best generalization that can be made concerning virus classification is that it depends on analysis of a number of features, and the importance of such features may vary depending upon the use being made of the classification. A classification scheme that combines the Baltimore basis along with the nature of the host and detailed genetic characterization of critical viral proteins can generate a global view of viruses as a virosphere, such as that shown in Figure 5.5.
Figure 5.5 The virosphere. Classification of a major portion of the currently known families of viruses (‐viridae) using criteria defined by the International Committee on Taxonomy of Viruses (ICTV). Major groupings are based on the nature of the viral genome and the nature of the host.
The features of viruses discussed in this chapter provide the basis for this comprehensive classification scheme, but they are not complete – for example, diseases caused by viruses cannot be readily listed. Further, relationships between virus families will often transcend the nature of the host – this would require a third dimension to the scheme (appropriate to the concept of a sphere). Since the concept of species in biology has always been a problem, it is no surprise that relationships between viruses pose a number of specific and profound problems. For more distantly related groups, the problems grow. Still, throughout this confusion, virus families made up of related species or types are clear, and it is possible to group some major virus families into superfamilies – we will see that this can be done for the Herpesviridae and certain bacteriophages. As a rationalization, it is useful to consider virus families and larger groupings as polythetic – a group whose members always have several properties in common, although no single common attribute is present in all of its members. As a result, no single property can be used as a defining property of a polythetic group on the basis that it is universally present in all the members and absent in the members of other groups. For viruses, it is impossible to use any one discriminating character for distinguishing related groups and families, because of the inherent variability of the members.
THE HUMAN VIROME
Recently, high‐throughput sequencing (Chapter 11) coupled with metagenetic analysis (Chapter 22) have expanded our understanding of the number and types of viruses that are a normal part of the flora of the human body. The so‐called human virome is in the process of being mapped for the blood, the gut, and various other locations that are a part of human anatomy, similar to work being done with bacterial species and the human microbiome. Since the techniques that lead to an understanding of the virome will be discussed later in this book, a detailed discussion of these explorations will be covered at that time. It suffices to say that the relationship we have with our normal viruses and their importance to our physiology are only partially understood.
QUESTIONS FOR CHAPTER 5
1 One structural form used in building virus particles is based on the icosahedron. Describe, either in words or in a diagram, the organization (number of capsomers, etc.) of the simplest virus particle of this form.
2 If a virus has a negative‐sense RNA genome, what enzymatic activity (if any) will be found as part of the virion, and what will be the first step in expression of the viral genome?
3 List three properties of a virus that might be used as criteria for classification (taxonomy).
4 What is the basis of the Baltimore classification scheme?
5 What are some examples of virus structural proteins? What are some examples of proteins that have enzymatic activity included as part of a virus structure?
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