube connected to a suction device. Through a microscope, he verified that the cotton wad contained «corpuscles whose shape and structure resembled that of organisms». He placed the cotton wad inside a special container (without letting in air) containing the sterile liquid. The result was the detection of microbes multiplying.


ON APRIL 7TH, 1864,

before an audience typical of great occasions (Alexandre Dumas, George Sand, Princess Mathilde) and with the use of theatrical effects, Pasteur presented the results of his research at the Sorbonne: «1. the atmosphere is always accompanied by [...] organized corpuscles [that] not even the most capable naturalist will be able to distinguish [...]. These, gentlemen, are the germs of microscopic beings; 2. is possible to seed the corpuscles in suspension in the air [...] which contain an organic infusion extremely subject to alteration upon contact with the atmosphere; the air now in the vials has, by virtue of having been introduced at an extremely high temperature, been made unsuited to the appearance of microscopic beings, but by [...] exposing the infusion to the atmosphere we find, after two or three days, that it has given rise to such microorganisms. [...] What conclusion should we draw from this test, so simple to perform, but of such grave consequence for the point in question?» Realizing that the committee was decisively oriented in favor of Pasteur, Pouchet did not present his research and left his adversary free reign. Pasteur concluded his conference in the midst of applause from the audience.



and their behavior led to Pasteur’s involvement in infectious diseases. But since he was a chemist and not a physician, he was received with hostility by clinicians. Among these, Edouard Chassaigne, who stated that «the laboratory kills many animals and saves few men», and the esteemed clinician Hermann Pidoux, who did not acknowledge disease as a consequence of external factors, of microorganisms present in the air, and sustained that «disease is inside of us». Pasteur was interested in diseases that involved livestock, in particular anthrax that effected sheep, horses and lambs, limiting its diffusion (the germ responsible for the disease will later be discovered by Robert Koch in 1876); he identified and found the cure for a disease that decimated the silkworm in the South of France; he developed the road to vaccination to combat disease in chickens affected by “fowl cholera”. Retracing the work of Edward Jenner, Pasteur, in 1885, with the contribution of his assistant Emile Roux, perfected a vaccine against rabies caused by dogs.

The encouragement and the exchange of experiences with English physician Joseph Lister, inventor of the antisepsis method, served as confirmation to Pasteur on what he had always retained - that there is a close connection between microbes and disease also in man - and led to the diffusion of the practice of sterilizing medical equipment.



Pasteur’s work allowed to profoundly improve life, promoting a positive thought in knowledge, allowing mankind to free itself of many diseases that had afflicted it for centuries and to identify the invisible beings responsible for them.

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