The Classification of Viruses

As we have noted, viruses can be classified into broad groups depending on their hosts. For instance, there are plant viruses, animal viruses, and bacterial viruses. A number of viruses infecting insects are also known and although viruses are known for fungi, protozoa, and algae, these viruses have been so little studied that no classification has been developed. In the present chapter, we discuss only the animal (primarily mammalian) and bacterial viruses, and we discuss here briefly how these two groups of viruses are classified. 

Classification of bacterial viruses In the bacterial viruses, a formal classification scheme is rarely used. Rather, each bacterial virus is designated in terms of its principal bacterial host, followed by an arbitrary alphanumeric. Thus, we speak of T4 virus of Escherichia coli or P22 virus of Salmonella typhimurium. An overview of some of the major types of bacterial viruses is given later. We should note, however, that although a bacterial virus may be designated in reference to its principal host, the actual host range of the virus may be broader. Thus, bacteriophage Mu, generally studied with Escherichia coli, also infects Citrobacter and Salmonella. 

Virus genera are designated by terms ending in - virus. Thus, among the Poxviridae those poxviruses which infect fowl are called by the genus name Avipoxvirus. Note that frequently in the animal viruses, the genus is defined based on the host which the virus infects. 

Except in a few cases, virus species have not been formally designated, but would refer to specific virus entities that have been recognized. At present, virus species are only designated by common names, such as mumps virus, poliovirus 1, and smallpox virus. For 

When contemplating the problem of virus classification, we can be truly impressed with the enormous diversity of viruses. Undoubtedly, many new viruses are awaiting discovery, although most undiscovered viruses will probably be considered members of existing virus families. 

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Virases are much simpler organisms than bacteria, and they are made from protein substances and nucleic acid. A single nucleoprotein molecule formed from molecules of nucleic acid that are chemically bound to a bulky protein molecule can be considered a simple viral particle. The protein molecule plays the role of a protective membrane. Thus the virus can be schematically described as a nucleic acid insert that is protected by a protein covering. A virus can contain either ribonucleic acid or deoxyribonucleic acid, but it never contains both of them together. The type of nucleic acid is the basis of one of the classifications of viruses. Viruses are obligatory intracellular parasites, which, upon entering a cell (i.e. after being infected) use many biochemical systems of the host cell. 

An initial approach to fullerene enumeration was based on point-group symmetry (Fowler 1986 Fowler et al. 1988) and involved an extension of Coxeter s (1971) work on icosahedral tessellations of the sphere and of methods for the classification of virus structures (Caspar Klug 1962). This approach led to magic numbers in fullerene electronic structure (Fowler Steer 1987 Fowler 1990) and will be described briefly here. 

The classification of viruses has undergone great change, as has bacterial taxonomy. Most viruses have not even been classified due to a lack of data concerning their reproduction and molecular biology. Estimates suggest that more than 30,000 viruses are being studies in laboratories and reference centers worldwide. 

Fig. 1 Classification of viruses by their genome replication strategy according to Baltimore (Baltimore 1971), Examples for important human pathogens falling into the respective class are listed above, Black DNA, gray RNA arrows to the right (+) strand polarity (i,e, corresponding to mRNA) arrows to the left (—(strands asterisk enveloped viruses...

Viruses are classified according to several criteria, including physical, biochemical, and pathogenic characteristics.18,62 The classifications of some of the more... 

An enormous number of viruses have been identified since 1892, when the Russian researcher Dmitri Ivanovski first isolated the tobacco mosaic virus. Because their origins and evolutionary history are unclear, the scientific classification of viruses has been difficult. Often, viruses have been assigned to groups according to such properties as their microscopic appearance (e.g., rhabdoviruses have a bullet-shaped appearance), the anatomic structures where they were first isolated (e.g., adenoviruses were discovered in the adenoids, a type of lymphoid tissue), or the symptoms they produce in a host organism (e.g., the herpes viruses cause rashes that spread). In recent years, scientists have attempted to develop a systematic classification system based primarily on viral structure, although several other factors are also important (e.g., host and disease caused). 

In the formation of oncogenes, depending on the action of still other viruses, or helper viruses, the end result will be either a solid tumor or a blood-related cancer such as leukania. In turn, solid tumors may be classified as carcinomas or sarcomas, depending on what kind of body tissue is involved. Furthermore, cancerous cells mimic the normal cells from which they originated, resulting in the classification of more than 100 different kinds of cancer, with some figures above 300, or even as many as 600 different kinds. 

In general, viruses are essentially made up of a nucleic acid core having either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) that provides the genetic material and also forms the basis for classification of viruses. 

It is interesting that the presence or absence of poly(c) coincides with the classification of enteroviruses, cardioviruses, rhinoviruses and PM) viruses as separate genera, proposed in (4). Frisby (l 5) has suggested that this implies a non-involvement of the poly(c) tract in a universal function such as replication, and that it might have some specific role in assembly, as a nucleation site, interacting with the capsid proteins of those viruses which contain it. The capacity of poly(C) to form double-stranded helices at pH 5 7 or below mi t contribute to the known instability of cardioviruses and FM) viruses at acid pH, which has been employed as a criterion in the classification of the picornavirus genera (4) ... 

The classification of biological risk is defined by Ordinance 3.214/78 of the Ministry of Labor and Employment, in its regulatory standards and job security (Brasil, 2002). Biohazards are represented by bacteria, fungi, parasites, viruses, among others. It is known that occupational exposure to potentially contaminated biological materials constitute a serious risk to health professionals in their workplaces. 

Barski, G., 1962, The significance of in vitro cellular lesions for classification of viruses. Virology 18 152. 

Classification of animal viruses Most of the animal viruses which have been studied in any detail have been those which have been amenable to cultivation in cell cultures. As seen, animal viruses are known with either single-stranded or doublestranded DNA or RNA. Some animal viruses are enveloped, others are naked. Size varies greatly, from those large enough to be just visible in the light microscope, to those so tiny that they are hard to see well even in the electron microscope. In the following sections, we will discuss characteristics and manner of multiplication of some of the most important and best-studied animal viruses. 

This classification is based on various criteria, including shape of the bacterium (rod, cocci) and the ability of the bacteria to be stained with a crystal violet-iodine complex in the presence of alcohol (Gram staining). This classification system also contains rickettsiae and chlamydiae both of these tend to be at the interface with viruses in that they are intracellular bacteria. 

Several methods of viral classification are in use. Classification based upon epidemiological criteria, such as enteric or respiratory viruses, is useful, but of more significance are schemes based upon the morphology of the virion (symmetry, envelope, etc.) and lype of nucleic acid (DNA, RNA, number of strands, polarity, etc.)... 

The classification in Table 14.1 shows 16 groups of animal viruses and is based on (a) the nature of the nucleic acid, (b) the structural symmetry of the virus particle, (c) the presence of an envelope, and (d) the size of the virion. The classification is simplified from that of Fraenkel-Conrat (1974) and resembles that of Wildy (1971), but details of classifications differ as precise taxonomic relationships have not been established. 

Van Etten JL (1995) Phycodnaviridae. In Murphy FA, Fauquet CM, Mayo MA, Jarvis AW, Ghabrial SA, Summers MD, Martelli GP, Bishop DHL (eds) The classification and nomenclature of viruses. Sixth Report of the International Committee on Taxonomy of Viruses Archives of Virology. Springer Verlag, Wien/ New York... 

Terms used for virus classification related to the biological of interest and the manufacturing processing are relevant viruses, specific model viruses, and nonspecific model viruses. The first refers to a virus that is likely to be present in the initial crude starting biological material, the second is a model... 

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