The Origin of Microorganisms
Thanks to his discovery of molecular asymmetry and other significant contributions, Louis Pasteur was appointed Dean of the Faculty of Sciences in Lille, France, in 1854. Shortly after, in 1857, he assumed the position of Director of Scientific Studies at the École Normale Supérieure, the same institution where he had been a student.
During the early years of his career, chemistry was, for Pasteur, a vast field in which he sought to contribute both fundamental knowledge—such as in the case of molecular chirality—and practical solutions to contemporary problems. At that time, chemistry was still a relatively new science. While its earliest ideas traced back to Aristotle, it wasn't until the 16th and 17th centuries, through the development of alchemy, that it began to take shape. Alchemy's pursuit of transforming metals into gold laid the groundwork for what would later become experimental chemistry.
Chemistry, as we know it today, began to develop in the 17th century, which is why by the 19th century, Pasteur still had many discoveries to make and old ideas to challenge.
One such idea, deeply rooted in history, was the Theory of Spontaneous Generation. Initially proposed by Aristotle, this theory was upheld for centuries by prominent thinkers like René Descartes, Francis Bacon, and Isaac Newton. It claimed that certain living beings (especially insects and small animals) could spontaneously emerge from non-living matter. For instance, it was believed that fleas arose from dust, flies or worms from rotting meat, and mice from garbage or crop residues.
Prior to Pasteur, scientists like Francesco Redi (1626-1697) and Lazzaro Spallanzani (1729-1801) had already cast doubt on this theory, even conducting experiments that undermined its validity. They demonstrated that flies laid eggs on meat, from which larvae (not spontaneously generated flies) emerged. However, Pasteur took it further by proving that the decomposition or alteration of substances, such as food, was caused by invisible microorganisms present in the air, which settled, grew, and multiplied, eventually leading to the appearance of flies or other creatures.
Thus, it was only in the mid-19th century—2,000 years after Aristotle—that Pasteur embarked on an experiment that definitively disproved the theory of spontaneous generation, proposing instead that all living things come from other living beings. This led to the famous Latin phrase used by Francesco Redi: Omne vivum ex vivo ("All life comes from life"). Pasteur particularly demonstrated this by showing the presence of germs in the air.
At that time, in the late 1850s, the scientific community was split. Some believed that certain airborne substances caused food decomposition, while others clung to the theory of spontaneous generation. In 1862, the Paris Academy of Sciences aimed to resolve the dispute by offering a prize for experiments that would determine the truth. Despite resistance from supporters of spontaneous generation—led by naturalist Félix-Archimède Pouchet—Pasteur designed an experiment to prove his hypothesis.
In his laboratory, Pasteur used two glass containers with long, curved necks, each containing boiled meat broth. The curved neck was meant to limit air contact with the liquid. After several days, Pasteur observed no changes in the broth. He then broke the neck of one container, allowing air to enter, and waited again. Soon, microorganisms appeared in the exposed broth, while the broth in the sealed container remained unchanged, proving that airborne germs, not spontaneous generation, were responsible for the growth.
With this experiment, Pasteur demonstrated that microorganisms did not appear spontaneously within the broth, but rather developed from invisible life forms present in the air, which ultimately caused the decomposition of the substance.
This discovery paved the way for Pasteur to formulate the Germ Theory of Disease, a groundbreaking concept that would later have a transformative impact, influencing fields ranging from winemaking to medicine.