Considered to be the father of modern bacteriology, Ferdinand Cohn (1828-1898) began his studies as a botanist and ultimately made discoveries which led to the creation of a new field of study. He was the first scientist who believed that bacteria should be classified as plants. His contributions include a system of classification and the discovery of spores, with its implication for spontaneous generation. His journal, Beitrage zur Biologie der Pflanzen, contained the first essays on modern bacteriology.
Ferdinand Julius Cohn was born in the German Jewish ghetto of Breslau, Silesia (now Wroclaw, Poland), on January 24, 1828. His father, Issak Cohn, became a successful merchant and was able to support his son's intellectual talents. A child prodigy, Cohn could read at the age of two, began school at four, and entered the Breslau Gymnasium (high school) in 1835. A hearing defect slowed his progress in school and contributed to his shyness and sensitivity as a young man. Still, in 1842, he was able to enter the University of Breslau. Influenced by professors Heinrich Goeppert and Christian Nees von Esenbeck, Cohn developed an interest in botany. Because he was Jewish, he was barred from taking the degree examinations at Breslau. He applied for an exemption from this restriction, but was refused. Cohn went to Berlin in 1846, and studied under Eilhard Mitscherlich, Karl Kunth, Johannes Muller, and Christian Ehrenberg, who introduced him to the study of microscopic animals. On November 13, 1848, at the age of nineteen, he received his doctorate in botany. His thesis dealt with the concept that each country must establish institutes for plant physiology.
In March 1848, Berlin was engulfed in a rebellion. Cohn supported the revolutionaries in spirit, although he did not actively participate. Because of his political opinions, and possibly because he was Jewish, Cohn was refused a teaching position in Berlin. He returned to Breslau in 1849 and obtained a teaching position at the University of Breslau, where he would remain for the rest of his life. He was appointed associate professor of botany in 1859 and married Pauline Reichenbach eight years later. In 1872, Cohn was became a full professor.
The mid-nineteenth century was an exciting time for botanists. Scientists such as Matthias Schleiden investigated cell theory and Hugo von Mohl described the protoplasm in a plant cell. Cohn decided to study the smallest organisms with a particular focus on protoplasm. Through his work on the unicellular algae, Protococcus pluvialis, he determined that the protoplasm in plants and the "sarcode" in animals were very similar. Cohn concluded that protoplasm contained the basic characteristics of all life. This view gained him a considerable amount of fame. Only at the end of the nineteenth century did scientists understand that protoplasm was a dynamic emulsion that could be further broken down into several different substances.
During this period Cohn, at the request of his former teacher Goeppert, did an extensive study of algae. By 1854, he had put together a work on the developmental history of microscopic algae and fungi. His conclusion, that algae and fungi belong to one class, turned out to be false. However, the section of the work which had lasting value dealt with a bacterium called Vibronia. Cohn insisted that Vibronia were plants because of their similarity to the development of algae. Vibronia had long been thought to be animals because they propelled themselves quickly by cilia or long tendrils. Cohn recognized that Vibronia were similar, yet different from fungi and algae. Also, they developed in much the same manner as algae. In an article on the unicellular algae, Sphaeroplea annulina, published in 1855, Cohn explored the sexuality of the algae, following the spermatozoa all the way to the egg.
One of Cohn's top priorities for twenty years had been to create an institute of plant physiology. In 1866, the university obtained an old building that had been a prison and allowed him to develop the first institute for plant physiology in the world. Cohn was the director of the institute from the time it opened in 1869 until his death. Using a small marine aquarium, he cultivated and studied marine plants. Here he drew much of the material for his later work. Because the red algae of the Oscillaria family could survive in primitive environments in which other plants could not, Cohn believed that they must have been the first inhabitants of earth and the first plants. This led him to the classification of lower plants. His system of classification was a pioneering attempt, though not entirely successful
In order to publicize the work of his institute, Cohn began a journal, Beitrage zur Biologie der Pflazen, in 1872. This journal contained the first essays on modern bacteriology and provided an outlet for other pioneers in the field to publish their research. In 1872, Cohn published a paper that defined bacteria as "chlorophyll-free cells of spherical oblong, or cylindrical form, sometimes twisted or bent, which multiply exclusively by transverse division and occur either isolated or in cell families." He classified bacteria into four groups, based on their constancy of external form. They included sphaerobacteria (round), microbacteria (short rods or cylinders), desmobacteria (longer rods or threads), and spirobacteria (screw or spiral). Cohn recognized six genera of bacteria, with at least one genus belonging to each group. He reiterated his conclusion of 1854 that bacteria belong to the plant kingdom because of their similarity to algae. He also suggested that there was no genetic relationship between bacteria and the fungi with which they were often grouped. This ground-breaking paper brought order to the new field of bacteriology.
Cohn studied plant nutrition and concluded that bacteria obtained their nitrogen from simple ammonia compounds, much like green plants. However, they were unable to take their carbon from carbonic acid, using carbohydrates and their derivatives instead. He also found that bacteria could be frozen without being killed. When thawed, they then returned to their former state. He also discovered that most bacteria would die if heated to 80 degrees Celsius. Cohn's conclusions were not universally accepted, and he continued to defend his research in subsequent essays published in his journal.
In 1875, Cohn published his second essay on bacteria and defended the theories outlined in his 1872 essay. New material included a long section on Bastian's experiments on turnip-cheese infusions. Bastian discovered that some bacteria survived boiling after ten minutes in a closed flask. Cohn theorized that there might be a special developmental stage or germ that survived the boiling. The bacteria that appeared after boiling in cheese infusions were not the common putrefactive bacteria, (B. terma), but rather, bacillus rods or threads, which he called Bacillus subtilis. After a short time many of them swelled at one end and became filled with oval, strongly refractive little bodies that multiplied continuously. Cohn believed that these bodies represented a stage in the life cycle of the bacilli and suggested that they were "real spores, from which new Bacilli may develop." Since it was known that spores survived high temperature, he concluded that these must also be spores that survived the boiling and then germinated to form bacteria.
Cohn's last important contribution to bacteriology was published in 1876. He proved that thermoresistant endospores in Bacillus subtilis were capable of surviving strong heat and germinating to form new bacilli. There was no spontaneous generation in this process. He also showed that the presence of air was necessary for the formation of these spores. Therefore, distinct genera of bacteria had different courses of development, different biological properties, and different fermentative activities. He also showed that spores that had already formed in heating of hay infusions of less than 100 degrees Celsius survived and retained their ability to develop even after three or four days of heating. After meeting with Robert Koch in April 1876, Cohn supported his paper on Bacillus anthracis. Koch brought his specimens and records and, for three days, showed Cohn his methods and results. Cohn immediately published what he had learned in his journal.
Although he broke no more new ground in the field of bacteriology, Cohn continued to publish monographs and treatises, lectures, and one book Die Pflanzie, in 1882. His book was very popular and contained history, biographical notes, and some poetry, as well as botany. Cohn returned to the study of plant physiology in the last fifteen years of his life. By the time of his death he had published the first three volumes of his Cryptogam-flora of Silesia. In 1887, the University of Breslau built a new institute of plant physiology in the Breslau botanical gardens. In the final years of his life, Cohn received many honors including an honorary doctorate from the faculty of medicine at the University of Tubingen. He was named a corresponding member of the Academia dei Lincei in Rome, the Institut de France in Paris, and the Royal Society of London. He also received the Leeuwenhoek Gold Medal in 1885 and the Gold Medal of the Linnean Society in 1895. Cohn died in Breslau on June 25, 1898, having made major contributions in the new field of bacteriology.
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