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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 passesthrough a filter that blocks bacterial agents and causestobacco mosaic disease is generally recognized as theearliest distinct piece of virus research. These initialobservations date to a report in 1892 by Ivanovski and,independently, another report 6 years later by Beije-rinck, who described tobacco mosaic virus (TMV) as a“contagium vivum fluidum.” Beijerinck, in recognizingthis infectious agent as living but noncorpuscular,distinguished it from bacteria, which were consideredto be more complex in their organization.

These moments in the history of virus research, andespecially Beijerinck’s work, are widely considered thestart of virology. However, a curious paradox existshere. In 1953, the Australian microbiologist and immu-nologist Macfarlane Burnet claimed that virology didnot become an independent science until the 1950s.

Scholarly activities during the 1950s certainly makeit tempting to designate these years as the dawningperiod of virology. For instance, several journals dedi-cated to virology, including ViroZogy (1955), Advancesin Virus Research (1953), Voprosy Virusologii (1956),Acta Virologica (1957), Progress in Medical Virology(1958), and Perspectives in Virology (1959), were start-ed during that period. Moreover, the original edition ofSalvador Luria’s seminal textbook, GeneraZ Virology,was published early during that decade. Critical tothese conceptual developments was the widely ac-cepted realization that viruses replicate within hostcells during a non-infectious phase, since then knownas the “eclipse” period.

On the other hand, a quarter century earlier, therehad been a similar burst of scholarly activity, including

Ton van Helvoort, a biochemist who completed hisPh.D. in the history of science, works as a technicaltranslator and a scientific writer in The Netherlands,near Maastricht. This article is based on the author’sHistory of Microbiology Lecture presented at the ASM95th General Meeting, held May 1995 in Washington,D. C.

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publication in 1928 of the collection of essays FiLterabLeViruses, edited by Thomas Rivers; introduction in 1939of the journal Archiv fir die gesamte Virusforschung bySpringer Verlag in Vienna (continued as Archives ofVirology); and publication of more than a dozen schol-arly monographs on plant and animal viruses. Duringthis earlier period, viruses were viewed as replicatingin the same way as bacteria and other microorganismsby binary fission but differed from them by being“filterable.”

Abrupt Conceptual Shift or ProgressiveUnveiling?

There are two main arguments to put the birth ofvirology in the 1950s. First, the latter period saw theemergence of the concept of an “eclipse” of the virionduring the multiplication phase, a concept that setsviruses apart from bacteria. Second, the definition ofthe virus that developed during the latter period uni-fied studies of animal, plant, and bacterial viruses.Indeed much of the research conducted on filterableviruses between 1920 and 1950 was held together onlyloosely under the somewhat crudely developed um-brella definition that was based primarily on the filter-ability of the infectious agents.

If we consider the definition of viruses as filterableagents and the modern concept of viruses as agentswith an “eclipse” phase, one can speak of two para-digms, to use Thomas Kuhn’s terminology. In thissense, an abrupt conceptual shift or scientific revolu-tion took place in the 1950s. However, Anthony Water-son, who was a virology professor at the ~University ofLondon, wrote that the history of virus research is “thestory of the progressive unveiling of the nature of thevirus particle.“.

I have come to believe that, despite its widespreadappearance in textbooks and journals of that era, theearly concept of the “filterable virus” lacked clarity andcertainty. More importantly, I also believe that duringthe 1930s and 194Os, the links between the study of

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filterable viruses and bacteriology were so strong thatviruses were still considered merely another form ofbacteria-not conceptually distinct, as they now are.Indeed, the critical and defining point came whenbiologists realized that viruses multiply in host cells,following a biological process for replication that setsthem apart from other microorganisms.

The consensus laid to rest the dichotomy betweenthe exogenous and endogenous interpretations of virusmultiplication. According to the first interpretation, avirus was an exogenous and autonomous agent. Thisview was pitted against the idea that a virus was anendogenous product of the host cell. Many researchers,particularly those who studied bacteriophage, wereadherents of the endogenous school of thought. Mostimportantly, those who conceived of bacteriophage as aproduct of bacterial cells did not consider bacterio-phage to be viruses.

If one describes the history of virus research as theprogressive unveiling of the nature of viruses, oneignores the deep controversies in virus research duringthe first half of the 20th century. These conflicts areillustrated by the history of theories of virus multipli-cation.

Viruses Defined as Filterable, Invisible,Unculturable Agents

Soon after the first reports on TMV, publicationsappeared establishing the filterability of other infec-tious agents responsible for diseases in both plants andanimals. By 1931, nearly two dozen such agents hadbeen associated with specific diseases, including yellowfever, rabies, fowl pox, and foot-and-mouth disease incattle.

These newer filterable agents differed from bacteriain other ways. For one thing, bacteria could be ob-served directly in light microscopes or made visible bymeans of staining procedures. For another, bacteriacould be cultured on plates, forming colonies that arevisible to the unaided eye. The filterable viruses, how-ever, remained unculturable on inert media and invis-ible by staining or upon direct examination in lightmicroscopes.

Because culturing of microorganisms was consid-ered a standard technique, some early investigatorsquickly concluded that viruses must be obligatory par-asites that depend on other cells for growth. However,not all investigators shared this view. Moreover, gen-eralization was complicated because not even all kindsof bacteria could be cultured readily. Frequently, growthfactors were needed for recalcitrant bacteria, suggest-ing to some early workers that the difficult-to-cultureviruses were merely fastidious forms of small bacteriaand, with patient efforts to find appropriate growthfactors, they could be cultured in much the same wayas could other once-difficult-to-grow bacteria.

Early Technical Advances Brought AdditionalInsights

Eventually, microbiologists realized that none of

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the viruses could be grown on ordinary nutrient mediabecause they are obligate parasites that depend on hostcells to replicate. An important practical breakthroughtoward this realization came from the early studies ofErnest Goodpasture, who grew fowl pox viruses on thechorioallantoic membranes of chicken embryos. Later,Macfarlane Burnet developed techniques for usingother types of tissues and membranes as host cells forgrowing various viruses.

The other early defining criteria for viruses werealso subject to skepticism and misinterpretation. Fil-terability, for example, depended upon the techniquesand filters being used. As early as 1908, StanislausProwazek noted that “one cannot express a judgementon the nature of the virus on the basis of filtrationexperiments, as has nowadays become a dogma, be-cause every filter is subject to individual fluctuations inrelation to its tightness.” When porcelain filters werereplaced by graded collodion membranes, the perfor-mance of such systems became a good deal morereliable.

The seeming invisibility of viruses eventually fellprey to better microscopes and newer techniques. Inthe 1920s and 193Os, dark field illumination and UVmicroscopy enabled some of the larger viruses to bevisualized. For example, Joseph Barnard in Englandused UV microscopes to view several of the poxvirusesduring this period.

Also during this era, several investigators beganusing newly available ultracentrifuges to study thefilterable viruses. From such studies, Wendell Stanleyput together a chart comparing the sizes of selectedviruses and those of various bacteria and proteins. Onthe basis of such comparisons, investigators came tounderstand that viruses have discrete sizes, rangingfrom that of the smallest bacteria to two- to threefoldlarger than several proteins found in serum.

Is Bacteriophage a Virus?

The Phage Group was instrumental in makingbacteriophage the model for virus studies. In the 1950sand 196Os, members of this group helped to establishthe modern field of molecular genetics. Although bac-teriophage are now well accepted as the class of virusesthat infect ,bacteria, many investigators early on con-sidered bacteriophage to be distinct from the filterableviruses associated with diseases in plants and animals.According to one school of thought, phage were lyticproteins, or enzymes, rather than living parasites. Butearlier, during the 1920s and 193Os, the impact ofphage research reached far beyond the study of thephenomenon itself.

To Ernest Goodpasture, uncertainties about bacte-riophage raised serious questions about the fundamen-tal nature of viruses and of viral diseases. “Two inter-pretations have been offered in explanation of themultiplication of viruses. . .namely, that they are livingthings and reproduce themselves by vital activity, orthat they are inanimate substances and are reproducedthrough an interaction between themselves and the

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cells which they alter,” he wrote. Other investigatorsagreed, noting that the enzyme-like behavior of phagecast doubt on the fundamental nature of viruses, whichhad once seemed more clearly to be ultramicroscopicliving organisms.

Thomas Rivers summarizedthe confused state of affairs con-cerning viruses in an articlepublished in PhysiologicaL Re-views in 1932. In addition toproposing a mechanism for theetiology of malignancy, he pre-sented three possible mecha-nisms for the production of vi-ruses by a host cell. In the firstand second mechanisms, a stim-ulus induces a normal cell tomake a substance X. This x mayremain free or become closelybound to a part of the cell. In the

Rivers

third mechanism Rivers mentioned, x is a minuteliving organism. It enters cells, multiplies, and pro-duces disease. Rivers concluded that x in the first andsecond mechanisms was distinct from the third case. Inthe former instances, x is an inanimate agent and theproduct of cellular perversion. In the latter case, x isviewed as an autonomous organism.

Thus, in outlining these alternative processes forvirus infection of a host cell, Rivers distinguishedbetween the notions of exogenous and endogenousformation of viruses.

Virus Multiplication as an Endogenous Process

It is important to realize that the exogenous andendogenous interpretations of virus multiplicationsharply divided virus researchers into two camps. To alarge extent, this division resulted from studies ofbacteriophage. The possibility that viruses are prod-ucts of host cells was not an idea limited to thosestudying phage. Robert Doerr, one of the outstandingscientists of that period, became an influential de-fender of this notion. Perhaps all filterable viruses areproducts of host cells, he pointed out.

Doerr, whose own research focused on herpesvi-ruses, cited in a 1938 publication several observationsas consistent with the intracellular formation of vi-ruses, including (i) generation of viral diseases fromlatent virus infections but without external contactwith the infectious $gent, (ii) generation of viral dis-eases through nonspecific causes (e.g., chemical irrita-tion), (iii) serologic relationships between host andviral proteins, (iv) association of virus multiplicationwith enhanced host cell metabolism, and (v) “lifeless”viral properties that contradict those of living organ-isms and point to endogenous formation of viruses inhost cells.

Within the field of plant virus research, FrederickBawden and Bill Pirie defended the position that avirus infection could be understood best as a distur-bance of host metabolism. They criticized Wendell M.

Stanley, who claimed that viruses were nucleopro-teinaceous particles of specific, characteristic lengths.Bawden and Pirie had observed that the mean lengthof particles in a virus preparation is influenced by the“past history and present environment in the prepara-tion.” In the late 1940s they stated that, in effect, noone physicochemical method could produce the virusparticle.

Although in general members of these two distinctcamps dominated the study of viruses, other research-ers tried to reconcile the two groups. For instance,Constantin Levaditi of the Pasteur Institute in Paristried to steer a middle course between the view thatviruses are exogenous agents or that they are anendogenous product of host cells. He viewed all cells asexisting amid two competing processes, called assimi-lation and dissimilation.

According to Levaditi’s model, a virus infectioncould hijack the control center of the cell and instruct itto multiply unrestrictedly or to produce viral offspring,which would lead to death (lysis) of the cell. The notionfor virus reproduction developed during the 1950s bySalvador Luria, known as “genetic parasitism,” is verymuch related to Levaditi’s concept. However, Levaditi’scontributions have been largely neglectedature describing th.e history of virology.

in the liter-

The Modern Concept of the Virus

Andre Lwoff at the Pasteur Institute brought newinsights to these questions when he reexamined theissue of lysogeny. Lysogeny was the apparently spon-taneous formation of bacteriophage from seeminglyphage-free bacteria. This phenomenon was crucial tothose investigators who held to the view that phagewas not a virus but a product of bacterial metabolism.In 1950, Lwoff asserted that the lysogenic function istransmitted from one generation of bacteria to the nextby an endomicrobial route.

Lwoff’s most remarkable proposal was that thephage was not infectious when transmitted from onegeneration of bacteria to the next. He called this phaseof the phage life cycle the probacteriophage, or pro-phage. Only when the phage is in the prophage stagecould it live in harmony with its bacterial host, henoted. And, by some process of induction (stimulus),the prophage again became an infectious particle.

Other important observations were incorporatedinto this model of the phage life cycle. For instance,August Doermann had reported that the phage particlewent into an “eclipse” during the multiplication cycle.Moreover, nucleic acid was recognized as the carrier ofgenetic information, in large part because of the Her-shey-chase experiment.

Less well known is the work from the late 1940s ofLeslie Hoyle on the eclipse phase of the influenza virus.For decades animal viruses had been believed to beultramicrobes that, like larger bacteria, multiplied bymeans of binary fission. The work of Hoyle was crucialfor the acceptance of an eclipse phase for the animalviruses. “Before 1948 it was almost universally be-

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lieved by animal virus workers that viruses hadevolved from bacteria by increasing parasitism. . .onlyretaining the ability to multiply by some process ofgrowth and fission,” he wrote in 1968. As other inves-tigators came to accept his views of the eclipse phaseduring animal virus multiplication and, also, Doer-mann’s similar observations for bacteriophage, earlierassumptions about viral replication had to be dis-carded.

Even though much was yet to be learned about howviruses multiply, there was no longer any doubt that itwas not by binary fission. “When the controversy [overthe eclipse of influenza virus] was finally over, thestudy of viruses was no longer regarded as a branch ofbacteriology, the similarity of plant, animal, and bac-terial viruses was established, and virology had be-come a science in its own right,” wrote Hoyle.

In the late 1950s Lwoff reformulated his model forphage and prophage again, widening it to includeviruses in general. He also incorporated a refinedunderstanding of the role played by nucleic acids incarrying genetic information. His definition from 1957,stating that viruses are infectious agents made up ofnucleic acids and proteins but unable to grow autono-mously or reproduce by binary fission, has stood upwell for nearly four decades.

This definition anchors the autonomous and exoge-nous character of a virus in the continuity of its geneticmaterial, while the dependence of virus multiplicationon host cell metabolism is grounded in the takeover ofthe cell’s metabolic machinery by the genetic materialof the virus. This takeover corresponds to what Luriadescribed in 1950 as “parasitism at the genetic level.”

Around the middle of the 20th century, importanttheoretical and social changes took place in virusresearch. These changes were reflected in the publica-tion of books and the launching of several new period-icals that centered on viral research.

In 1952 Wendell M. Stanley set up the Virus Lab-oratory at the University of California, Berkeley, thatbears his name. Two years later, the Max-Planck-Institut fur Virusforschung was established in Tubin-gen, Germany. Such events established virology as an

independent discipline, a development that was basedon 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 thebeginning of the discipline of virology, the centenary ofvirology is surely not at hand. If the earlier concept offilterable virus is called the starting point, then 1998will mark the centenary of Beijerinck’s publication.

Undoubtedly, many virologists will choose 1998 tocommemorate the birth of this discipline. However,this paper indicates that researchers were engaged forhalf a century in diverging interpretations and deepcontroversies before their conflicting views about bac-teriophage, plant viruses, and animal viruses werebrought together coherently as the modern concept ofthe virus. Because dynamic processes such as contro-versy and consensus formation lie at the heart of allscientific research, it may be useful to keep this historyin mind when virology’s anniversary celebrations areunder way. cl

Suggested ReadingCairns, J., G. S. Stent, and J. D. Watson (ed.). 1966. Phage and the

origins of molecular biology. Cold Spring Harbor Laboratory, ColdSpring Harbor, N.Y.

Doerr, R., and C. Hallauer (ed.). 1938. Handbuch der Virusfor-schung- erste Halfte. Springer, Vienna.

Fenner, F., and A. Gibbs (ed.). 1988. Portraits of virology: a history ofvirology. Karger, Basel.

Grafe, A. 1991. A history of experimental virology. American Chem-ical Society, Washington, D.C.

Levaditi, C., P. Lepine, et al. 1938. Les ultravirus des maladieshumaines. 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 mosaicdisease. Stud. Hist. Philos. Sci. 22557-588.

van Helvoort, T. 1994. History of virus research in the twentiethcentury: the problem of conceptual continuity. Hist. Sci. 32:185-235.

van Helvoort, T. 1994. The construction of bacteriophage as bacterialvirus: linking endogenous and exogenous thought styles. J. Hist.Biol. 27:91-139.

Waterson, A. P., and L. Wilkinson. 1978. An introduction to thehistory of virology. Cambridge University Press, Cambridge.

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