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When Did Virology Start? Despite discoveries of nearly a century ago, the unifying concept undey?inning this discipline dates more recently to the 1950s TONVANHELVOORT
The discovery of an infectious agent which passes through a filter that blocks bacterial agents and causes tobacco mosaic disease is generally recognized as the earliest distinct piece of virus research. These initial observations date to a report in 1892 by Ivanovski and, independently, another report 6 years later by Beijerinck, who described tobacco mosaic virus (TMV) as a “contagium vivum fluidum.” Beijerinck, in recognizing this infectious agent as living but noncorpuscular, distinguished it from bacteria, which were considered to be more complex in their organization. These moments in the history of virus research, and especially Beijerinck’s work, are widely considered the start of virology. However, a curious paradox exists here. In 1953, the Australian microbiologist and immunologist Macfarlane Burnet claimed that virology did not become an independent science until the 1950s. Scholarly activities during the 1950s certainly make it tempting to designate these years as the dawning period of virology. For instance, several journals dedicated to virology, including ViroZogy (1955), Advances in Virus Research (1953), Voprosy Virusologii (1956), Acta Virologica (1957), Progress in Medical Virology (1958), and Perspectives in Virology (1959), were started during that period. Moreover, the original edition of Salvador Luria’s seminal textbook, GeneraZ Virology, was published early during that decade. Critical to these conceptual developments was the widely accepted realization that viruses replicate within host cells during a non-infectious phase, since then known as the “eclipse” period. On the other hand, a quarter century earlier, there had been a similar burst of scholarly activity, including Ton van Helvoort, a biochemist who completed his Ph.D. in the history of science, works as a technical translator and a scientific writer in The Netherlands, near Maastricht. This article is based on the author’s History of Microbiology Lecture presented at the ASM 95th General Meeting, held May 1995 in Washington, D. C. 142
publication in 1928 of the collection of essays FiLterabLe Viruses, edited by Thomas Rivers; introduction in 1939 of the journal Archiv fir die gesamte Virusforschung by Springer Verlag in Vienna (continued as Archives of Virology); and publication of more than a dozen scholarly monographs on plant and animal viruses. During this earlier period, viruses were viewed as replicating in the same way as bacteria and other microorganisms by binary fission but differed from them by being “filterable.” Abrupt Conceptual Shift or Progressive Unveiling? There are two main arguments to put the birth of virology in the 1950s. First, the latter period saw the emergence of the concept of an “eclipse” of the virion during the multiplication phase, a concept that sets viruses apart from bacteria. Second, the definition of the virus that developed during the latter period unified studies of animal, plant, and bacterial viruses. Indeed much of the research conducted on filterable viruses between 1920 and 1950 was held together only loosely under the somewhat crudely developed umbrella definition that was based primarily on the filterability of the infectious agents. If we consider the definition of viruses as filterable agents and the modern concept of viruses as agents with an “eclipse” phase, one can speak of two paradigms, to use Thomas Kuhn’s terminology. In this sense, an abrupt conceptual shift or scientific revolution took place in the 1950s. However, Anthony Waterson, who was a virology professor at the ~University of London, wrote that the history of virus research is “the story of the progressive unveiling of the nature of the virus particle.“. I have come to believe that, despite its widespread appearance in textbooks and journals of that era, the early concept of the “filterable virus” lacked clarity and certainty. More importantly, I also believe that during the 1930s and 194Os, the links between the study of ASM News
Features filterable viruses and bacteriology were so strong that viruses were still considered merely another form of bacteria-not conceptually distinct, as they now are. Indeed, the critical and defining point came when biologists realized that viruses multiply in host cells, following a biological process for replication that sets them apart from other microorganisms. The consensus laid to rest the dichotomy between the exogenous and endogenous interpretations of virus multiplication. According to the first interpretation, a virus was an exogenous and autonomous agent. This view was pitted against the idea that a virus was an endogenous product of the host cell. Many researchers, particularly those who studied bacteriophage, were adherents of the endogenous school of thought. Most importantly, those who conceived of bacteriophage as a product of bacterial cells did not consider bacteriophage to be viruses. If one describes the history of virus research as the progressive unveiling of the nature of viruses, one ignores the deep controversies in virus research during the first half of the 20th century. These conflicts are illustrated by the history of theories of virus multiplication. Viruses Defined as Filterable, Invisible, Unculturable Agents Soon after the first reports on TMV, publications appeared establishing the filterability of other infectious agents responsible for diseases in both plants and animals. By 1931, nearly two dozen such agents had been associated with specific diseases, including yellow fever, rabies, fowl pox, and foot-and-mouth disease in cattle. These newer filterable agents differed from bacteria in other ways. For one thing, bacteria could be observed directly in light microscopes or made visible by means of staining procedures. For another, bacteria could be cultured on plates, forming colonies that are visible to the unaided eye. The filterable viruses, however, remained unculturable on inert media and invisible by staining or upon direct examination in light microscopes. Because culturing of microorganisms was considered a standard technique, some early investigators quickly concluded that viruses must be obligatory parasites that depend on other cells for growth. However, not all investigators shared this view. Moreover, generalization was complicated because not even all kinds of bacteria could be cultured readily. Frequently, growth factors were needed for recalcitrant bacteria, suggesting to some early workers that the difficult-to-culture viruses were merely fastidious forms of small bacteria and, with patient efforts to find appropriate growth factors, they could be cultured in much the same way as could other once-difficult-to-grow bacteria. Early Technical Advances Brought Additional Insights Eventually, microbiologists realized that none of VOL. 62, NO. 3, 1996
the viruses could be grown on ordinary nutrient media because they are obligate parasites that depend on host cells to replicate. An important practical breakthrough toward this realization came from the early studies of Ernest Goodpasture, who grew fowl pox viruses on the chorioallantoic membranes of chicken embryos. Later, Macfarlane Burnet developed techniques for using other types of tissues and membranes as host cells for growing various viruses. The other early defining criteria for viruses were also subject to skepticism and misinterpretation. Filterability, for example, depended upon the techniques and filters being used. As early as 1908, Stanislaus Prowazek noted that “one cannot express a judgement on the nature of the virus on the basis of filtration experiments, as has nowadays become a dogma, because every filter is subject to individual fluctuations in relation to its tightness.� When porcelain filters were replaced by graded collodion membranes, the performance of such systems became a good deal more reliable. The seeming invisibility of viruses eventually fell prey to better microscopes and newer techniques. In the 1920s and 193Os, dark field illumination and UV microscopy enabled some of the larger viruses to be visualized. For example, Joseph Barnard in England used UV microscopes to view several of the poxviruses during this period. Also during this era, several investigators began using newly available ultracentrifuges to study the filterable viruses. From such studies, Wendell Stanley put together a chart comparing the sizes of selected viruses and those of various bacteria and proteins. On the basis of such comparisons, investigators came to understand that viruses have discrete sizes, ranging from that of the smallest bacteria to two- to threefold larger than several proteins found in serum. Is Bacteriophage a Virus? The Phage Group was instrumental in making bacteriophage the model for virus studies. In the 1950s and 196Os, members of this group helped to establish the modern field of molecular genetics. Although bacteriophage are now well accepted as the class of viruses that infect ,bacteria, many investigators early on considered bacteriophage to be distinct from the filterable viruses associated with diseases in plants and animals. According to one school of thought, phage were lytic proteins, or enzymes, rather than living parasites. But earlier, during the 1920s and 193Os, the impact of phage research reached far beyond the study of the phenomenon itself. To Ernest Goodpasture, uncertainties about bacteriophage raised serious questions about the fundamental nature of viruses and of viral diseases. “Two interpretations have been offered in explanation of the multiplication of viruses. . .namely, that they are living things and reproduce themselves by vital activity, or that they are inanimate substances and are reproduced through an interaction between themselves and the 143
Features cells which they alter,” he wrote. Other investigators agreed, noting that the enzyme-like behavior of phage cast doubt on the fundamental nature of viruses, which had once seemed more clearly to be ultramicroscopic living organisms. Thomas Rivers summarized the confused state of affairs concerning viruses in an article published in PhysiologicaL Reviews in 1932. In addition to proposing a mechanism for the etiology of malignancy, he presented three possible mechanisms for the production of viruses by a host cell. In the first and second mechanisms, a stimulus induces a normal cell to make a substance X. This x may Rivers remain free or become closely bound to a part of the cell. In the third mechanism Rivers mentioned, x is a minute living organism. It enters cells, multiplies, and produces disease. Rivers concluded that x in the first and second mechanisms was distinct from the third case. In the former instances, x is an inanimate agent and the product of cellular perversion. In the latter case, x is viewed as an autonomous organism. Thus, in outlining these alternative processes for virus infection of a host cell, Rivers distinguished between the notions of exogenous and endogenous formation of viruses. Virus Multiplication as an Endogenous Process It is important to realize that the exogenous and endogenous interpretations of virus multiplication sharply divided virus researchers into two camps. To a large extent, this division resulted from studies of bacteriophage. The possibility that viruses are products of host cells was not an idea limited to those studying phage. Robert Doerr, one of the outstanding scientists of that period, became an influential defender of this notion. Perhaps all filterable viruses are products of host cells, he pointed out. Doerr, whose own research focused on herpesviruses, cited in a 1938 publication several observations as consistent with the intracellular formation of viruses, including (i) generation of viral diseases from latent virus infections but without external contact with the infectious $gent, (ii) generation of viral diseases through nonspecific causes (e.g., chemical irritation), (iii) serologic relationships between host and viral proteins, (iv) association of virus multiplication with enhanced host cell metabolism, and (v) “lifeless” viral properties that contradict those of living organisms and point to endogenous formation of viruses in host cells. Within the field of plant virus research, Frederick Bawden and Bill Pirie defended the position that a virus infection could be understood best as a disturbance of host metabolism. They criticized Wendell M. 144
Stanley, who claimed that viruses were nucleoproteinaceous particles of specific, characteristic lengths. Bawden and Pirie had observed that the mean length of particles in a virus preparation is influenced by the “past history and present environment in the preparation.” In the late 1940s they stated that, in effect, no one physicochemical method could produce the virus particle. Although in general members of these two distinct camps dominated the study of viruses, other researchers tried to reconcile the two groups. For instance, Constantin Levaditi of the Pasteur Institute in Paris tried to steer a middle course between the view that viruses are exogenous agents or that they are an endogenous product of host cells. He viewed all cells as existing amid two competing processes, called assimilation and dissimilation. According to Levaditi’s model, a virus infection could hijack the control center of the cell and instruct it to multiply unrestrictedly or to produce viral offspring, which would lead to death (lysis) of the cell. The notion for virus reproduction developed during the 1950s by Salvador Luria, known as “genetic parasitism,” is very much related to Levaditi’s concept. However, Levaditi’s contributions have been largely neglected in the literature describing th.e history of virology. The Modern Concept of the Virus Andre Lwoff at the Pasteur Institute brought new insights to these questions when he reexamined the issue of lysogeny. Lysogeny was the apparently spontaneous formation of bacteriophage from seemingly phage-free bacteria. This phenomenon was crucial to those investigators who held to the view that phage was not a virus but a product of bacterial metabolism. In 1950, Lwoff asserted that the lysogenic function is transmitted from one generation of bacteria to the next by an endomicrobial route. Lwoff’s most remarkable proposal was that the phage was not infectious when transmitted from one generation of bacteria to the next. He called this phase of the phage life cycle the probacteriophage, or prophage. Only when the phage is in the prophage stage could it live in harmony with its bacterial host, he noted. And, by some process of induction (stimulus), the prophage again became an infectious particle. Other important observations were incorporated into this model of the phage life cycle. For instance, August Doermann had reported that the phage particle went into an “eclipse” during the multiplication cycle. Moreover, nucleic acid was recognized as the carrier of genetic information, in large part because of the Hershey-chase experiment. Less well known is the work from the late 1940s of Leslie Hoyle on the eclipse phase of the influenza virus. For decades animal viruses had been believed to be ultramicrobes that, like larger bacteria, multiplied by means of binary fission. The work of Hoyle was crucial for the acceptance of an eclipse phase for the animal viruses. “Before 1948 it was almost universally beASM News
lieved by animal virus workers that viruses had evolved from bacteria by increasing parasitism. . .only retaining the ability to multiply by some process of growth and fission,” he wrote in 1968. As other investigators came to accept his views of the eclipse phase during animal virus multiplication and, also, Doermann’s similar observations for bacteriophage, earlier assumptions about viral replication had to be discarded. Even though much was yet to be learned about how viruses multiply, there was no longer any doubt that it was not by binary fission. “When the controversy [over the eclipse of influenza virus] was finally over, the study of viruses was no longer regarded as a branch of bacteriology, the similarity of plant, animal, and bacterial viruses was established, and virology had become a science in its own right,” wrote Hoyle. In the late 1950s Lwoff reformulated his model for phage and prophage again, widening it to include viruses in general. He also incorporated a refined understanding of the role played by nucleic acids in carrying genetic information. His definition from 1957, stating that viruses are infectious agents made up of nucleic acids and proteins but unable to grow autonomously or reproduce by binary fission, has stood up well for nearly four decades. This definition anchors the autonomous and exogenous character of a virus in the continuity of its genetic material, while the dependence of virus multiplication on host cell metabolism is grounded in the takeover of the cell’s metabolic machinery by the genetic material of the virus. This takeover corresponds to what Luria described in 1950 as “parasitism at the genetic level.” Around the middle of the 20th century, important theoretical and social changes took place in virus research. These changes were reflected in the publication of books and the launching of several new periodicals that centered on viral research. In 1952 Wendell M. Stanley set up the Virus Laboratory at the University of California, Berkeley, that bears his name. Two years later, the Max-PlanckInstitut fur Virusforschung was established in Tubingen, Germany. Such events established virology as an
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independent discipline, a development that was based on a new definition of viruses formulated at this time. Is the Centenary of Virology at Hand? If we take the modern concept of the virus as the beginning of the discipline of virology, the centenary of virology is surely not at hand. If the earlier concept of filterable virus is called the starting point, then 1998 will mark the centenary of Beijerinck’s publication. Undoubtedly, many virologists will choose 1998 to commemorate the birth of this discipline. However, this paper indicates that researchers were engaged for half a century in diverging interpretations and deep controversies before their conflicting views about bacteriophage, plant viruses, and animal viruses were brought together coherently as the modern concept of the virus. Because dynamic processes such as controversy and consensus formation lie at the heart of all scientific research, it may be useful to keep this history in mind when virology’s anniversary celebrations are cl under way. Suggested Reading Cairns, J., G. S. Stent, and J. D. Watson (ed.). 1966. Phage and the origins of molecular biology. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. Doerr, R., and C. Hallauer (ed.). 1938. Handbuch der Virusforschung- erste Halfte. Springer, Vienna. Fenner, F., and A. Gibbs (ed.). 1988. Portraits of virology: a history of virology. Karger, Basel. Grafe, A. 1991. A history of experimental virology. American Chemical Society, Washington, D.C. Levaditi, C., P. Lepine, et al. 1938. Les ultravirus des maladies humaines. Maloine, Paris. Luria, S. E. 1953. General virology. Wiley, New York. Rivers, T. M. (ed.). 1928. Filterable viruses. Williams & Wilkins, \ Baltimore. van Helvoort, T. 1991. What is a virus? The case of tobacco mosaic disease. Stud. Hist. Philos. Sci. 22557-588. van Helvoort, T. 1994. History of virus research in the twentieth century: the problem of conceptual continuity. Hist. Sci. 32:185235. van Helvoort, T. 1994. The construction of bacteriophage as bacterial virus: linking endogenous and exogenous thought styles. J. Hist. Biol. 27:91-139. Waterson, A. P., and L. Wilkinson. 1978. An introduction to the history of virology. Cambridge University Press, Cambridge.
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