A Neglected Visionary
Recognizing that systematics should not be totally inflexible, Ferdinand Cohn would have little patience with modern arguments over the significance of bacterial species
When I read about threats to the species concept posed by modern knowledge of microbial genomes and horizontal gene transfer, I think of Ferdinand Julius Cohn. Though he died over a century ago, the German botanist/bacteriologist would have had no difficulties in coping with pragmatic changes to the very notions which he did much to evolve. He would, I suspect, be wryly amused by today's spirited debates over the blurring of species boundaries.
The real puzzle surrounding Cohn, who was born in Breslau, Silesia, in 1828, is why his name is almost totally eclipsed in accounts of the emergence of microbiology by those of Robert Koch, Paul Ehrlich, and Louis Pasteur. He does not, for example, appear in The Microbe Hunters, in which Paul de Kruif paints vivid pen portraits of all the other pioneers. Just as perplexing, A History of Medical Bacteriology and Immunology, an apparently scholarly book by W. D. Foster (Heinemann, 1970), describes Cohn as “one of the founders of bacteriology” but then says nothing else whatever about him.
The main sources of this neglect are probably Cohn's personality and his chosen field. Unlike Pasteur, who loved theatricality (exemplified by his public demonstration of the efficacy of chicken cholera vaccine at Pouilly-le-Fort in 1881), Cohn was not a showman. Also, he worked in a discipline which many biologists and historians seem to find rather boring. He was a systematist, classifying organisms into tidy groups.
Ferdinand Cohn's principal achievement, recorded in Untersuchungen über Bacterien in 1872, was to show that bacteria could be categorized, like plants and animals, into genera and species. He demonstrated that bacilli, for example, did not transmute capriciously into cocci, nor vice versa.
The experiments he conducted to establish these facts occupied many years of intensive research at Breslau, where he was professor of botany. They went far to disentangle a mass of previous, confusing reports. Far from being of mere academic interest, his findings were vital in firmly establishing the concept of specific aetiology: particular infections are caused by particular microrganisms, which do not vary between one case and another but which can be isolated and studied in the laboratory.
We now know, of course, that bacteria and other animalcules do change their form. Mutation and what the New England Journal of Medicine recently called “genetic gymnastics” alter their characteristics, providing the raw material for evolution. Yet despite the increasing use of molecular technologies for detecting and identifying pathogens, species labels remain as valid and useful as they were in Cohn's time. Indeed, we still use the basic classification he developed.
It is difficult now to appreciate the visionary importance of Cohn's work for his own time. Hitherto, bacteriologists had meddled with mixed cultures and put forward innumerable theories to explain the alleged pleomorphism of the microbial world. Ernst Hallier, another German botanist/bacteriologist who made major contributions to the subject, was certain that vaccinia sometimes turned into a fungus, and that the micrococci responsible for enteric fever were a stage in the life history of the mold Rhizopus nigricans.
Cohn swept away such notions, and the sloppy experimental work which often went with them. This was an essential step in efforts to relate specific diseases to specific organisms. Cohn was also surprisingly modern in his approach to the classification of bacteria.
Here too, many writers have failed to appreciate his true significance. When Cohn does receive a footnote on an early page of a microbiology primer, he is usually represented as the man who grouped bacteria simply by their morphology. In fact, he did not apply this criterion rigidly or simplistically. He went out of his way to emphasize that bacteria which look alike may differ from each other in their physiology. He argued, therefore, that metabolism should be an important factor in classification.
It is these insights which persuade me that Ferdinand Cohn would have enthusiastically taken on board successive advances that have since modified our approach to systematics. Consider firstly the difficulties created for microbiologists by slavish adherence to Ernst Mayr's definition of species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” (Systematics and the Origin of Species, Cambridge University Press, 1942). I cannot imagine Cohn following those who, because this definition is strictly applicable only to sexually reproducing organisms, have reached the reductio ad absurdum that species do not exist.
Other problems have stemmed from genome mapping over the past decade, which has shown that horizontal gene transfer (between not only strains but also species) is far commoner than previously supposed. About a quarter of the Escherichia coli genome, for example, seems to have been acquired from other species. Yet again, this need not pose insuperable problems for systematists trying to fathom the relationships between different organisms. Neither does it compromise the identification of clinical isolates in hospital laboratories.
Just as Ferdinand Cohn, ahead of his time, grasped that morphology should combine with physiology in classifying organisms, so he would have welcomed the new computational methods that are beginning to resolve the taxonomic dilemmas posed by gene swapping. He would not, however, be impressed by current proposals to reject Linnaean binomials altogether in favor of so- called “PhyloCodes.”
Practical motives, seen behind Cohn's systematic work, can be discerned in many of his other contributions to microbiology. Together with Pasteur, for example, Cohn denounced the idea of spontaneous generation. But he went further, becoming the first person to demonstrate that organisms such as Bacillus subtilis can form spores which are resistant to heat and other physical agents.
He showed that many bacteria can be killed by boiling, but that spores are more resistant than vegetative forms. This was a crucial discovery in invalidating the work of Henry Bastian, primarily an English neurologist but principally remembered for his vigorous opposition to Pasteur. Bastian's apparent demonstrations of heterogenesis were actually attributable to the presence of sporing organisms in infusions supposedly sterilized by heat.
By all accounts, Ferdinand Cohn was also a nice man. It was he who encouraged Robert Koch, at the age of 33, to pursue his historic work on anthrax. Cohn even published Koch's first anthrax paper, in the journal he founded in 1876. Their friendship began during the spring of that year when Koch, then working in a primitive laboratory at home in Wollstein (a small town in Polish Prussia, where he was a district medical officer), felt that he had solved the major problems of the etiology of anthrax, but needed advice on what to do next.
“Esteemed Professor,” he wrote to Cohn, “I would be most grateful if you, as the leading authority on bacteria, would give me your criticism of my work before I submit it for publication.” Cohn received many such communications from dilettantes, and felt pessimistic about this approach by an unknown doctor from an obscure address. Nevertheless, he acceded to Koch's request to visit him in Breslau to demonstrate his experiments.
“Within the first very hour I recognized that he was an unsurpassed master of scientific research,” wrote Cohn of his reaction on that first day. Cohn invited other observers to witness subsequent demonstrations, and afterwards sent Koch home in a highly elated state. Koch's report of his experiments was complete three weeks later, and Cohn published it the following year. But he did not leave matters there. He continued to help Koch, helping him to move out of medical practice and establishing him on the scientific career for which he is now renowned.
Ferdinand Cohn deserves our admiration and gratitude for many reasons. When will someone write a full-scale biography of this innovative and prescient man?