Ferdinand Cohn and the Development of Modern Bacteriology

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Ferdinand Cohn and the Development of Modern Bacteriology


Ferdinand Julius Cohn (1828-1898) is recognized as one of the founders of modern bacteriology. He contributed to the creation of this discipline in two important ways. First, he invented a new system for classifying bacteria, which provided microbiologists with a more standardized nomenclature with which to work. Secondly, his drive to understand the life cycles of microorganisms, combined with his research into the effect of heat on bacteria, contributed to the downfall of spontaneous generation arguments. These achievements took place in the context of advances in experimental techniques and laboratory equipment.

In the nineteenth century the terms bacteriology and microbiology were used in often confusing and contradictory ways. For the purpose of this essay, the term bacteriology is limited to mean simply the study of bacteria, although it often refers to their relationship to disease or other medical questions. Microbiology is the general study of bacteria and other microbes with a focus on their morphology and physiology.


The discipline of bacteriology originated with the recognition that bacteria are organisms in their own right—that they are different from algae, fungi, and other single-celled microorganisms. This idea is central to Cohn's belief in the constancy of bacterial species and his creation of an extensive classification system for microorganisms, in which bacteria are given their own place.

He was not the first to develop a classification system of microorganisms that included bacteria. Otto Friedrich Müller (1730-1784) did so in the late eighteenth century and Christian Ehrenberg (1795-1876), later Cohn's teacher, built upon Müller's system in the early nineteenth century. Ehrenberg described several species of what we would recognize today as bacteria. Advances in microscope technology and staining methods improved the ability to observe these very small and quickly moving organisms. As a result, better descriptions, in the form of words and illustrations, appeared in the zoological and botanical literature of the nineteenth century. Cohn drew upon these in making his own system. His most important contribution to the founding of bacteriology, was in recognizing that bacteria, because of their peculiar shapes, motions, and methods of reproduction, represented separate genera and species.

He first addressed these issues in his On the Developmental History of Microscopic Algae and Fungi (1853), in which, opposing previous investigators, he placed bacteria in the plant rather than animal kingdom. Drawing upon his botanical education, he made the novel suggestion that, although they differed in color, bacteria were very similar in shape and method of reproduction to microscopic algae. His ability to draw analogies between larger (and thus easier to see) organisms, and the nearly invisible bacteria, allowed him to interpret his observations in a new way. Over the next 20 years, culminating in the 1870s with three articles entitled Untersuchungen der Bakterien (Researches on Bacteria), his influence on the scientific community grew steadily.

During this period, a debate raged over the nature of microbe species. Some contended that the many different types of organisms they observed were actually only a few species that could adopt many shapes. On the opposing side of the argument, others contended that each species was constant and thus defined by one form. Cohn supported the second group, and argued during the 1860s and 1870s for clarity in the usage of nomenclature. He, for example, criticized the French chemist Louis Pasteur (1822-1896) for using several names, such as cryptogames, animalcules, infusorien, monads, and more, for the same microorganisms.

Cohn made his classification system public in 1872. For him, bacteria were "chlorophyllless cells of spherical, oblong, or cylindrical shapes, which are now and then rotated or crooked, which multiply exclusively through transverse division and are either isolated or vegetate into colonies." He arranged the bacteria into four "tribes" based on their external characters, and each tribe contained one or more genera. The first tribe, Sphaerobacteria, were round and contained the genus Micrococcus; the second, Microbacteria, were rod-shaped and contained the genus Bacterium; Desmobacteria, the third tribe, were thread-like and contained the genera Bacillus (staff) and Vibrio (vibrating); the final tribe, Spirobacteria were coil-shaped and contained the two genera Spirillium and Spirochaete. Cohn continued to make order of the different organisms, describing many species for each genus. For example, in Micrococcus alone he placed 10 different types.

During the 1870s, microscope technology and staining methods were still quite undeveloped. This forced Cohn to build his system around morphological traits. He did, however, emphasize the necessity of using physiological characters in order to fully understand the relationships between the different species of bacteria. He challenged microscope builders to design instruments that would clear the fog away from the microscopic world he and his fellow microscopists were exploring.


The notion, promoted by Cohn and others, that bacterial species were constant, led to methods of growing pure cultures. Pasteur was using pure cultures to support his claims that different types of fermentations were caused by specific microorganisms. German physician Robert Koch (1843-1910) would later apply similar reasoning in developing the germ theory of disease, which suggests that each disease is caused by a specific bacterium. The pure culture methods coming to the fore in the 1870s required the use of sterile organic solutions (infusions), which were made from a wide variety of plants and animals. Sterilizing these infusions usually required only boiling them for five minutes; however, at times this did not prove sufficient. In an effort to discover the reasons for this, researchers such as William Roberts, John Tyndall (1820-1893), and Cohn conducted elaborate investigations on how heat effected the growth of bacteria.

Cohn, drawing on Robert's work, was the first to discover that some bacteria possess a life cycle that includes a spore stage. This life cycle included an incubation period, a stage of progressive growth, a fastigium (the period of maximum development of a disease), and a period of remission. During this last stage, the bacteria are resistant to very high and low temperatures. On a visit to England, Cohn visited Tyndall and presented him with a copy of his report on this matter. Tyndall incorporated Cohn's discovery to invent a process of discontinuous heating, which successfully killed bacteria and their spores. This process came to be known as "Tyndallization" and is still used in microbiology laboratories.

By contributing to Tyndall's work, Cohn contributed to ending any serious scientific debate concerning the origin of life by spontaneous generation. In France, by 1870, spontaneous generation had become a dead issue for most serious scientists. In England, however, the influential Henry Charlton Bastion (1837-1915) kept the issue active. Using what were considered at the time to be sound experimental procedures, he observed that bacteria would appear in turnipcheese infusions that had been sterilized by boiling and then sealed. Since boiling should have killed everything in the infusion, the bacteria must have, in Bastion's view, been produced by spontaneous generation. Through the 1870s, a series of extensive investigations on turnip-cheese were conducted by supporters of the germ theory. They strove to prove that Bastion's bacteria had somehow found its way into his infusions after they were boiled, or that his investigative procedures were faulty and the microbes had not been completely killed off in the first place.

Among those challenging Bastion were Pasteur, Tyndall, and Cohn. Throughout the 1860s Pasteur had challenged many supporters of spontaneous generation on the basis of their faulty procedures. In the next decade, joined by Roberts and Tyndall, the criticisms came from a new direction. Tyndall had shown in 1877 that five minutes of boiling was insufficient to sterilize some types of organic infusions. It was Cohn's paper on Bacillus subtilis (1876) that inspired this line of investigation for Tyndall. Although Bastion continued to argue for spontaneous generation into the first decade of the twentieth century, his claims were mostly ignored by the scientific community.

In 1872 Cohn published "Bacteria: The Smallest of Living Things," an article written for a popular audience. Here he discussed the possibility that the presence of bacteria in blood and other bodily tissues during epidemics and in cases of disease could mean that bacteria are the conveyors and originators of infection and contagion. He drew upon the work of several earlier researchers, including that of Casimar Davaine (1812-1882), who had shown that anthrax was always accompanied by the presence of the same microorganism, to build a case for the importance of bacteria. Cohn's article, with its poetic language and its exploring questions, introduced the bacteria and their still unknown, yet crucial role in all life, to a wide public audience.

Cohn contributed to bacteriology, not only through his popular literature and scientific investigations, but also by using the influence his institutional powers gave him. As director of the plant physiology institute at the University of Breslau and editor of its journal, he promoted the people and ideas that he respected. One such person was Robert Koch. Koch visited Cohn for several days in 1876, in order to demonstrate his claim that the bacteria Bacillus anthracis was the cause of the disease anthrax. Cohn, convinced of the correctness of Koch's ideas and procedures, published his work in the 1876 volume of the institute's journal. Koch's work and the students he later trained ushered a "Golden Age of Bacteriology" during the decade of the 1880s. This era of great productivity was built, in part, upon the foundations of Cohn's numerous discoveries in bacterial life cycles and the classification system he constructed for them.


Further Reading

Bulloch, William. The History of Bacteriology. London: Oxford University Press, 1960.

Cohn, Ferdinand J. Bacteria: The Smallest of Living Organisms. Baltimore, MD: Johns Hopkins University Press, 1939.

Vandervliet, Glenn. Microbiology and the Spontaneous Generation Debate During the 1870s. Kansas: Coronado Press, 1971.

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