A prodigy in Breslau
Ferdinand Julius Cohn was born on January 24, 1828, in the Jewish quarter of Breslau, the capital of Prussian Silesia, now Wrocław in Poland. He was the eldest son of Isaac Cohn, a merchant and oil manufacturer who for a time served as Austro-Hungarian consul in the city, and of Amalie Nissen. His younger brothers also made their mark: Oscar, who wrote comedies and humorous sketches under the name Oscar Justinus, and Max, a distinguished jurist who published as Max Conrat.
Ferdinand was remarkably precocious. Family tradition held that he could read at the age of two. He entered Breslau’s Maria Magdalena Gymnasium as a small child, finished school at sixteen in 1844 and went straight on to study natural science, above all botany, at the University of Breslau. His teachers there included the botanists Heinrich Göppert and Christian Nees von Esenbeck, who encouraged his gift for close observation.
Doors closed to a Jew
Cohn soon met the limits that Prussia placed on its Jewish citizens. Because he was Jewish, the University of Breslau would not allow him to take a doctorate. In 1846 he moved to the University of Berlin, where he studied with leading scientists of the day, among them the microscopist Christian Ehrenberg, a pioneer in the study of microscopic life, and he received his doctorate in 1847, at nineteen, with a dissertation on the physiology of seeds. In it he argued for botanical gardens devoted to the experimental study of how plants live, a vision he would later help to realise.
He returned to Breslau in 1848 and qualified in 1850 as a Privatdozent, an unpaid university lecturer. Even then, as the Jewish Encyclopedia records, a year passed before he was permitted to take the required oath and begin teaching, because he was a Jew. He remained loyal to Breslau for the rest of his life, rising slowly through the ranks: he became an associate professor and director of the university’s botanical museum in 1859, and full professor of botany in 1872, succeeding his teacher and friend Göppert.
Life in a drop of water
Cohn’s early fame came from the microscopic world of algae and fungi. In 1851 he published a study of the single-celled alga Protococcus pluvialis, tracing its life cycle and drawing attention to protoplasm, the living substance inside cells, as something common to plant and animal life alike. In 1855 he described sexual reproduction in the green alga Sphaeroplea, and he went on to study Volvox, the spinning colonial alga that fascinates every student with a microscope.
Through the 1860s he worked on plant physiology more broadly, and in 1866 he founded at Breslau an Institute of Plant Physiology, described as the first of its kind in the world. He was the first to classify algae firmly as plants and to define what set them apart from the green land plants. He also brought science to the public, writing popular books such as Die Pflanze (The Plant), and he served for decades in the botanical section of the Silesian Society for National Culture, which he led from 1856.
Bringing order to the bacteria
From about 1870 Cohn turned to the smallest living things then known: bacteria. At the time they were a confusion of names and guesses. Many naturalists believed that one microbe could transform into another, or that bacteria arose spontaneously from decaying matter. Cohn set out to treat them like any other group of organisms, to be observed, described and classified with care.
In 1870 he founded his own journal, Beiträge zur Biologie der Pflanzen (Contributions to the Biology of Plants), and in its pages in 1872 he published his landmark Untersuchungen über Bacterien (Researches on Bacteria). He grouped bacteria by shape into spheres, short rods, threads and spirals, a scheme that still underlies the way microbiologists describe them, and he argued that bacterial forms were stable species, not shape-shifters. The name Bacillus, now attached to one of the best-known groups of bacteria, goes back to him. He also used sterile nutrient media to grow bacteria, an essential step toward controlled laboratory study. In 1870 he identified a thread-forming bacterium, Crenothrix, in drinking-water wells, an early sign of how microbes affect public water supplies.
Just as important was his attitude. Cohn insisted that bacteria be studied as living organisms with their own life cycles, and that claims about them be tested by careful observation rather than speculation. That disciplined approach, applied to creatures that most scientists of his day dismissed as too small and too chaotic to classify, helped turn the study of microbes into a science.
Spores that survive boiling
Cohn’s most important single discovery concerned the hay bacillus, now called Bacillus subtilis. In the mid-1870s he showed that it could form tiny endospores, dormant bodies that survive temperatures which kill ordinary bacterial cells, and that the spores could later return to active growth. Britannica credits him as the first to note the heat resistance of endospores.
The discovery settled a stubborn argument. Experimenters had boiled broths, sealed them and still found bacteria growing, which seemed to support spontaneous generation. Cohn showed that the bacteria had simply survived as spores. His work, alongside that of Louis Pasteur and John Tyndall, helped bury the theory of spontaneous generation. It also had lasting practical consequences: because some spores survive boiling, safe sterilisation of surgical instruments, laboratory media and canned food requires higher temperatures, pressure or repeated heating. The sterilisation standards used in hospitals and food processing today are designed precisely to kill such spores.
The country doctor from Wollstein
In 1876 Cohn received a letter from an unknown district physician named Robert Koch, who worked in the small town of Wollstein and had been studying anthrax, a disease that killed livestock and sometimes people. Koch asked to come to Breslau and show his evidence. Cohn, then the leading German authority on bacteria, agreed. Over three days in Breslau Koch laid out his evidence that the anthrax bacillus, which could survive as spores, was the sole cause of the disease. Cohn and his colleagues were convinced.
Cohn published Koch’s paper on the cause of anthrax in his own journal in 1876, giving an obscure country doctor a platform before the scientific world, and he supported Koch’s further work. It was the first time a specific microbe had been shown to cause a specific disease, the foundation of the germ theory that would transform medicine. Koch went on to identify the causes of tuberculosis and cholera and to win the Nobel Prize. Without Cohn’s generosity and judgement, that career might have begun much later.
A Jewish citizen of Breslau
Cohn was not an observant Jew, but he never left the Jewish community. According to the Jewish Encyclopedia, he and his wife belonged to several Jewish societies. In 1867 he married Pauline Reichenbach, who after his death compiled a memorial volume of recollections, Blätter der Erinnerung (1901).
Breslau, with one of the largest Jewish communities in Germany, was proud of him. In 1888 the university opened a new Botanical Institute built largely through his efforts, and he received the title of Privy Councillor. Honours came from abroad: the Leeuwenhoek Medal of the Royal Netherlands Academy in 1885, the Linnean Medal in London in 1895 and foreign membership of the Royal Society in 1897. On his seventieth birthday in January 1898, the city of Breslau made him an honorary citizen, a distinction the historian Till van Rahden has described as making him the first Jewish honorary citizen of Breslau.
His standing in world science was reflected in a long list of memberships. He was elected to the Leopoldina, Germany’s ancient academy of natural scientists, in 1849, when he was barely twenty-one. Later came the Royal Prussian Academy of Sciences, the Société de biologie in France, the Royal Microscopic Society in Britain, the Natural History Society of Boston, the Manchester Literary and Philosophical Society, Italy’s Accademia dei Lincei and, in 1895, the French Academy of Sciences as a corresponding member. For a scientist whose career had begun with a closed door, it was a remarkable reversal.
Death in his home city
Ferdinand Cohn died in Breslau on June 25, 1898, a few months after that celebration, at the age of seventy. He had spent half a century at the university that had once refused him a doctorate. He left more than 150 papers, essays and books. He was buried in the row of honour in the Jewish cemetery of Breslau; his grave can still be found in the Old Jewish Cemetery of Wrocław, now preserved as a museum.
The city he loved changed beyond recognition after his death. Breslau’s Jewish community was destroyed in the Holocaust, and after 1945 the city became Polish Wrocław. Cohn’s grave is one of the quiet witnesses to the German-Jewish world that had produced him.
Legacy: a founder of microbiology
Cohn is remembered, with Pasteur and Koch, as one of the founders of modern bacteriology. He gave the field its first workable classification, proved that bacteria are distinct and stable living forms, discovered the heat-resistant spores that shaped all later sterilisation practice, and opened the door for Koch’s proof that germs cause disease. Each of these steps fed into the medical revolution that followed: antiseptic surgery, sterile laboratories, safe food preservation and the control of infectious disease.
German microbiologists honour him with the Ferdinand Cohn Medal, awarded by the German Society for Hygiene and Microbiology for outstanding contributions to the field, and a genus of bromeliads, Deuterocohnia, carries his name. But his deepest legacy may be the example he set: a scientist who was told, because he was a Jew, that he could not earn a degree in his own city, and who answered by building that city a world-class institute, and by lifting up a younger colleague whose discoveries would save millions of lives.
Sources & further reading
Ferdinand Cohn, Wikipedia · Ferdinand Cohn, German Wikipedia · Cohn, Ferdinand Julius, Jewish Encyclopedia (1906) · Ferdinand Julius Cohn, EBSCO Research Starters · Researched from the sources above and checked against them. Spotted an error? Tell us.
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