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2 Redefining Viruses 2 Redefining viruses The development and reception of the virus concept in the Netherlands The [tobacco mosaic] infection is not caused by microbes, but by a contagium vivum fluidum. − Martinus Beijerinck (1898) Viruses are viruses. − André Lwoff (1957) In the previous chapter we considered the history of viral disease in the Netherlands in an era before anyone knew what a virus really was. Such an approach naturally begs the question as to when such a thing became known, but the answer is not straightforward at all. Much has been written on this subject, and for full expositions, we refer readers to the classic works of Sally Smith Hughes (1977) and A.P. Waterson and Lise Wilkinson (1978). In this chapter we offer but a brief tour, highlighting those aspects of the discovery and understanding of viruses that are most relevant to the history of understanding viral disease in the Netherlands. The word virus has been in use for a very long time, long before anything was known about the existence of any sort of ultramicroscopic or even microscopic, agent of infectious disease was discovered. It appears to have been imported into the English language from Latin as a word for ‘poison’ or ‘venom’ or noxious substance, sometime in the fifteenth or sixteenth centuries (Waterson and Wilkinson, 1978, p. 3). Perhaps the best-known reference is Edward Jenner’s famous exposition on the prevention of smallpox in 1798. The term ‘virus’ appears multiple times in this treatise, variously representing the ‘morbid matter’ of the disease – i.e., the material from the pustules of the pox – or the causative disease agent itself. After the advent of the germ theory in the nineteenth century the word was used rather non-specifically to denote agents of infectious disease. This idea is implicit, for example, in Louis Pasteur’s 1890 declaration: ‘En résumé, tout virus est un microbe.’ This statement, which translates as ‘In summary, every virus is a microbe,’ conveys his opinion that all infectious agents were microscopic living entities. Since that time the term ‘virus’ has undergone multiple ‘variances’ in meaning until various aspects were consolidated 68 LEEUWENHOEK’S LEGATEES AND BEIJERINCK’S BENEFICIARIES Figure 5 M.W. Beijerinck (1859-1931) M. W. Beijerinck (1859-1931) shortly before his retirement from the chair in Delft, at the age of 70 Photo reproduced from G. van Iterson, L.E. den Dooren de Jong, A.J. Kluyver (eds), Verzamelde Geschriften van M.W. Beijerinck, Zesde Deel, 1940 by the French microbiologist André Lwoff to denote an entity that was an Aristotelian natural kind in its own right: an obligate parasite of living ReI DEF NING VIRUSES 69 host cells composed of a core of a single species of nucleic acid encased in a proteinaceous coat (Lwoff, 1957; Lwoff and Tournier, 1966; Löwy, 1990, p. 88; Van Helvoort and Sankaran, 2019). The discovery of a remarkable anomaly The Netherlands can justly claim a place of pride in the foundational chapter of the history of virology – not just medical virology but the discipline in its entirety – because the first person who suggested that viruses were different from other disease agents was Dutch. In 1886, German agricultural scientist Adolf Mayer, director of the Agricultural Experimental Station in Wageningen, the Netherlands, described a new ‘mosaic’ disease of tobacco plants (TMD), which he was investigating upon the request of Dutch farmers affected by losses due to this disease. Although he was unable to find the causative agent, he believed it to be an infectious disease of bacterial cause (Mayer, 1942/1886). A few years later a Russian graduate student studying the same disease reported that it appeared to be caused by an invisible and filter- able entity, but like Mayer, was unable to isolate a causative bacterial agent (Ivanowski, 1942/1892).22 Then in 1898, the Dutch microbiologist, Martinus Willem Beijerinck, Mayer’s former colleague, by then in Delft, proposed an entirely new type of causative agent for the disease. He first presented his experiments and ideas in 1898 to the Koninklijke Nederlandse Akademie van Wetenschappen (the Royal Netherlands Academy of Sciences) – an enlarged23 version of this lecture was published in German the following year – where he announced: ‘We were dealing here with a disease which was caused by a contagium which was not a contagium fixum in the usual sense of the words’ (Beijerinck, 1961/1899, p. 154). Instead, he described the tobacco mosaic agent as a contagium vivum fluidum, a Latin phrase conveying the meaning of a living and fluid infectious agent. Beijerinck’s phrase was something of a contradiction in terms. At the time, the conception of life entailed that any being deemed living – vivum – had 22 This researcher’s name has been variously translated into English by different authors in different ways, e.g., Ivanovski, Ivanovsky, Ivanowski, and Iwanowsky. For the sake of convenience and consistency, the version used in the text here is Ivanowski, which was the spelling used in the Phytopathological Classics translation by J. Johnson, 1942. Direct quotes and titles of papers bearing his name, however, use the spelling used by the author of the publication quoted. 23 The English translation of the proceedings published by the KNAW was a verbal translation of the Dutch text. In contrast, the German version contained a translation of an enlarged text that was used by J. Johnson for the translation published in Phytopathological Classics 7 (1942). 70 LEEUWENHOEK’S LEGATEES AND BEIJERINCK’S BENEFICIARIES to be organized, which meant it was corpuscular or particulate (fixum) and therefore certainly not fluid! Although the translation for the word fluidum is fluid or liquid in English, Beijerinck used the expressions ‘liquid state’ and ‘dissolved state’ interchangeably in his original Dutch paper to describe the contagium (Kluyver, 1983, p. 119; Bos, 1999, p. 678). His ideas were heterodox for his times, but Beijerinck had experimental and theoretical back-ups for different elements of conception. For instance, he designed a series of experiments to test his hypothesis regarding the solubility of the infectious agent. First, he spread some material (extracts of crushed leaves) from diseased plants on the surface of a thick layer of agar and allowed a diffusion time of about ten days. After washing the surface of the plate, he infected healthy new plants with material extracted from the deeper layers of agar, taking great care to avoid contamination from the upper layers. By his reasoning: A virus, consisting of discrete particles, would need remain on the surface of the agar and consequently be in the impossibility of rendering the agar virulent; a virus, really dissolved in water would, on the contrary, be able to penetrate to a certain depth into the agar. (Beijerinck, 1899b, pp. 171-172) If the material from the deeper layers did indeed produce an infection identical to the original disease when injected into healthy new plants, Beijerinck felt justified in concluding that it contained a soluble, disease- producing agent, i.e. a contagium fluidum. As for the living nature of the agent, Beijerinck was quite clear. To him, the fact that a soluble agent retained its infectivity through serial transfers from a diseased plant to an unaffected one was evidence of multiplication. And, as he responded to a question from the famous botanist Hugo de Vries about the appropriateness of the adjective vivum at the Royal Academy meeting, he ‘considered the ability to reproduce to be the major characteristic of life’ (Beijerinck, 1898a; Bos, 1999, p. 678). In addition, he proposed a completely new mechanism for the multiplication of the disease agent. ‘Propagation results only when the virus is connected with the living and growing protoplasm of the host plant,’ he reported, noting too that the replication occurred only in young, dividing cells of the tobacco plant: The method of reproduction of the virus reminds one in certain ways of that of the amyloplasts and chloroplasts, which also grow only with the growing cell protoplasm, but can also exist and function independently. (Beijerinck, 1942/1898, p. 39) ReI DEF NING VIRUSES 71 Given the novelty of his ideas, it was not to be expected that Beijerinck would be allowed to present them uncontested. One of his main opponents was none other than Ivanowski, the first person who had demonstrated the filterability of the tobacco mosaic agent in 1892, but whose interpretation of the results had been quite different. His own conclusion was that the tobacco mosaic disease was ‘explained most simply by the assumption of a toxin secreted by the bacteria present, which is dissolved in the filtered sap’ (Ivanowski, 1942/1892, p. 30). His ideas were drawn from the example of the French bacteriologists Émile Roux and Alexandre Yersin, who had demonstrated that the major symptoms of diphtheria were caused by a toxin produced by the bacteria, rather than the live bacteria themselves (Roux and Yersin, 1888). Upon reading Beijerinck’s reports, Ivanowski was not only understandably piqued that the latter had made no mention of his own earlier discoveries, but also critical of the idea of a soluble contagion. He repeated the agar diffusion experiments and obtained similar results, but extended the studies by testing the infectivity of material from different stages of a fractionated filtration, due to which he maintained his belief that the agent was particulate: We see that there is not a single fact which supports the hypothesis on the soluble character of the infectious agent of mosaic disease. On the contrary, the experiment with the diffusion into agar and especially the fractionated filtration clearly indicates that we are dealing with a contagium fixum. (Ivanowski, 1903, quoted in Lechevalier, 1972, pp.
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