1.8 Filter systems used to characterize/purify virus particles. (A) The Berkefeld filter, invented in Germany in 1891, was a “candle”-style filter comprising diatomaceous earth (called Kieselguhr), pressed into a hollow candle shape. The white candle in the upper chamber is open at the top to receive the liquid to be filtered. The smallest pore size retained bacteria and let virus particles pass through. Such filters were probably used by Ivanovsky, Loeffler, and Frosch to isolate the first viruses. (B) Modern-day filter systems are made of disposable plastic with the upper and lower chambers separated by a biologically inert membrane, available in a variety of pore sizes. Such filtration approaches may have limited our detection of giant viruses. Image courtesy of EMD Millipore Corporation.
The discovery of the first virus, tobacco mosaic virus, is often attributed to the work of Ivanovsky in 1892. However, he did not identify the tobacco mosaic disease pathogen as a distinctive agent, nor was he convinced that its passage through bacterial filters was not the result of some technical failure. It may be more appropriate to attribute the founding of the field of virology to the astute insights of Beijerinck, Loeffler, and Frosch, who recognized the distinctive nature of the plant and animal pathogens they were studying more than 120 years ago.
The pioneering work on tobacco mosaic and foot-and-mouth disease viruses was followed by the identification of viruses associated with specific diseases in many other organisms. Important landmarks from this early period include the identification of viruses that cause leukemias or solid tumors in chickens by Vilhelm Ellerman and Olaf Bang in 1908 and Peyton Rous in 1911, respectively. The study of viruses associated with cancers in chickens, particularly Rous sarcoma virus, eventually led to an understanding of the molecular basis of cancer (Volume II, Chapter 6).
The fact that bacteria could also be hosts to viruses was first recognized by Frederick Twort in 1915 and Félix d’Hérelle in 1917. d’Hérelle named such viruses bacteriophages because of their ability to cause their bacterial host cells to rupture (a phenomenon called lysis; “phage” is derived from the Greek for “eating”). In an interesting twist of serendipity, Twort made his discovery of bacterial viruses while testing the smallpox vaccine virus to see if it would grow on simple media. He found bacterial contaminants, some of which proved to be infected by a bacteriophage. As discussed below, investigation of bacteriophages established not only the foundations for the field of molecular biology but also fundamental insights into how viruses interact with their host cells.
The Defining Properties of Viruses
Throughout the early period of virology when many viruses of plants, animals, and bacteria were cataloged, ideas about the origin and nature of these distinctive infectious agents were quite controversial. Arguments centered on whether viruses originated from parts of a cell or were built from unique components. Little progress was made toward resolving these issues and establishing the definitive properties of viruses until the development of new techniques that allowed their visualization or propagation in cultured cells.
The Structural Simplicity of Virus Particles
Dramatic confirmation of the structural simplicity of virus particles came in 1935, when Wendell Stanley obtained crystals of tobacco mosaic virus. At that time, nothing was known of the structural organization of any biologically important macromolecules, such as proteins and DNA. Indeed, the crucial role of nucleic acids as genetic material had not even been recognized. The ability to obtain an infectious agent in crystalline form, a state that was more generally associated with inorganic material, created much wonder and speculation about whether a virus is truly a life form. In retrospect, it is obvious that the relative ease with which this particular virus could be crystallized was a direct result of its structural simplicity.
The 1930s saw the introduction of the instrument that rapidly revolutionized virology: the electron microscope. The great magnifying power of this instrument (eventually more than 100,000-fold) allowed direct visualization of virus particles for the first time. It has always been an exciting experience for investigators to obtain images of viruses, especially as they appear to be remarkably elegant (Fig. 1.9). Images of many different virus particles confirmed that these agents are very small (Fig. 1.10) and that most are far simpler in structure than any cellular organism. Many appeared as regular helical or spherical particles. The description of the morphology of virus particles made possible by electron microscopy also opened the way for the first rational classification of viruses.
The Intracellular Parasitism of Viruses
Organisms as Hosts
A defining characteristic of viruses is their absolute dependence on a living host for reproduction: they are obligate parasites. Transmission of plant viruses such as tobacco mosaic virus can be achieved readily, for example, by applying extracts of an infected plant to a scratch made on the leaf of a healthy plant. Furthermore, as a single infectious particle of many plant viruses is sufficient to induce a characteristic lesion (Fig. 1.11), the concentration of the infectious agent could be measured. Plant viruses were therefore the first to be studied in detail. Some viruses of humans and other species could also be propagated in laboratory animals, and methods were developed to quantify them by determining the lethal dose. The transmission of yellow fever virus to mice by Max Theiler in 1930 was an achievement that led to the isolation of an attenuated strain, still considered one of the safest and most effective ever produced for the vaccination of humans.
After specific viruses and appropriate host organisms were identified, it became possible to produce sufficient quantities of virus particles for study of their physical and chemical properties and the consequences of infection for the host. Features such as the incubation period, symptoms of infection, and effects on specific tissues and organs were investigated. Laboratory animals remain an essential tool in investigations of the pathogenesis of viruses that cause disease. However, real progress toward understanding the mechanisms of virus reproduction was made only with the development of cell culture systems. The first and the simplest, but crucial to both virology and molecular biology, were cultures of bacterial cells.
Lessons from Bacteriophages
In the late 1930s and early 1940s, the bacteriophages, or “phages,” received increased attention as a result of controversy centering on how they might have arisen. John Northrup, a biochemist at the Rockefeller Institute in Princeton, NJ, championed the theory that a phage was a metabolic product of a bacterium. On the other hand, Max Delbrück, in his work with Emory Ellis and later with Salvador Luria, regarded phages as autonomous, stable, self-replicating entities characterized by heritable traits. According to this paradigm, phages were seen as ideal tools with which to investigate the nature of genes and heredity. Probably the most critical early contribution of Delbrück and Ellis was the perfection of the “one-step growth” method for synchronization of the reproduction of phages, an achievement that allowed analysis of a single cycle of phage reproduction in a population of bacteria. This approach introduced highly quantitative methods to virology, as well as an unprecedented rigor of analysis. The first experiments showed that phages indeed multiplied in the bacterial host and were liberated in a “burst” following disruption of the cell.
Figure 1.9 Electron micrographs of virus particles following negative staining. (A) The complex,
