Beijerinck Discovers Viruses
In the late 19th century, significant advances were made in understanding infectious diseases, primarily through the work of scientists like Robert Koch, who established the germ theory of disease linked to bacteria. However, the nature of certain infectious agents remained elusive, particularly in the case of viral diseases. The Russian biologist Dmitri Ivanovski and Dutch scientist Martinus Beijerinck played crucial roles in this discovery, particularly concerning tobacco mosaic disease (TMD). Ivanovski demonstrated that this disease could be transmitted through a filtrate devoid of bacterial cells, but he incorrectly attributed the agent to a bacterium.
In 1898, Beijerinck expanded upon Ivanovski's work and concluded that the causative agent of TMD was neither bacterial nor a toxin, proposing that it was a "contagium vivum fluidum," a living fluid capable of reproduction only in living tissue. Beijerinck's findings established a distinction between these agents and toxins, marking a pivotal moment in the understanding of viruses. His work laid the groundwork for future research into viruses, which would later be identified as unique entities requiring living cells for replication. The significance of Beijerinck's contributions is underscored by the subsequent identification of similar filterable agents in animal diseases, leading to the recognition of viruses as a distinct form of life in scientific discourse.
Beijerinck Discovers Viruses
Date 1898
The Dutch bacteriologist Martinus W. Beijerinck demonstrated that cell-free extracts prepared from plants with tobacco mosaic disease could transmit the infection to healthy plants. The preparation contained what Beijerinck called contagium vivum fluidum, infectious material that would replicate only in living tissue, and represented the first evidence for the existence of what became known as viruses.
Locale Wageningen, the Netherlands
Key Figures
Martinus W. Beijerinck (1851-1931), Dutch bacteriologistAdolf Mayer (1843-1942), Dutch agriculturalistDmitri Ivanovski (1864-1920), Russian biologistFriedrich August Johannes Löffler (1852-1915), German professor of hygiene at Greifswald and bacteriologistPaul Frosch (1860-1928), German bacteriologistRobert Koch (1843-1910), German bacteriologist
Summary of Event
During the last decades of the nineteenth century, Robert Koch and others discovered that bacteria represented the etiological agents behind many human illnesses. This discovery became the basis for the germ theory of disease, the idea that most illnesses were caused by bacteria. The ability to grow these organisms on laboratory media played a major role in formation of Koch’s Postulates, a series of experimental steps linking a disease with a specific organism. The growing list of individual diseases found to be associated with bacterial infections gave rise to a general belief that most diseases were the result of infection by such microscopic agents.
Vaccines had been developed by the 1880’s against what we now know to be virally induced diseases, most notably against smallpox and rabies, but there was a significant reluctance to carry out infection in humans of extracts from infected tissues. As a result, the soluble nature of these agents was overlooked, and Koch’s Postulates could not be applied. The difficulty in growing many viral agents in the laboratory, as well as lack of animal models, would remain a problem in applying the postulates to viral diseases well into the twentieth century.
The first experimental transmission of a viral disease could arguably be attributed to Adolf Mayer, director of the agricultural station at Wageningen, in the Netherlands. In the 1880’s, Mayer studied tobacco mosaic disease (TMD), a name he coined, which was having a significant economic impact on tobacco growers. Mayer demonstrated that one could transmit the disease to healthy plants by spraying them with sap extracted from diseased plants. He attempted to link bacterial agents he isolated from the diseased plants with TMD by applying Koch’s Postulates, but his inability to culture any specific organism made this impossible. Despite being unable to isolate the infectious agent, Mayer found that the agent seemed to be removed from tobacco samples by the filtration process he used to look for it. He incorrectly suggested that this was an indication that the agent was a bacterium.
The Russian biologist Dmitri Ivanovski repeated and extended Mayer’s work in 1892. Mayer had used a double layer of filter paper to remove any bacteria or other cells from his preparations. Ivanovski, instead of using filter paper, prepared cell-free filtrates from diseased tobacco plants using newly developed porcelain Chamberland filter candles. He was able to transmit TMD to healthy plants even in the absence of bacteria. Ivanovski’s conclusion was that a toxin was probably associated with the disease, reflecting the recent discovery by Emil von Behring of the relationship between a toxin and human diphtheria. As late as 1903, however, Ivanovski maintained that the agent behind TMD was probably a bacterium that could not be cultured.
Martinus W. Beijerinck was probably unaware of Ivanovski’s earlier work on the nature of the TMD agent. In 1898, he was collaborating with Mayer in the study of the disease and unknowingly repeated the filtration experiments first carried out six years earlier by Ivanovski. Beijerinck’s conclusion was that the sap contained a contagium vivum fluidum, a contagious living fluid. In a more detailed analysis of the agent, Beijerinck first demonstrated that it would not grow on the culture media generally used to grow or maintain bacteria. Nor would the agent grow in the sap itself. Beijerinck concluded the agent could not be a bacterium. Furthermore, he found that the agent was capable of diffusing through agar, indicating it was a soluble substance, and that it was stable over a period of months even when dried. His work was reported in a publication later that same year.
Beijerinck also carried out studies on the development of TMD itself. He observed that the disease’s agent spread through the plant through the phloem and that it had a preference for young growing leaves. By passing the sap from plant to plant, Beijerinck demonstrated that it was capable of reproduction—unlike a toxin, which would have lost viability as it became increasingly diluted. Beijerinck’s conclusion that the TMD agent was neither bacterial in nature nor a toxin but that it required living tissue in which to reproduce set his work apart from that carried out earlier by Ivanovski. Thus, Beijerinck has correctly been given priority in the discovery of viruses.
It was shortly after the work by Beijerinck on TMD was reported that the first demonstration of a disease in animals that could be transmitted by cell-free extracts was carried out. Friedrich August Johannes Löffler, head of a Prussian research commission for the study of foot-and-mouth disease, and his collaborator Paul Frosch, a colleague of Robert Koch at Koch’s Institute of Infectious Diseases in Berlin, transmitted the disease using extracts from vesicles isolated from infected cattle. Together, Beijerinck, Löffler, and Frosch demonstrated that some diseases were associated with agents that were too small to be observed with standard microscopes and that required living tissue in which to replicate.
Significance
Beijerinck was not the first to observe that a filterable agent could serve as an etiological agent for (plant) disease. He demonstrated, however, that such an agent could not be grown in culture media, which likely meant that it was not a bacterium. Furthermore, the fact that the filterable agent could be shown to multiply eliminated the possibility of its being a toxin. Beijerinck’s definition of the contagium vivum fluidum, however, did not imply a full understanding of the nature and functioning of viruses. The modern concept of a virus required a leap in understanding that was not yet available to the science of the day.
Beijerinck’s discoveries were particularly significant in that he demonstrated that the agent required living tissue in which to reproduce. When, shortly afterward, Löffler and Frosch reported that an analogous agent was associated with foot-and-mouth disease in animals, a twenty-five-year debate began over the nature of such “viruses”: Were they particles or enzymes? This question was settled only with the independent codiscovery of bacterial viruses by Frederick Twort and Felix d’Herelle, as well as with the development of the electron microscope, which allowed viruses to be visualized.
During these same decades, filterable agents were also demonstrated to be etiological agents of human and other animal diseases, such as yellow fever, polio, rabies, and possibly even cancer. None of these “organisms” could be grown on laboratory media: It was shown that they could replicate only in the animal itself. Scientists gradually came to the conclusion that viruses represented a form of life that could not be considered as either animal or plant.
Studies of tobacco mosaic virus (TMV) have played a major role in the nascent field of virology. TMV was the first virus to be purified free from host tissue (1935). Unlike most organisms, in which deoxyribonucleic acid (DNA) was determined by the 1940’s to be the genetic material, TMV was the first agent found to contain ribonucleic acid (RNA) as a genome.
Bibliography
Dimmock, N. J., A. J. Easton, and K. N. Leppard. Introduction to Modern Virology. 5th ed. Malden, Mass.: Blackwell Science, 2001. Abbreviated discussion of the molecular biology of viruses. Introduces the subject with a review of the early history and discovery of viruses.
Fraenkel-Conrat, Heinz. “The History of Tobacco Mosaic Virus and the Evolution of Molecular Biology.” In The Plant Viruses, edited by M. H. V. van Regenmortel and Heinz Fraenkel-Conrat. New York: Plenum Press, 1986. Discusses the early history of the work carried out by Beijerinck, as well as the role played by TMV in the study of molecular biology of viruses.
Helvoort, Ton van. “When Did Virology Start?” ASM News 62 (1996): 142-145. Synopsis of the early years of virology and the changing concept of viral agents.
Knipe, David, Peter Howley, and Diane Griffin, eds. Fields’ Virology. 2 vols. New York: Lippincott Williams & Wilkins, 2001. Covers most major groups of viruses. The introductory chapter provides extensive coverage of the origin and early history of the subject.
Rott, Rudolf, and Stuart Siddell. “One Hundred Years of Animal Virology.” Journal of General Virology 79 (1998): 2871-2874. Presentation at the centenary meeting commemorating the discovery of the foot-and-mouth disease virus. Gives the early history of the discovery of filterable agents associated with plant and animal diseases.
Zaitlin, Milton. “The Discovery of the Causal Agent of the Tobacco Mosaic Disease.” In Discoveries in Plant Biology, edited by S. Kung and S. Yang. Hong Kong: World, 1998. Mayer, Ivanovski, and Beijerinck each played a role in demonstrating the association of a cell-free agent in plant disease. Places their work in the context of contemporary scientific thought.