Louis Pasteur

Louis Pasteur (27 December 1822 – 28 September 1895) was a French chemist, microbiologist, and pioneering scientific figure whose discoveries transformed medicine, public health, and biological chemistry. His research provided key evidence for germ theory, established principles of vaccination, and produced the technique known as pasteurisation, all of which have saved millions of lives. His work also contributed significantly to stereochemistry, industrial microbiology, and the understanding of infectious disease. Pasteur is widely regarded as a founding figure in modern bacteriology, sharing this status with contemporaries such as Robert Koch, and is often honoured as the “father of microbiology”.

Early Life and Education

Pasteur was born in Dole, in the Jura region of France, into a humble Catholic family; his father was a tanner by profession. During his early upbringing the family moved to Marnoz and then to Arbois, where Pasteur completed his primary education. He was not an exceptional pupil during childhood and was known more for his interest in fishing and sketching than for academic achievement. His talent for drawing was notable, and he produced numerous pastel portraits of friends, family, and neighbours.
He attended the Collège d’Arbois for secondary schooling. In 1838 he travelled to Paris to enrol in a boarding school but soon returned home due to homesickness. Continuing his studies locally, he earned a Bachelor of Letters in 1840 before beginning advanced studies in science. After initial difficulties, including a failed examination in 1841, he successfully completed his Bachelor of Science in Mathematics in 1842, although his chemistry performance was unremarkable.
Determined to pursue higher scientific training, he twice prepared for the entrance examination of the École Normale Supérieure in Paris. After a low ranking in 1842 he deferred admission and strengthened his preparation through coursework at the Lycée Saint-Louis and lectures at the University of Paris. In 1843 he achieved a high enough rank to enter the École Normale Supérieure, where he later worked under the notable chemists Jean-Baptiste Dumas and Jean-Baptiste Boussingault. His early academic work culminated in two doctoral theses in 1847: one in chemistry focusing on arsenic acid salts, and one in physics on optical rotation in liquids.

Academic Career and Institutional Roles

Pasteur’s professional appointments began with positions in physics at Dijon and later chemistry at the University of Strasbourg. In Strasbourg he met and married Marie Laurent, with whom he had five children; only two survived to adulthood due to widespread infectious diseases of the era.
In 1854 he became dean of the newly established Faculty of Sciences at the University of Lille, where he initiated his influential studies on fermentation. During this period he coined one of his most famous statements: “In the field of observation, chance favours only the prepared mind.” His research productivity and administrative leadership led to further appointments.
From 1857 to 1867 he served as director of scientific studies at the École Normale Supérieure in Paris, instituting reforms that raised academic standards but also provoked student unrest due to strict discipline. In 1863 he accepted a professorship in geology, physics, and chemistry at the École Nationale Supérieure des Beaux-Arts. Later, in 1867, he was appointed to the chair of organic chemistry at the Sorbonne, although declining health forced him to withdraw from active teaching.
Pasteur was instrumental in establishing the laboratory of physiological chemistry at the École Normale and served as its director for over two decades. In 1887 he founded the Pasteur Institute in Paris, which became a premier centre for microbiological research and vaccine development. He remained its director until his death.

Research on Molecular Asymmetry and Chirality

One of Pasteur’s earliest and most influential scientific contributions was his work on molecular asymmetry. While studying crystallography, he discovered that solutions of sodium ammonium tartrate contained two types of crystals that were mirror images of one another and rotated polarised light in opposite directions. By manually separating these crystals and examining their optical properties, he provided the first empirical demonstration of molecular chirality. This work laid fundamental groundwork for stereochemistry and had long-term implications for structural chemistry and pharmaceuticals.
His investigations into optical activity, racemic mixtures, and the relationship between crystal structure and molecular configuration established principles that continue to underpin modern chemical science.

Disproof of Spontaneous Generation

Pasteur played a decisive role in overturning the long-held belief that microorganisms could arise spontaneously. Under the direction of the French Academy of Sciences he designed experiments using swan-necked flasks, demonstrating that sterilised nutrient broths remained free of microbial growth unless exposed to airborne contamination. This work provided compelling evidence that living organisms originate from existing life rather than from non-living matter.
For these contributions he received the prestigious Alhumbert Prize in 1862. The experiments formed a key foundation of germ theory by showing that microorganisms were responsible for processes once thought intrinsic to the substances themselves.

Fermentation, Pasteurisation, and Industrial Microbiology

Pasteur’s studies on fermentation established that microbial organisms—not chemical catalysts—were responsible for processes such as the souring of wine and the production of alcohol. He identified that specific microorganisms produced specific fermentation products and that contamination could spoil industrial production. To mitigate spoilage, he developed the process now known as pasteurisation, which involves controlled heating to destroy harmful microbes while maintaining product quality.
Pasteurisation quickly became central to the safety of milk, wine, and other beverages, and remains widely used across the world.

Germ Theory and Contributions to Medicine

Pasteur’s work provided essential scientific support for the germ theory of disease, which at the time was considered a marginal idea. Through rigorous experimentation he showed that infectious diseases could be prevented by killing or halting microbial agents. This led directly to major advances in hygiene, sterilisation, and public health, influencing clinical practice across Europe and beyond.
He later applied these principles to immunology. His development of vaccines for anthrax and rabies represented watershed moments in preventive medicine. The rabies vaccine, in particular, earned global recognition for saving the lives of countless individuals and demonstrated the potential of laboratory-based vaccine development.

Controversies and Later Reputation

Although Pasteur’s scientific contributions were monumental, later historical examination of his notebooks revealed several instances in which he selectively presented data or withheld methodological details to gain priority over rivals. These findings have led to reassessments of aspects of his scientific conduct, though they have not diminished the lasting value of his research.

Legacy and Influence

Pasteur’s influence on science and public health is immeasurable. His work underpins biotechnology, clinical microbiology, immunology, and food safety. The Pasteur Institute, founded in 1887, continues to be a leading research institution, contributing significantly to global health.