Many epidemics of viral diseases occurred before anyone understood the nature of the causative agents of those diseases
Edward Jenner (1798) published case reports of successful attempts to prevent disease (smallpox) by vaccination; these attempts were made even though Jenner did not know that the etiological agent of the disease was a virus
The word virus, which is Latin for poison, was used to describe diseases of unknown origin; filtering devices, which trapped bacteria but not viruses, were used by several scientists (Ivanowski, Beijerinck, Loeffler, Frosch, and Reed) to study a number of infectious agents; their recognition of an entity that was filterable (i.e., passed through a filter) led to the modern use of the term virus
The role of viruses in causing malignancies was established by Ellerman and Bang (1908), who showed that leukemia in chickens was caused by a filterable virus, and Peyton Rous (1911), who showed that muscle tumors in chickens were caused by a filterable virus
The existence of bacterial viruses was established by the work of Frederick Twort (1915), who first isolated bacterial viruses, and Felix díHerelle (1917), who devised a method for enumerating them and demonstrated that they could reproduce only in live bacteria
W.M. Stanley (1935) helped demonstrate the chemical nature of viruses when he crystallized the tobacco mosaic virus and showed that it was mostly composed of protein; subsequently, F. C. Bawden and N. W. Pirie (1935) separated tobacco mosaic virus particles into protein and nucleic acid components
General Properties of Viruses
They have a simple, acellular organization, consisting of one or more molecules of DNA or RNA enclosed in a coat of protein, and sometimes in more complex layers
With one known exception, virions contain either DNA or RNA, but not both
They are obligate intracellular parasites
The Cultivation of Viruses
Cultivation requires a suitable host B. Hosts for animal viruses
Suitable host animals
Embryonated eggs
Tissue (cell) cultures-monolayers of animal cells
Cell destruction can be localized if infected cells are covered with a layer of agar; the areas of localized cell destruction are called plaques
Viral growth does not always result in cell lysis to form a plaque; microscopic (or macroscopic) degenerative effects can sometimes be seen; these are referred to as cytopathic effects
Bacteriophages (viruses that infect bacteria) are usually cultivated in broth or agar cultures of suitable, young, actively growing host cells; broth cultures usually clear, while plaques form in agar cultures
Plant viruses can be cultivated in
Plant tissue cultures
Cultures of separated plant cells
Whole plants-may cause localized necrotic lesions or generalized symptoms of infection
Plant protoplast cultures
Virus Purification and Assays
Virus purification
Centrifugation of virus particles
Differential centrifugation separates according to size
Gradient centrifugation separates according to density or to sedimentation rate (size and density), and is more sensitive to small differences between various viruses
Differential precipitation with ammonium sulfate or polyethylene glycol separates viruses from other components of the mixture
Denaturation and precipitation of contaminants with heat, pH, or even organic solvents can sometimes be used
Enzymatic degradation of cellular proteins and/or nucleic acids can sometimes be used because viruses tend to be more resistant to these types of treatment
Virus assays
Particle count
Direct counts can be made with an electron microscope
Indirect counts can be made using methods such as hemagglutination (virus particles can cause red blood cells to clump together or agglutinate)
Measures of infectivity
Plaque assays involve plating dilutions of virus particles on a lawn of host cells; clear zones result from viral damage to the cells; results are expressed as plaque-forming units (PFU)
Infectious dose assays are an end point method for determining the smallest amount of virus needed to cause a measurable effect, usually on 50% of the exposed target units; results are expressed as infectious dose (ID50) or lethal dose (LD50)
The Structure of Viruses
Virion size-ranges from 10 nm to 400 nm
General Structural Properties
Nucleocapsid-the nucleic acid plus the surrounding capsid (protein coat that surrounds the genome); for some viruses this may be the whole virion; other viruses may possess additional structures
Four morphological types of capsids and virions
Icosahedral
Helical
Enveloped-having an outer membranous layer surrounding the nucleocapsid
Complex-having capsid symmetry that is neither purely icosahedral or helical
Viral capsids are constructed from many copies of one or a few types of proteins (protomers), which are assembled, together with the viral genome, by a process called self-assembly
Helical capsids-hollow tube with a protein wall shaped as a helix or spiral; may be either rigid or flexible;
Icosahedral capsids-regular polyhedron with 20 equilateral triangular faces and 12 vertices; appears spherical; constructed of capsomeres (ring or knob-shaped units), each usually made of five or six protomers
Nucleic acids
Viral genome may be either RNA or DNA, single- or double-stranded, linear or circular
DNA viruses
Most use double stranded DNA as genome
Many have one or more unusual bases (e.g., hydroxymethylcytosine instead of cytosine)
RNA Viruses-most have single-stranded RNA (ssRNA) as their genome
Plus strand viruses have a genomic RNA with the same sequence as the viral mRNA; the genomic RNA molecules may have other features (5¢ cap, poly-A tail, etc.) common to mRNA and may direct the synthesis of proteins immediately after entering the cell
Negative strand viruses have a genomic RNA complementary to the viral mRNA
Segmented genomes are those in which the virion contains more than one RNA molecule; each segment is unique and frequently encodes a single protein
Viral envelopes and enzymes
Envelopes are membrane structures surrounding some (but not all) viruses
Lipids and carbohydrates are usually derived from the host membranes
Proteins are virus specific
Many have protruding glycoprotein spikes (peplomeres)
Enzymes-some viruses have capsid-associated enzymes; many are involved in viral nucleic acid replication
Viruses with capsids of complex symmetry
Poxviruses are large (200 to 400 nm) with an ovoid exterior shape
Some bacteriophages have complex, elaborate shapes composed of heads (icosahedral symmetry) coupled to tails (helical symmetry); the structure of the tail regions are particularly variable; such viruses are said to have binal symmetry
Principles of Virus Taxonomy
In 1971, the International Committee for Taxonomy of Viruses developed a uniform classification system, which places the greatest weight on these properties:
Nucleic acid type
Nucleic acid strandedness (double or single stranded)
The sense of ssRNA genomes
The presence or absence of an envelope
The host
In addition, other characteristics (capsid symmetry, diameter of capsid or nucleocapsid, number of capsomeres in icosahedral viruses, immunological properties, gene number and genomic map, intracellular location of virus replication, presence or absence of a DNA intermediate in the replication of ssRNA viruses, type of virus release, and disease caused by the virus) can be considered
