Cohn, Ferdinand Julius (1828-1898)

German microbiologist

Ferdinand Cohn, a founder of modern microbiology, became the first to recognize and study bacteriology as a separate science. Cohn developed a system for classifying bacteria and discovered the importance of heat-resistant endospores. Additionally, Cohn recognized that both pathogens and nonpathogens could be found in drinking water and spoke of the importance of analyzing drinking water. Finally, Cohn worked with Robert Koch on the development of the etiology of the anthrax bacillus.

Cohn initially began his studies in botany at the University of Breslau in 1844. After being denied entry into the doctoral program in 1846 because of his Jewish heritage, Cohn moved to Berlin. There he completed his doctoral degree in 1847, at the age of 19, on the structure and germination of seeds.

After returning to Breslau in 1849, Cohn was presented with a top of the line microscope from his father. There he studied the cell biology of plants including the growth and division of plant cells, plasma streaming, cell differentiation, and cellular structures. In time, Cohn's studies were redirected toward algae, protozoa, fungi, and bacteria. His efforts on the developmental and sexual cycles of these microorganisms led to important advancements in cell biology.

At that time, bacteriology was an emerging field and although scientists knew that bacteria existed, they had failed to isolate bacteria in pure cultures. Scientists began to name bacteria without regard for someone else that had already observed and named the very same bacteria. Moreover, scientists believed bacteria to be a single species and that variations observed were due to different stages of development. Cohn recognized that bacteria could not be classified as a single species and developed a system for classifying them. He proposed that bacteria could be divided into groups based on whether they had similar development, chemical make-up, or descent. In 1875, he defined bacteria as "chlorophyll-less cells of characteristic shape that multiply by cross division and live as singe cells, filamentous cell chains, or cell aggregates." Eventually he extended his definition to include that "bacteria can be divided into distinct species with typical characteristics, which are transmitted to the following generations when bacteria multiply and that variations exist within each species."

After comprehensive studies of bacteria, Cohn believed that bacteria were related to algae and should thus be classified in the plant kingdom. Additionally, Cohn studied the growth of bacteria and found that in some bacteria organic substances were broken down in the presence of nitrogen. He also claimed that carbon dioxide could not be utilized as a carbon source in bacteria. It was not until 1890 when Sergei N. Winogradsky disproved this statement and discovered autotrophy.

Cohn's initial classification of bacteria consisted of four groups based on shape: Sphaerobacteria (sphere-shaped), Microbacteria (rod-shaped), Desmobacteria (filamentous), and Spirobacteria (screw-like shaped). Of those four groups the genus Micrococcus was classified as Sphaerobacteria, Bacterium was classified as Microbacteria, Bacillus and Vibrio were classified as Desmobacteria, and Spirillum and Spirochaeta were classified as Spirobacteria. Some of the genera could be further divided into subcategories.

Through the studies of Bacillus subtilis Cohn was able to disprove the earlier theory of spontaneous generation. Cohn recognized that some solutions were easily sterilized by heat, requiring only a few minutes of boiling, while other solutions required several hours of boiling. He found that still others, such as hay infusions, could not be sterilized at all. Cohn discovered heat-resistant structures called endospores, not spontaneous generation, were responsible for tainting sterilized cultures. Endospores are not killed in boiling water while the vegetative cells are. It was the heat resistant endospores from which bacteria grew, discounting the old theory of spontaneous generation.

Early on Cohn assisted in diagnosing fungal infections of crops and provided treatment options to the farmers for these plant diseases. Additionally, Cohn recognized that water sources were capable of harboring and transferring infectious diseases to humans. It was Robert Koch who first identified the pathogen that caused cholera in the drinking water; however, Cohn also analyzed the drinking water and found disease and non-disease causing microorganisms. Cohn developed a system for chemical analysis of water and claimed that drinking water should be monitored for microorganisms on a regular basis.

Later when Robert Koch was studying anthrax bacillus, Koch sought the help of Cohn. Cohn realized the importance of studying the disease causing anthrax bacillus and worked with Koch to further investigate the etiology of the bacteria. In 1875, Cohn founded the journal Beitrage zur Biologie der Pflanzen and published Koch's findings on anthrax bacillus in 1877.

See also Water quality; Cell cycle and cell division; History of microbiology