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SECTION IV VIROLOGY 29 C H...

SECTION IV VIROLOGY 29 C H A P T E R General Properties of Viruses Viruses are the smallest infectious agents (ranging from TERMS AND DEFINITIONS IN VIROLOGY about 20 to 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as their genome. The nucleic Schematic diagrams of viruses with icosahedral and helical acid is encased in a protein shell, which may be surrounded symmetry are shown in Figure 29-1. Indicated viral compo- by a lipid-containing membrane. The entire infectious nents are described below. unit is termed a virion. Viruses are parasites at the genetic Capsid: The protein shell, or coat, that encloses the level, replicating only in living cells and are inert in the nucleic acid genome. extracellular environment. The viral nucleic acid contains Capsomeres: Morphologic units seen in the electron information necessary to cause the infected host cell to microscope on the surface of icosahedral virus particles. synthesize virus-specific macromolecules required for the Capsomeres represent clusters of polypeptides, but the mor- production of viral progeny. During the replicative cycle, phologic units do not necessarily correspond to the chemi- numerous copies of viral nucleic acid and coat proteins cally defined structural units. are produced. The coat proteins assemble together to form Defective virus: A virus particle that is functionally the capsid, which encases and stabilizes the viral nucleic deficient in some aspect of replication. acid against the extracellular environment and facilitates Envelope: A lipid-containing membrane that surrounds the attachment and penetration by the virus upon contact some virus particles. It is acquired during viral maturation with new susceptible cells. The virus infection may have by a budding process through a cellular membrane (see little or no effect on the host cell or may result in cell dam- Figure 29-3). Virus-encoded glycoproteins are exposed on age or death. the surface of the envelope. These projections are called The spectrum of viruses is rich in diversity. Viruses vary peplomers. greatly in structure, genome organization and expression, Nucleocapsid: The protein–nucleic acid complex repre- and strategies of replication and transmission. The host range senting the packaged form of the viral genome. The term is for a given virus may be broad or extremely limited. Viruses commonly used in cases in which the nucleocapsid is a sub- are known to infect unicellular organisms, such as mycoplas- structure of a more complex virus particle. mas, bacteria, and algae, and all higher plants and animals. Structural units: The basic protein building blocks of General effects of viral infection on the host are considered the coat. They are usually a collection of more than one non- in Chapter 30. identical protein subunit. The structural unit is often referred Much information on virus–host relationships has been to as a protomer. obtained from studies on bacteriophages, the viruses that Subunit: A single folded viral polypeptide chain. attack bacteria. This subject is discussed in Chapter 7. Proper- Virion: The complete virus particle. In some instances ties of individual viruses are discussed in Chapters 31–44. (eg, papillomaviruses and picornaviruses), the virion is 413 Riedel_CH29_p413-p436.indd 413 05/04/19 4:47 PM  414   SECTION IV  Virology Capsomere CLASSIFICATION OF VIRUSES Basis of Classification Nucleic acid core The following properties have been used as a basis for the Nucleocapsid Core classification of viruses. The amount of information avail- Capsid able in each category is not the same for all viruses. Genome sequencing is now often performed early in virus identifi- Envelope cation, and comparisons with databases provide detailed A information on the viral classification, predicted protein composition, and taxonomic relatedness to other viruses. Matrix protein 1. Virion morphology, including size, shape, type of sym- Nucleocapsid metry, presence or absence of peplomers, and presence or absence of membranes. 2. Virus genome properties, including type of nucleic acid Lipid envelope (DNA or RNA), size of the genome, strandedness (single or double), whether linear or circular, sense (positive, negative, ambisense), segments (number, size), nucleotide Glycoprotein spikes sequence, percent GC content, and presence of special fea- B tures (repetitive elements, isomerization, 5′-terminal cap, FIGURE 29-1 Schematic diagram illustrating the components 5′-terminal covalently linked protein, 3′-terminal poly(A) of the complete virus particle (the virion). A: Enveloped virus with tract). icosahedral symmetry. Not all icosahedral viruses have envelopes. 3. Genome organization and replication, including gene B: Virus with helical symmetry. order, number and position of open reading frames, strat- egy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assem- identical with the nucleocapsid. In more complex virions bly, virion release). (herpesviruses, orthomyxoviruses), this includes the nucleo- 4. Virus protein properties, including number, size, amino capsid plus a surrounding envelope. This structure, the acid sequence, modifications (glycosylation, phosphory- virion, serves to transfer the viral nucleic acid from one cell lation, myristoylation), and functional activities of struc- to another. tural and nonstructural proteins (transcriptase, reverse transcriptase, neuraminidase, fusion activities). 5. Antigenic properties, particularly reactions to various EVOLUTIONARY ORIGIN OF VIRUSES antisera. 6. Physicochemical properties of the virion, including molec- The origin of viruses is not known. There are profound differ- ular mass, buoyant density, pH stability, thermal stability, ences among the DNA viruses, the RNA viruses, and viruses and susceptibility to physical and chemical agents, espe- that use both DNA and RNA as their genetic material during cially solubilizing agents and detergents. different stages of their life cycle. It is possible that different 7. Biologic properties, including natural host range, mode types of agents are of different origins. Two theories of viral of transmission, vector relationships, pathogenicity, tissue origin can be summarized as follows: tropisms, and pathology. 1. Viruses may be derived from DNA or RNA nucleic acid components of host cells that became able to replicate autonomously and evolve independently. They resemble Universal System of Virus Taxonomy genes that have acquired the capacity to exist indepen- A system has been established in which viruses are separated dently of the cell. Some viral sequences are related to into major groupings—called families—on the basis of virion portions of cellular genes encoding protein functional morphology, genome structure, and strategies of replication. domains. It seems likely that at least some viruses evolved Virus family names have the suffix -viridae. Table 29-1 sets in this fashion. forth a convenient scheme used for classification. Diagrams 2. Viruses may be degenerate forms of intracellular parasites. of animal virus families are shown in Figure 29-2. There is no evidence that viruses evolved from bacteria, Within each family, subdivisions called genera are usually although other obligately intracellular organisms (eg, rick- based on biological, genomic, physicochemical, or serologic ettsiae and chlamydiae) presumably did so. However, pox- differences. Criteria used to define genera vary from family to viruses are so large and complex that they might represent family. Genus names carry the suffix -virus. In several families evolutionary products of some cellular ancestor. (Herpesviridae, Paramyxoviridae, Parvoviridae, Poxviridae, Riedel_CH29_p413-p436.indd 414 05/04/19 4:47 PM  CHAPTER 29 General Properties of Viruses   415 TABLE 29-1 Families of Animal Viruses That Contain Members Able to Infect Humans Virus Size of Nucleic Virion: Particle Nucleic Acid Physical Type Acid Capsid Enveloped Ether Number of Size in Virion of Nucleic Core Symmetry or Naked Sensitivity Capsomeres (nm)a (kb/kbp) Acidb Virus Family DNA Icosahedral Naked Resistant 32 18–26 5.6 ss Parvoviridae 12 30 2.0–3.9 ss circular Anelloviridae 72 45 5 ds circular Polyomaviridae 72 55 8 ds circular Papillomaviridae 252 70–90 26–45 ds Adenoviridae Enveloped Sensitive 180 40–48 3.2 ds circular c Hepadnaviridae 162 150–200 125–240 ds Herpesviridae Complex Complex Resistant d 230 × 400 130–375 ds Poxviridae coats RNA Icosahedral Naked Resistant 32 28–30 7.2–8.4 ss Picornaviridae 32 28–30 6.4–7.4 ss Astroviridae 32 27–40 7.4–8.3 ss Caliciviridae 180 27–34 7.2 ss Hepeviridae 12 35–40 4 ds segmented Picornaviridae 32 60–80 16–27 ds segmented Reoviridae Enveloped Sensitive 42 50–70 9.7–11.8 ss Togaviridae Unknown Enveloped Sensitive 40–60 9.5–12.5 ss segmented Flaviviridae or complex 50–300 10–14 ss Arenaviridae 120–160 27–32 ss Coronaviridae 80–110 7–11 e ss diploid Retroviridae Helical Enveloped Sensitive 80–120 10–13.6 ss segmented Orthomyxoviridae 80–120 11–21 ss segmented Bunyaviridae 80–125 8.5–10.5 ss Bornaviridae 75 × 180 13–16 ss Rhabdoviridae 150–300 16–20 ss Paramyxoviridae 80 × 1000 f 19.1 ss Filoviridae a Diameter, or diameter × length. b ds, double stranded; ss, single stranded. c The negative-sense strand has a constant length of 3.2 kb; the other varies in length, leaving a large single-stranded gap. d The genus Orthopoxvirus, which includes the better-studied poxviruses (eg, vaccinia), is ether resistant; some of the poxviruses belonging to other genera are ether sensitive. e Size of monomer. f Filamentous forms vary greatly in length. Reoviridae, Retroviridae), a larger grouping called subfamilies of 2017, the International Committee on Taxonomy of Viruses has been defined, reflecting the complexity of relationships had organized more than 4400 virus species into 122 families among member viruses. Virus orders may be used to group and 735 genera. virus families that share common characteristics. For exam- Properties of the major families of animal viruses that ple, order Mononegavirales encompasses the Bornaviridae, contain members important in human disease are summa- Filoviridae, Paramyxoviridae, and Rhabdoviridae families. As rized in Table 29-1. They are discussed briefly below in the Riedel_CH29_p413-p436.indd 415 05/04/19 4:47 PM  416   SECTION IV  Virology DNA viruses dsDNA ssDNA Parvoviridae Iridoviridae Asfarviridae Poxviridae dsDNA (RT) Polyomaviridae Circoviridae Hepadnaviridae Herpesviridae Papillomaviridae Adenoviridae RNA viruses dsRNA ssRNA (–) ssRNA (RT) Orthomyxoviridae Rhabdoviridae Retroviridae Reoviridae Bornaviridae Arenaviridae Paramyxoviridae Bunyaviridae Filoviridae Picobirnaviridae ssRNA (+) Hepeviridae Caliciviridae Astroviridae Picornaviridae 100 nm Coronaviridae Arteriviridae Togaviridae Flaviviridae FIGURE 29-2 Shapes and relative sizes of animal viruses of families that infect vertebrates. In some diagrams, certain internal structures of the particles are represented. Only those families that include human pathogens are listed in Table 29-1 and described in the text. (Reproduced with permission from van Regenmortel MHV, Fauquet CM, Bishop DHL, et al [editors]: Virus Taxonomy: Classification and Nomenclature of Viruses. Seventh Report of the International Committee on Taxonomy of Viruses. Academic Press, 2000.) Riedel_CH29_p413-p436.indd 416 05/04/19 4:47 PM  CHAPTER 29 General Properties of Viruses   417 order shown in Table 29-1 and are considered in greater detail cubic symmetry, with fiber spikes protruding from cap- in the chapters that follow. somers that aid in host attachment. The genome is linear, double-stranded DNA, 26–48 kb in size. Replication occurs in the nucleus. Complex splicing patterns produce mRNAs. Survey of DNA-Containing Virus Families At least 67 types infect humans, especially in mucous mem- branes, and some types can persist in lymphoid tissue. Ade- A. Parvoviridae noviruses can cause acute respiratory diseases, conjunctivitis, Parvoviruses (from Latin parvus meaning small) are very and gastroenteritis. Some human adenoviruses can induce small viruses with a particle size of about 18–26 nm. The par- tumors in newborn hamsters. Many serotypes infect animals ticles have cubic symmetry, with 32 capsomeres, but they have (see Chapters 32 and 43). no envelope. The genome is linear, single-stranded DNA, aver- aging 5 kb in size. Replication occurs only in actively dividing F. Hepadnaviridae cells; capsid assembly takes place in the nucleus of the infected cell. Human parvovirus B19 replicates in immature erythroid Hepadnaviruses (from Latin hepa meaning liver) are small cells and causes several adverse consequences, including aplas- (40–48 nm), enveloped viruses containing circular, partially tic crisis, fifth disease, and fetal death (see Chapter 31). double-stranded DNA molecules that are about 3.2 kbp in size. Replication involves repair of the single-stranded gap in the DNA, transcription of RNA, and reverse transcrip- B. Anelloviridae tion of the RNA to make genomic DNA. The virus consists Anelloviruses (from Latin anello meaning ring) are small of a 27-nm icosahedral nucleocapsid core within a closely (~30 nm in diameter), icosahedral virions that lack an adherent envelope that contains lipid and the viral surface envelope. The viral genome is negative sense, circular, antigen. The surface protein is characteristically overpro- single-stranded DNA, 2–4 kb in size. Anelloviruses include duced during replication of the virus, which takes place in the torque teno viruses, and are globally distributed in the the liver, and is shed into the bloodstream. Hepadnaviruses human population and many animal species. No specific dis- such as Hepatitis B virus can cause acute and chronic hepa- ease associations have been proven. titis; persistent infections are associated with a high risk of developing liver cancer. Viral types are known that infect C. Polyomaviridae mammals and ducks (see Chapter 35). Polyomaviruses are small (45 nm), nonenveloped, heat-stable, solubilization-resistant viruses exhibiting cubic symmetry, G. Herpesviridae with 72 capsomeres. The name derives from Greek poly- Herpesviruses are family of large viruses 150–200 nm in (many) and –oma (tumor) and refers to the ability of some diameter. The name refers to Latin herpes (creep), describ- of these viruses to produce tumors in infected hosts. The ing the spreading nature of skin lesions caused by these genome is circular, double-stranded DNA, about 5 kb in size. viruses. The nucleocapsid is 100 nm in diameter, with cubic These agents have a slow growth cycle, stimulate cell DNA symmetry and 162 capsomeres, surrounded by a lipid- synthesis, and replicate within the nucleus. The most well- containing envelope. The genome is linear, double-stranded known human polyomaviruses are JC virus, the causative DNA, 120–240 kb in size. Latent infections may last for the agent of progressive multifocal leukoencephalopathy; BK life span of the host, usually in ganglial or lymphoblastoid virus, associated with nephropathy in transplant recipients; cells. Human herpesviruses include herpes simplex types 1 and Merkel cell virus, found associated with the majority of and 2 (oral and genital lesions), varicella-zoster virus (chick- Merkel cell skin carcinomas. SV40, a primate virus, can also enpox and shingles), cytomegalovirus, Epstein-Barr virus infect humans. Most animal species harbor chronic infec- (infectious mononucleosis and association with human neo- tions with one or more polyomaviruses (see Chapter 43). plasms), human herpesviruses 6 and 7 (T cell lymphotropic), and human herpesvirus 8 (associated with Kaposi sarcoma). D. Papillomaviridae Other herpesviruses occur in many animals (see Chapters 33 Papillomaviruses are similar to polyomaviruses in some and 43). respects but with a larger genome (8 kb) and particle size (55–60 nm). The name refers to Latin papilla (nipple) and H. Poxviridae Greek –oma (tumor) and describes wart-like lesions produced Poxviruses are large brick-shaped or ovoid viruses by these viral infections. There are many types of human 220–450 nm long × 140–260 nm wide × 140–260 nm thick. papillomaviruses, and certain high-risk types are causative The particle structure is complex, with a lipid-containing agents of genital cancers in humans (see Chapter 43). envelope. The name derives from Anglo-Saxon pokkes mean- ing pouch, referring to their characteristic vesicular skin E. Adenoviridae lesions. The genome is linear, covalently closed, double- Adenoviruses (from Latin adenos meaning gland) are stranded DNA, 130–375 kb in size. Poxvirus particles con- medium-sized (70–90 nm), nonenveloped viruses exhibiting tain about 100 proteins, including many with enzymatic Riedel_CH29_p413-p436.indd 417 05/04/19 4:47 PM  418   SECTION IV  Virology activities, such as a DNA-dependent RNA polymerase. have two or three protein shells with channels extending from Replication occurs entirely within the cell cytoplasm. Some the surface to the core; short spikes extend from the virion are pathogenic for humans (smallpox, vaccinia, molluscum surface. The genome is linear, double-stranded, segmented contagiosum); others that are pathogenic for animals can RNA (10–12 segments), totaling 18–30 kbp in size. Individual infect humans (cowpox, monkeypox) (see Chapter 34). RNA segments range in size from 200 to 3000 bp. Replica- tion occurs in the cytoplasm; genome segment reassortment occurs readily. Reoviruses of humans include rotaviruses, Survey of RNA-Containing Viruses which have a distinctive wheel-shaped appearance and cause A. Picornaviridae gastroenteritis. Antigenically similar reoviruses infect many Picornaviruses are small (28–30 nm), ether-resistant viruses animals. The genus Coltivirus includes Colorado tick fever exhibiting cubic symmetry. The RNA genome is single virus of humans. (See Chapter 37.) stranded and positive sense (ie, it can serve as an mRNA) and is 7.2–8.4 kb in size. The groups infecting humans are entero- G. Arboviruses and Rodent-Borne Viruses viruses (polioviruses, coxsackieviruses, echoviruses, parecho- Arboviruses and rodent-borne viruses are ecologic group- viruses, and rhinoviruses [more than 100 serotypes causing ings (not a virus family) of viruses with diverse physical and common colds]) and hepatovirus (hepatitis A). Rhinoviruses chemical properties. The arboviruses (there are more than are acid labile and have a high density; other enteroviruses are 350 of them) have a complex cycle involving arthropods as generally acid stable and have a lower density. Picornaviruses vectors that transmit the viruses to vertebrate hosts by their infecting animals include foot-and-mouth disease of cattle and bite. Viral replication does not seem to harm the infected encephalomyocarditis of rodents (see Chapter 36). arthropod. Arboviruses infect humans, mammals, birds, and reptiles and use mosquitoes and ticks as vectors. Human B. Astroviridae pathogens include dengue, yellow fever, West Nile fever, and Astroviruses are similar in size to picornaviruses (28–30 nm), encephalitis viruses. Rodent-borne viruses establish persis- but particles display a distinctive star-shaped outline on tent infections in rodents and are transmitted without an their surfaces. The genome is linear, positive-sense, single- arthropod vector. Human diseases include hantavirus infec- stranded RNA, 6.8–7.0 kb in size. These agents are associated tions and Lassa fever. The viruses in these ecologic groupings with gastroenteritis in humans and neurological disease in belong to several virus families, including arenaviridae, bun- some animals (see Chapter 37). yaviridae, flaviviridae, reoviridae, rhabdoviridae, and toga- viridae (see Chapter 38). C. Caliciviridae Caliciviruses are similar to picornaviruses but slightly larger H. Togaviridae (27–40 nm). The particles appear to have cup-shaped depres- Many arboviruses that are major human pathogens, called sions on their surfaces. The genome is single-stranded, positive- alphaviruses—as well as rubella virus—belong to this group. sense RNA, 7.3–8.3 kb in size; the virion has no envelope. They have a lipid-containing envelope and are ether sensi- Important human pathogens are the noroviruses (eg, Norwalk tive, and their genome is single-stranded, positive-sense virus), the cause of epidemic acute gastroenteritis. Other agents RNA, 9.7–11.8 kb in size. The enveloped virion measures infect cats and sea lions as well as primates (see Chapter 37). 65–70 nm. The virus particles mature by budding from host cell membranes. An example is eastern equine encephalitis D. Hepeviridae virus. Rubella virus has no arthropod vector (see Chapters 38 Hepeviruses are similar to caliciviruses. The particles are and 40). small (32–34 nm) and ether resistant. The genome is sin- gle-stranded, positive-sense RNA, 7.2 kb in size. It lacks I. Flaviviridae a genome-linked protein (VPg). Human hepatitis E virus Flaviviruses are enveloped viruses, 40–60 nm in diameter, belongs to this group (see Chapter 35). containing single-stranded, positive-sense RNA. Genome sizes vary from 9.5 to 12 kb. Mature virions accumulate E. Picobirnaviridae within cisternae of the endoplasmic reticulum. This group of Picobirnaviruses are small (35–40 nm) nonenveloped viruses arboviruses includes yellow fever virus and dengue viruses. with icosahedral structure. The genome is linear, double- Most members are transmitted by blood-sucking arthropods. stranded, segmented (bipartite) RNA (two segments), total- Hepatitis C virus is a flavivirus with no known vector (see ing about 4 kb. Human disease associations remain unclear. Chapters 35 and 38). F. Reoviridae J. Arenaviridae Reoviruses are medium-sized (60–80 nm), ether-resistant, Arenaviruses are pleomorphic, enveloped viruses ranging nonenveloped viruses having icosahedral symmetry. Particles in size from 60 to 300 nm (mean, 110–130 nm). The genome Riedel_CH29_p413-p436.indd 418 05/04/19 4:47 PM  CHAPTER 29 General Properties of Viruses   419 is segmented, circular, single-stranded RNA that is nega- hemagglutinin or neuraminidase activity. The genome is tive sense and ambisense, 10–14 kb in total size. Replication linear, segmented, negative-sense, single-stranded RNA, occurs in the cytoplasm with assembly via budding on the totaling 10–13.6 kb in size. Segments range from 890 to 2350 plasma membrane. The virions incorporate host cell ribo- nucleotides each. The virus matures by budding at the cell somes during maturation, which gives the particles a “sandy” membrane. Orthomyxoviruses include influenza viruses that appearance. Most members of this family are unique to tropi- infect humans or animals. The segmented nature of the viral cal America (ie, the Tacaribe complex). Arenaviruses patho- genome permits ready genetic reassortment when two influ- genic for humans cause chronic infections in rodents. Lassa enza viruses infect the same cell, presumably fostering the fever virus of Africa is one example. These viruses require high rate of natural variation among influenza viruses. Viral maximum containment conditions in the laboratory (see reassortment and transmission from other species is thought Chapter 38). to explain the emergence of new human pandemic strains of influenza A viruses (see Chapter 39). K. Coronaviridae Coronaviruses are enveloped 120- to 160-nm particles con- N. Bunyaviridae taining an unsegmented genome of positive-sense, single- Bunyaviruses are spherical or pleomorphic, 80- to 120-nm stranded RNA, 27–32 kb in size. Coronaviruses resemble enveloped particles. The genome is made up of a triple- orthomyxoviruses but have petal-shaped surface projections segmented, single-stranded, negative-sense or ambisense arranged in a fringe, similar to a solar corona. Coronavirus RNA, 11–19 kb in overall size. Virion particles contain three nucleocapsids develop in the cytoplasm and mature by bud- circular, helically symmetric nucleocapsids about 2.5 nm in ding into cytoplasmic vesicles. These viruses have narrow diameter and 200–3000 nm in length. Replication occurs in host ranges. Classically, human coronaviruses cause mild the cytoplasm, and an envelope is acquired by budding into acute upper respiratory tract illnesses—“colds”—but more the Golgi. The majority of these viruses are transmitted to recently discovered coronaviruses cause severe acute respira- vertebrates by arthropods (arboviruses). Hantaviruses are tory syndrome (SARS) and Middle East respiratory syndrome transmitted not by arthropods but by persistently infected (MERS). Toroviruses, which cause gastroenteritis, form a dis- rodents via aerosols of contaminated excreta. They cause tinct genus. Coronaviruses of animals readily establish per- hemorrhagic fevers and nephropathy as well as a severe pul- sistent infections and include mouse hepatitis virus and avian monary syndrome (see Chapter 38). infectious bronchitis virus (see Chapter 41). O. Bornaviridae L. Retroviridae Bornaviruses are enveloped, spherical (70–130 nm) viruses. Retroviruses are spherical, enveloped viruses (80–110 nm The genome is linear, single-stranded, nonsegmented, neg- in diameter) whose genome contains two copies of linear, ative-sense RNA, 8.5–10.5 kb in size. Unique among non- positive-sense, single-stranded RNA. Each monomer RNA is segmented, negative-sense RNA viruses, replication and 7–11 kb in size. Particles contain a helical nucleocapsid within transcription of the viral genome occur in the nucleus. Borna an icosahedral capsid. Replication is unique; the virion con- disease virus is neurotropic in animals; a postulated associa- tains a reverse transcriptase enzyme that produces a DNA tion with neuropsychiatric disorders of humans is unproven copy of the RNA genome. This DNA becomes circularized (see Chapter 42). and integrated into host chromosomal DNA. The virus is then replicated from the integrated “provirus” DNA copy. Virion assembly occurs by budding from plasma membranes. P. Rhabdoviridae Hosts remain chronically infected. Retroviruses are widely Rhabdoviruses are enveloped virions resembling a bul- distributed; there are also endogenous proviruses resulting let, flat at one end and round at the other, measuring about from ancient infections of germ cells transmitted as inher- 75 × 180 nm. The envelope has 10-nm spikes. The genome is ited genes in most species. Leukemia and sarcoma viruses of linear, single-stranded, nonsegmented, negative-sense RNA, animals and humans (see Chapter 43), foamy viruses of pri- 11–15 kb in size. Particles are formed by budding from the mates, and lentiviruses (human immunodeficiency viruses; cell membrane. Viruses have broad host ranges. Rabies virus visna of sheep) (see Chapters 42 and 44) are included in this is a member of this group (see Chapter 42). group. Retroviruses cause acquired immunodeficiency syn- drome (AIDS) (see Chapter 44) and make possible the identi- Q. Paramyxoviridae fication of cellular oncogenes (see Chapter 43). Paramyxoviruses are similar to but larger (150–300 nm) than orthomyxoviruses. Particles are pleomorphic. The internal M. Orthomyxoviridae nucleocapsid measures 13–18 nm, and the linear, single- Orthomyxoviruses are medium-sized, 80- to 120-nm envel- stranded, nonsegmented, negative-sense RNA is 16–20 kb oped viruses exhibiting helical symmetry. Particles are either in size. Both the nucleocapsid and the hemagglutinin are round or filamentous, with surface projections that contain formed in the cytoplasm. Those infecting humans include Riedel_CH29_p413-p436.indd 419 05/04/19 4:47 PM  420   SECTION IV  Virology mumps, measles, parainfluenza, metapneumovirus, and the mechanisms of certain processes such as the interaction respiratory syncytial viruses. These viruses have narrow host of virus particles with cell surface receptors and neutralizing ranges. In contrast to influenza viruses, paramyxoviruses are antibodies. It may lead also to the rational design of antivi- genetically stable (see Chapter 40). ral drugs capable of blocking viral attachment, uncoating, or assembly in susceptible cells. R. Filoviridae Filoviruses are enveloped, pleomorphic viruses that may appear very long and threadlike. They typically are 80 nm Types of Symmetry of Virus Particles wide and about 1000 nm long. The envelope contains large Electron microscopy, cryoelectron microscopy, and x-ray peplomers. The genome is linear, negative-sense, single- diffraction techniques have made it possible to resolve fine stranded RNA, 18–19 kb in size. Marburg and Ebola viruses differences in the basic morphology of viruses. The study of cause severe hemorrhagic fever in Africa. These viruses viral symmetry by standard electron microscopy requires the require maximum containment conditions (Biosafety Level 4) use of heavy metal stains (eg, potassium phosphotungstate) for handling (see Chapter 38). to emphasize surface structure. The heavy metal adsorbs to virus particles and brings out the surface structure of viruses S. Emerging Viruses by virtue of “negative staining.” The typical level of resolution Novel viruses are being discovered with increasing frequency; is 3–4 nm. (The size of a DNA double helix is 2 nm.) However, most belong to existing families but rarely agents are not clas- conventional methods of sample preparation often cause dis- sifiable. Some of these are associated with human disease, tortions and changes in particle morphology. Cryoelectron while many affect other species (see Chapter 48). microscopy uses virus samples quickly frozen in vitreous ice; fine structural features are preserved, and the use of negative T. Viroids stains is avoided. Three-dimensional structural information Viroids are small infectious agents that cause diseases of can be obtained by the use of computer image processing pro- plants. Viroids are agents that do not fit the definition of clas- cedures. Examples of image reconstructions of virus particles sic viruses. They are nucleic acid molecules without a protein are shown in following chapters (see Chapters 32 and 37). coat. Plant viroids are single-stranded, covalently closed cir- X-ray crystallography can provide atomic resolution cular RNA molecules consisting of about 360 nucleotides and information, generally at a level of 0.2–0.3 nm. The specimen with a highly base-paired rodlike structure. Viroids replicate must be crystalline, and this has only been achieved with by an entirely novel mechanism. Viroid RNA does not encode small, nonenveloped viruses. However, it is possible to obtain any protein products; the devastating plant diseases induced high-resolution structural data on well-defined substruc- by viroids occur by an unknown mechanism. Hepatitis D tures prepared from the more complex viruses. virus in humans has properties similar to viroids. Genetic economy requires that a viral structure be made from many identical molecules of one or a few proteins. Viral architecture can be grouped into three types based on the U. Prions arrangement of morphologic subunits: (1) cubic symmetry Prions are infectious particles composed solely of protein (eg, adenoviruses), (2) helical symmetry (eg, orthomyxovi- with no detectable nucleic acid. They are highly resistant to ruses), and (3) complex structures (eg, poxviruses). inactivation by heat, formaldehyde, and ultraviolet light that inactivate viruses. The infectious prion protein is misfolded A. Cubic Symmetry and able to change the conformation of the native prion pro- tein which is encoded by a single cellular gene. Prion diseases, All cubic symmetry observed with animal viruses is of the called “transmissible spongiform encephalopathies,” include icosahedral pattern, the most efficient arrangement for sub- scrapie in sheep, mad cow disease in cattle, and kuru and units in a closed shell. The icosahedron has 20 faces (each Creutzfeldt-Jakob disease in humans (see Chapter 42). an equilateral triangle), 12 vertices, and fivefold, threefold, and twofold axes of rotational symmetry. The vertex units have five neighbors (pentavalent), and all others have six PRINCIPLES OF VIRUS STRUCTURE (hexavalent). There are 60 identical subunits on the surface of an ico- Viruses come in many shapes and sizes. Structural informa- sahedron. To build a particle size adequate to encapsidate tion is necessary for virus classification and for establishing viral genomes, viral shells are often composed of multiples structure–function relationships of viral proteins. The par- of 60 structural units. Larger capsid structures are formed in ticular structural features of each virus family are determined some cases to accommodate the size of the viral genome with by the functions of the virion: morphogenesis and release the association of additional protein subunits. from infected cells; transmission to new hosts; and attach- Most viruses that have icosahedral symmetry do not ment, penetration, and uncoating in newly infected cells. have an icosahedral shape—rather, the physical appearance Knowledge of virus structure furthers our understanding of of the particle is spherical. Riedel_CH29_p413-p436.indd 420 05/04/19 4:47 PM  CHAPTER 29 General Properties of Viruses   421 The viral nucleic acid is condensed within the isomet- vary in size (10–100 nm). Some are spherical or hexagonal and ric particles; virus-encoded core proteins—or, in the case of have short or long tails. (3) Representative protein molecules polyomaviruses and papillomaviruses, cellular histones—are range in diameter from serum albumin (5 nm) and globu- involved in condensation of the nucleic acid into a form suit- lin (7 nm) to certain hemocyanins (23 nm). (4) Eukaryotic able for packaging. “Packaging sequences” on viral nucleic ribosomes are about 25–30 nm in size, with mitochondria acid are involved in assembly into virus particles. There are being much larger (1–10 μm). (5) Red blood cells are about size constraints on the nucleic acid molecules that can be 6–8 μm in diameter. (6) The width of a human hair is about packaged into a given icosahedral capsid. Icosahedral capsids 100 μm. are formed independently of nucleic acid. Most preparations The relative sizes and morphology of various virus fami- of isometric viruses contain some “empty” particles devoid of lies are shown in Figure 29-2. Particles with a twofold dif- viral nucleic acid. Expression of capsid proteins from cloned ference in diameter have an eightfold difference in volume. genes often results in self-assembly and formation of empty Thus, the mass of a poxvirus is about 1000 times greater than “virus-like particles.” Both DNA and RNA viral groups that of the poliovirus particle, and the mass of a small bacte- exhibit examples of cubic symmetry. rium is 50,000 times greater. B. Helical Symmetry In cases of helical symmetry, protein subunits are bound in a CHEMICAL COMPOSITION OF VIRUSES periodic way to the viral nucleic acid, winding it into a helix. The filamentous viral nucleic acid–protein complex (nucleo- Viral Protein capsid) is then coiled inside a lipid-containing envelope. The structural proteins of viruses have several important Thus, as is not the case with icosahedral structures, there is functions. Their major purpose is to facilitate transfer of the a regular, periodic interaction between capsid protein and viral nucleic acid from one host cell to another. They serve to nucleic acid in viruses with helical symmetry. It is not pos- protect the viral genome against inactivation by nucleases, par- sible for “empty” helical particles to form. ticipate in the attachment of the virus particle to a susceptible All known examples of animal viruses with helical sym- cell, and provide the structural symmetry of the virus particle. metry contain RNA genomes and, with the exception of rhab- The proteins determine the antigenic characteristics of doviruses, have flexible nucleocapsids that are wound into a the virus. The host’s protective immune response is directed ball inside envelopes (see Figures 29-1B, 29-2, and 42-1). against antigenic determinants of proteins or glycoproteins exposed on the surface of the virus particle. Some surface proteins may also exhibit specific activities (eg, influenza C. Complex Structures virus hemagglutinin agglutinates red blood cells). Some virus particles do not exhibit simple cubic or helical Some viruses carry protein enzymes inside the virions. symmetry but are more complicated in structure. For exam- The enzymes are present in very small amounts and are prob- ple, poxviruses are brick shaped, with ridges on the external ably not important in the structure of the virus particles; how- surface and a core and lateral bodies inside (see Figures 29-2 ever, they are essential for the initiation of the viral replicative and 34-1). cycle when the virion enters a host cell. Examples include an RNA polymerase carried by viruses with negative-sense RNA Measuring the Sizes of Viruses genomes (eg, orthomyxoviruses and rhabdoviruses) that is needed to copy the first mRNAs, and reverse transcriptase, an Small size and the ability to pass through filters that hold back enzyme in retroviruses that makes a DNA copy of the viral bacteria are classic attributes of viruses. However, because RNA, an essential step in replication and transformation. some bacteria may be smaller than the largest viruses, filter- At the extreme in this respect are the poxviruses, the cores ability is not regarded as a unique feature of viruses. of which contain a transcriptional system; many different Direct observation in the electron microscope is the enzymes are packaged in poxvirus particles. most widely used method for estimating particle size. Viruses can be visualized in preparations from tissue extracts and in ultrathin sections of infected cells. Another method that Viral Nucleic Acid can be used is sedimentation in the ultracentrifuge. The rela- Viruses contain a single kind of nucleic acid—either DNA tionship between the size and shape of a particle and its rate or RNA—that encodes the genetic information necessary for of sedimentation permits determination of particle density. replication of the virus. The genome may be single or double stranded, circular or linear, and segmented or nonsegmented. A. Comparative Measurements The type of nucleic acid, its polarity, and its size are major Viruses range in diameter from about 20 to 300 nm (see characteristics used for classifying viruses into families (see Table 29-1). For purposes of comparison, the following data Table 29-1). should be recalled: (1) Staphylococcus species have a diameter The size of the viral DNA genome ranges from 3.2 kbp of about 1000 nm (1 μm). (2) Bacterial viruses (bacteriophages) (hepadnaviruses) to 375 kbp (poxviruses). The size of the Riedel_CH29_p413-p436.indd 421 05/04/19 4:47 PM  422   SECTION IV  Virology viral RNA genome ranges from about 4 kb (picobirnaviruses) Matrix protein Ribonucleoprotein to 32 kb (coronaviruses). Plasma Neuraminidase All major DNA viral groups in Table 29-1 have genomes membrane that are single molecules of DNA and have a linear or circular Hemagglutinin configuration. Viral RNAs exist in several forms. The RNA may be a single linear molecule (eg, picornaviruses). For other viruses (eg, orthomyxoviruses), the genome consists of several seg- ments of RNA that may be loosely associated within the virion. The isolated RNA of viruses with positive-sense genomes (ie, picornaviruses, togaviruses) is infectious, and the molecule functions as an mRNA within the infected cell. The isolated RNA of the negative-sense RNA viruses, such as rhabdoviruses and orthomyxoviruses, is not infectious. For these viral families, the virions carry an RNA polymerase that in the cell transcribes the genomic RNA molecules into several complementary RNA molecules, each of which may serve as an mRNA template. The sequence and composition of nucleotides of each viral nucleic acid are distinctive. Many viral genomes have been sequenced. The sequences can reveal genetic relation- ships among isolates, including unexpected relationships between viruses not thought to be closely related. The num- ber of genes in a virus can be estimated from the open read- ing frames deduced from the nucleic acid sequence. Molecular techniques such as polymerase chain reaction assays and nucleic acid sequencing permit the study of tran- FIGURE 29-3 Release of influenza virus by plasma membrane scription of the viral genome within the infected cell as well budding. First, viral envelope proteins (hemagglutinin and as comparison of the relatedness of different viruses. Viral neuraminidase) are inserted into the host plasma membrane. Then nucleic acid may be characterized by its GC content, pro- the nucleocapsid approaches the inner surface of the membrane file based on use of restriction endonucleases, enzymes that and binds to it. At the same time, viral proteins collect at the site, cleave DNA at specific nucleotide sequences, and genome and host membrane proteins are excluded. Finally, the plasma sequence. Comparison to nucleic acid or protein sequence membrane buds to simultaneously form the viral envelope and databases can be used to classify viral agents. release the mature virion. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw-Hill, 2008. © McGraw-Hill Education.) Viral Lipid Envelopes A number of different viruses contain lipid envelopes as part of their structure. The lipid is acquired when the viral nucleo- Nonlipid-containing viruses are generally resistant to ether capsid buds through a cellular membrane during maturation. and detergents. Budding occurs only at sites where virus-specific proteins have been inserted into the host cell membrane. The budding process varies markedly depending on the replication strat- Viral Glycoproteins egy of the virus and the structure of the nucleocapsid. Bud- Viral envelopes contain glycoproteins. In contrast to the lip- ding by influenza virus is illustrated in Figure 29-3. ids in viral membranes, which are derived from the host cell, The phospholipid composition of a virion envelope is the envelope glycoproteins are virus encoded. However, the determined by the specific type of cell membrane involved sugars added to viral glycoproteins typically reflect the host in the budding process. For example, herpesviruses bud cell in which the virus is grown. through the nuclear membrane of the host cell, and the phos- The surface glycoproteins of an enveloped virus attach pholipid composition of the purified virus reflects the lipids the virus particle to a target cell by interacting with a cel- of the nuclear membrane. The acquisition of a lipid-contain- lular receptor. They are also often involved in the membrane ing membrane is an integral step in virion morphogenesis in fusion step of infection. The glycoproteins are also important some viral groups (see Replication of Viruses). viral antigens. As a result of their position at the outer surface Lipid-containing viruses are sensitive to treatment with of the virion, they are frequently involved in the interaction ether and other organic solvents (see Table 29-1), indicating of the virus particle with neutralizing antibody. Extensive that disruption or loss of lipid results in loss of infectivity. glycosylation of viral surface proteins may prevent effective Riedel_CH29_p413-p436.indd 422 05/04/19 4:48 PM  CHAPTER 29 General Properties of Viruses   423 neutralization of a virus particle by specific antibody. The three-dimensional structures of the externally exposed regions of some viral glycoproteins have been determined by x-ray crystallography (see Figure 39-2). Such studies provide insights into the antigenic structure and functional activities of viral glycoproteins. CULTIVATION AND DETECTION OF VIRUSES Cultivation of Viruses Many viruses can be grown in cell cultures or in fertile eggs under strictly controlled conditions. Virus growth in animals A B is still used for the primary isolation of certain viruses and for studies of the pathogenesis of viral diseases and of viral oncogenesis. Diagnostic laboratories may attempt to recover viruses from clinical samples to establish disease causes (see Chapter 47). Research laboratories cultivate viruses as the basis for detailed analyses of viral replication and protein function. Cells grown in vitro are central to the cultivation and characterization of viruses. There are three basic types of cell cultures. Primary cultures are made by dispersing cells (usually with trypsin) from freshly removed host tissues. In general, they are unable to grow for more than a few pas- sages in culture. Diploid cell lines are secondary cultures that have undergone a change that allows their limited cul- C D ture (up to 50 passages) but that retain their normal chromo- some pattern. Continuous cell lines are cultures capable of FIGURE 29-4 Cytopathic effects produced in monolayers of more prolonged—perhaps indefinite—growth that have been cultured cells by different viruses. The cultures are shown as they derived from diploid cell lines or from malignant tissues. would normally be viewed in the laboratory, unfixed and unstained These have altered and irregular numbers of chromosomes. (60×). A: Enterovirus—rapid rounding of cells progressing to The type of cell culture used for viral cultivation depends on complete cell destruction. B: Herpesvirus—focal areas of swollen, the sensitivity of the cells to a particular virus. rounded cells. C: Paramyxovirus—focal areas of fused cells (syncytia). D: Hemadsorption. Erythrocytes adhere to those cells in the monolayer that are infected by a virus that causes a hemagglutinin A. Detection of Virus-Infected Cells to be incorporated into the plasma membrane. Many enveloped Multiplication of a virus can be monitored in a variety of viruses that mature by budding from cytoplasmic membranes ways: produce hemadsorption. (Courtesy of I Jack.) 1. Development of cytopathic effects (ie, morphologic changes in the cells). Types of virus-induced cytopathic are visible and in some cases occurs in the absence of cyto- effects include cell lysis or necrosis, inclusion body forma- pathic effects (Figure 29-4D). tion, giant cell formation, and cytoplasmic vacuolization 5. Viral growth in an embryonated chick egg may result in (Figure 29-4A, B, and C). death of the embryo (eg, encephalitis viruses), production 2. Appearance of a virus-encoded protein, such as the hem- of pocks or plaques on the chorioallantoic membrane (eg, agglutinin of influenza virus. Specific antisera can be used herpes, smallpox, and vaccinia), or development of hemag- to detect the synthesis of viral proteins in infected cells. glutinins in the embryonic fluids or tissues (eg, influenza). 3. Detection of virus-specific nucleic acid. Molecular-based assays such as polymerase chain reaction provide rapid, B. Inclusion Body Formation sensitive, and specific methods of detection. In the course of viral multiplication within cells, virus- 4. Adsorption of erythrocytes to infected cells, called hemad- specific structures called inclusion bodies may be produced. sorption, caused by the presence of virus-encoded hemag- They become far larger than the individual virus particle and glutinin (parainfluenza, influenza) in cellular membranes. often have an affinity for acid dyes (eg, eosin). They may be This reaction becomes positive before cytopathic changes situated in the nucleus (herpesvirus; see Figure 33-3), in the Riedel_CH29_p413-p436.indd 423 05/04/19 4:48 PM  424   SECTION IV  Virology cytoplasm (poxvirus, rabies virus), or in both (measles virus; (eg, poliovirus) to 2 weeks or more (eg, SV40). Under con- see Figure 40-5). In many viral infections, the inclusion bod- trolled conditions, a single plaque can arise from a single ies are the site of development of the virions (the viral fac- clonal infectious virus particle, termed a plaque-forming tories). Variations in the appearance of inclusion material unit. The cytopathic effect of infected cells within the plaque depend on the tissue fixative and stain used. can be distinguished from uninfected cells of the monolayer. A more rapid method of assay is based on determination of the number of infected cells producing a viral antigen, such Quantitation of Viruses as by immunofluorescence. Certain viruses (eg, herpes and vaccinia) form pocks A. Physical Methods when inoculated onto the chorioallantoic membrane of an Quantitative nucleic acid-based assays such as the poly- embryonated egg. Such viruses can be quantitated by relating merase chain reaction can determine the number of viral the number of pocks counted to the viral dilution inoculated. genome copies in a sample. Both infectious and noninfec- tious genomes are detected. Virus sequence variation may reduce virus detection and quantitation by this method. A variety of serologic tests such as radioimmunoassays PURIFICATION AND IDENTIFICATION and enzyme-linked immunosorbent assays (see Chapter 47) OF VIRUSES can be standardized to quantitate the amount of virus in a sample. These tests do not distinguish infectious from non- Purification of Virus Particles infectious particles and sometimes detect viral proteins not Pure virus must be available in order for certain types of stud- assembled into particles. ies on the properties and molecular biology of the agent to be Certain viruses contain a protein (hemagglutinin) that carried out. For purification studies, the starting material is has the ability to agglutinate red blood cells of humans or usually large volumes of tissue culture medium, body fluids, some animal. Hemagglutination assays provide a method for or infected cells. The first step frequently involves concentra- quantitating the infective and noninfective particles of these tion of the virus particles by precipitation with ammonium types of viruses (see Chapter 47). sulfate, ethanol, or polyethylene glycol or by ultrafiltration. Virus particles can be counted directly in the electron Hemagglutination and elution can be used to concentrate microscope by comparison with a standard suspension of orthomyxoviruses (see Chapter 39). After concentration, latex particles of similar small size. However, a relatively con- virus can be separated from host materials by differential centrated preparation of virus is necessary for this procedure, centrifugation, density gradient centrifugation, column and infectious virus particles cannot be distinguished from chromatography, and electrophoresis. noninfectious ones. More than one step is usually necessary to achieve ade- quate purification. A preliminary purification will remove B. Biologic Methods most nonviral material. This first step may include centrif- End point biologic assays depend on the measurement of ani- ugation; the final purification step almost always involves mal death, animal infection, or cytopathic effects in tissue density gradient centrifugation. In rate-zonal centrifugation, culture at a series of dilutions of the virus being tested. The a sample of concentrated virus is layered onto a preformed titer is expressed as the 50% infectious dose (ID50), which is linear density gradient of sucrose or glycerol, and during cen- the reciprocal of the dilution of virus that produces infection trifugation the virus sediments as a band at a rate determined in 50% of the cells or animals inoculated. The ratio of the primarily by the density of the virus particle. number of infectious particles to the total number of virus Viruses can also be purified by high-speed centrifugation particles varies widely, from near unity to less than one per in density gradients of cesium chloride, potassium tartrate, 1000, but often is one per several hundred. Precise assays potassium citrate, or sucrose. The gradient material of choice require the use of a large number of replicates. is the one that is least toxic to the virus. Virus particles migrate A widely used assay for infectious virus is the plaque to an equilibrium position where the density of the solution is assay, although it can only be used for viruses that grow equal to their buoyant density and form a visible band. well in tissue culture. Monolayers of host cells are inocu- Additional methods for purification are based on the lated with suitable dilutions of virus and after adsorption are chemical properties of the viral surface. In column chroma- overlaid with medium containing agar or carboxymethyl- tography, virus is bound to a substance such as diethylami- cellulose to prevent virus spreading throughout the culture. noethyl or phosphocellulose and then eluted by changes in After several days, the cells initially infected have produced pH or salt concentration. Zone electrophoresis permits sepa- virus that spreads only to surrounding cells. Multiple cycles ration of virus particles from contaminants on the basis of of replication and cell killing produce a small area of infec- charge. Specific antisera also can be used to remove virus tion, or plaque. The length of time from infection to when particles from host materials. plaques can be visualized for counting depends on the rep- Icosahedral viruses are easier to purify than enveloped lication cycle of the virus and can range from a few days viruses. Because the latter usually contain variable amounts Riedel_CH29_p413-p436.indd 424 05/04/19 4:48 PM  CHAPTER 29 General Properties of Viruses   425 of envelope per particle, the viral population is heterogeneous Good biosafety practices include the following: in both size and density. (1) training in and use of aseptic techniques; (2) no eat- It is very difficult to achieve complete purity of viruses. ing, drinking, mouth pipetting, or smoking in the labora- Small amounts of cellular material tend to adsorb to particles tory; (3) use of personal protective equipment (eg, coats, and copurify. The minimal criteria for purity are a homoge- gloves, or masks) not to be worn outside the laboratory; neous appearance in electron micrographs and the failure of (4) sterilization of experimental wastes; (5) use of biosafety additional purification procedures to remove “contaminants” hoods; and (6) immunization if relevant vaccines are avail- without reducing infectivity. able. Additional precautions and special containment facili- ties (Biosafety Level 4) are necessary when personnel are performing research with high-risk agents such as the filovi- Identification of a Particle as a Virus ruses (see Chapter 38) and rabies virus (see Chapter 42). When a characteristic physical particle has been obtained, it should fulfill the following criteria before it is identified as a virus particle: REACTION TO PHYSICAL AND 1. The particle can be obtained only from infected cells or CHEMICAL AGENTS tissues. Heat and Cold 2. Particles obtained from various sources are identi- cal regardless of the cellular origin in which the virus is There is great variability in the heat stability of different grown. viruses. Icosahedral viruses tend to be stable, losing little 3. Particles contain nucleic acid (DNA or RNA), the sequence infectivity after several hours at 37°C. Enveloped viruses are of which is not the same as the species of host cells from much more heat labile, rapidly dropping in titer at 37°C. Viral which the particles were obtained. infectivity is generally destroyed by heating at 50–60°C for 4. The degree of infective activity of the preparation varies 30 minutes, although there are some notable exceptions (eg, directly with the number of particles present. hepatitis B virus and polyomaviruses). 5. Destruction of the physical particle by chemical or physi- Viruses can be preserved by storage at subfreezing tem- cal means is associated with a loss of viral activity. peratures, and some may withstand lyophilization and can 6. Certain properties of the particles and infectivity must be thus be preserved in the dry state at 4°C or even at room tem- shown to be identical (eg, their sedimentation behavior in perature. Enveloped viruses tend to lose infectivity after pro- the ultracentrifuge and their pH stability curves). longed storage even at −80°C and are particularly sensitive to 7. Antisera prepared against the infectious virus should repeated freezing and thawing. react with the characteristic particle and vice versa. Direct observation of an unknown virus can be accomplished by electron microscopic examination of aggregate formation Stabilization of Viruses by Salts in a mixture of antisera and crude viral suspension. Many viruses can be stabilized by salts in order to resist heat 8. The particles should be able to induce the characteristic inactivation, which is important in the preparation of vac- disease in vivo (if such experiments are feasible). cines. The ordinary nonstabilized oral polio vaccine must be 9. Passage of the particles in tissue culture should result in stored at freezing temperatures to preserve its potency. How- the production of progeny with biologic and antigenic ever, with the addition of salts for stabilization of the virus, properties of the virus. potency can be maintained for weeks at ambient tempera- tures even in the high temperatures of the tropics. LABORATORY SAFETY pH Many viruses are human pathogens, and laboratory-acquired Viruses are usually stable between pH values of 5.0 and 9.0. infections can occur. Laboratory procedures are often poten- Some viruses (eg, enteroviruses) are resistant to acidic con- tially hazardous if proper technique is not followed. Among ditions. All viruses are destroyed by alkaline conditions. the common hazards that might expose laboratory person- Hemagglutination reactions can be quite sensitive to changes nel to the risk of infection are the following: (1) aerosols— in pH. generated by homogenization of infected tissues, centrifuga- tion, ultrasonic vibration, or broken glassware; (2) ingestion— from mouth pipetting, eating or smoking in the laboratory, Radiation or inadequate washing of hands; (3) skin penetration—from Ultraviolet, x-ray, and high-energy particles inactivate needle sticks, broken glassware, hand contamination by leak- viruses. The dose varies for different viruses. Infectivity is ing containers, handling of infected tissues, or animal bites; the most radiosensitive property because replication requires and (4) splashes into the eye or mucous membranes. expression of the entire genetic contents. Irradiated particles Riedel_CH29_p413-p436.indd 425 05/04/19 4:48 PM  426   SECTION IV  Virology that are unable to replicate may still be able to express some Sterilization may be accomplished by steam under pres- specific functions in host cells. sure, dry heat, ethylene oxide, and γ-irradiation. Surface disinfectants include sodium hypochlorite, glutaraldehyde, formaldehyde, and peracetic acid. Skin disinfectants include Ether Susceptibility chlorhexidine, 70% ethanol, and iodophors. Vaccine produc- Ether susceptibility can be used to distinguish viruses that tion may involve the use of formaldehyde, β-propiolactone, possess an envelope from those that do not. Ether sensitivity psoralen + ultraviolet irradiation, or detergents (subunit vac- of different virus groups is shown in Table 29-1. cines) to inactivate the vaccine virus. Detergents REPLICATION OF VIRUSES: AN Nonionic detergents (eg, Nonidet P40 and Triton X-100) solu- OVERVIEW bilize lipid constituents of viral membranes. The viral proteins in the envelope are released (undenatured). Anionic detergents Viruses multiply only in living cells. The host cell provides (eg, sodium dodecyl sulfate) also solubilize viral envelopes; in the energy and synthetic machinery and the low-molecu- addition, they disrupt capsids into separated polypeptides. lar-weight precursors for the synthesis of viral proteins and nucleic acids. The viral nucleic acid carries the genetic speci- ficity to code for all of the virus-specific macromolecules in a Formaldehyde highly organized fashion. Formaldehyde destroys viral infectivity by reacting with For a virus to replicate, viral proteins must be synthe- nucleic acid. Viruses with single-stranded genomes are inac- sized by the host cell protein-synthesizing machinery. There- tivated much more readily than those with double-stranded fore, the virus genome must be able to produce a functional genomes. Formaldehyde has minimal adverse effects on the mRNA. Various mechanisms have been identified that allow antigenicity of proteins and therefore has been used fre- viral RNAs to compete successfully with cellular mRNAs to quently in the production of inactivated viral vaccines. produce adequate amounts of viral proteins. The unique feature of viral multiplication is that soon after interaction with a host cell the infecting virion is disrupted and Photodynamic Inactivation its measurable infectivity is lost. This phase of the growth cycle Viruses are penetrable to a varying degree by vital dyes such is called the eclipse period; its duration varies depending on as toluidine blue, neutral red, and proflavine. These dyes bind both the particular virus and the host cell, and it is followed by to the viral nucleic acid, and the virus then becomes suscep- an interval of rapid accumulation of infectious progeny virus tible to inactivation by visible light. particles. The eclipse period is actually one of intense synthetic activity as the cell is redirected toward fulfilling the needs of the viral parasite. In some cases, as soon as the viral nucleic Antibiotics and Other Antibacterial Agents acid enters the host cell, the cellular metabolism is redirected Antibacterial antibiotics and sulfonamides have no effect exclusively toward the synthesis of new virus particles and the on viruses. Some antiviral drugs are available, however (see cell is destroyed. In other cases, the metabolic processes of the Chapter 30). host cell are not altered significantly, although the cell synthe- Certain disinfectants, such as quaternary ammonium sizes viral proteins and nucleic acids, and the cell is not killed. and organic iodine compounds, are not effective against After the synthesis of viral nucleic acid and viral proteins, viruses. Larger concentrations of chlorine are required to the components assemble to form new infectious virions. The destroy viruses than to kill bacteria, especially in the presence yield of infectious virus per cell ranges widely, from modest of extraneous proteins. For example, the chlorine treatment numbers to more than 100,000 particles. The duration of the of stools adequate to inactivate typhoid bacilli is inadequate virus replication cycle also varies widely, from 6 to 8 hours to destroy poliomyelitis virus present in feces. Alcohols, such (picornaviruses) to more than 40 hours (some herpesviruses). as isopropanol and ethanol, are relatively ineffective against Not all infections lead to new progeny virus. Productive certain viruses, especially picornaviruses. infections occur in permissive cells and result in the produc- tion of infectious virus. Abortive infections fail to produce infectious progeny, either because the cell may be nonper- Common Methods of Inactivating Viruses missive and unable to support the expression of all viral for Various Purposes genes or because the infecting virus may be defective, lack- Viruses may be inactivated for various reasons, such as to ing some functional viral gene. A latent infection may ensue, sterilize laboratory supplies and equipment, disinfect sur- with the persistence of viral genomes, the expression of no faces or skin, make drinking water safe, and produce inac- or a few viral genes, and the survival of the infected cell. The tivated virus vaccines. Different methods and chemicals are pattern of replication may vary for a given virus, depending used for these purposes. on the type of host cell infected. Riedel_CH29_p413-p436.indd 426 05/04/19 4:48 PM  CHAPTER 29 General Properties of Viruses   427 General Steps in Viral Replication Cycles RNA virus are shown in Figure 29-5. Details are included in the following chapters devoted to specific virus groups. A variety of different viral strategies have evolved for accom- plishing multiplication in parasitized host cells. Although the details vary from group to group, the general outline of the A. Attachment, Penetration, and Uncoating replication cycles is similar. The growth cycles of a double- The first step in viral infection is attachment, interaction of stranded DNA virus and a positive-sense, single-stranded a virion with a specific receptor site on the surface of a cell. Viral proteins 3 Cytoplasm Viral DNA Virus Nuclear pore 1 1 Nucleus (1) 2 Viral RNA (1) 2 (1) Viral mRNA (2) 3 Viral 5 proteins 4 Mature 4 virus (1) (1) 5 (1) (1) Capsid Host Cytoplasm DNA 6 Replicated viral DNA Nucleus A B FIGURE 29-5 Example of viral growth cycles. A: The growth cycle of a nonenveloped, double-stranded DNA virus. In this example multiple steps in the replication cycle take place in the nucleus. (1) After penetrating the host cell, viral DNA is uncoated and enters the nucleus. (2) Viral genes are transcribed. (3) The mRNAs are translated in the cytoplasm. Newly synthesized proteins enter the nucleus. (4) Viral DNA is replicated in the nucleus, sometimes with the help of newly synthesized viral replication proteins. (5) Viral DNA and viral structura