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Prescott’s Microbiology, 9th Edition
6 Viruses and Other Acellular Infectious Agents CHAPTER OVERVIEW Viruses are small, acellular entities that usually possess only a single type of nucleic acid and must use the metabolic machinery of a living host in order to reproduce. Viruses have been and continue to be of tremendous importance because many human diseases have a viral etiology. The study of viruses has contributed greatly to our knowledge of molecular biology, and the blossoming field of genetic engineering is based on discoveries in the field of virology. This chapter focuses on the general properties of viruses, their structure, reproduction, infectivity, and cultivation. The discussion concludes with the features of viroids, virusoids, and prions.
LEARNING OUTCOMES After reading this chapter students should be able to: • define the terms virology, bacteriophages and phages • list organisms that are hosts to viruses • state the size range of virions • identify the parts of a virion and describe their function • distinguish enveloped from noneveloped viruses • describe the types of capsid symmetry • describe the five steps common to the life cycles of all viruses • discuss the roles of receptors, capsid proteins, and envelope proteins in the life cycles of virues • describe the two most common methods for virion release from a host cell • compare and contrast the major steps of the life cycles of virulent phages and temperate phages • list examples of lysogenic conversion • differentiate among the types of viral infections of the eukaryotic cells • summarize the current understanding of how oncoviruses cause cancer • list the types of approaches used to cultivate viruses, noting which viruses are cultivated by each method • describe three direct counting methods and two indirect counting methods used to enumerate viruses • outline the events that lead to the formation of a plaque in a lawn of bacterial cells • distinguish lethal dose from infectious dose • describe the structure of a viroid and the discuss the practical importance of viroids • distinguish satellite viruses from satellite nucleic acids • describe prion structure and how prions are thought to replicate • list characteristics common to all animal diseases caused by prions • name at least two human diseases caused by prions • describe the mechanisms by which a prion protein might first appear in a human brain cell
GUIDELINES FOR ANSWERING THE MICRO INQUIRY QUESTIONS Figure 6.1 Which capsids are icosahedral? Which are helical? Which have complex symmetry? 1 © 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition Viruses pictured with icosahedral symmetry include human papilloma virus, herpes virus, and X174 phage. Those with helical capsids include Rhabdovirus and tobacco mosaic virus. Those with complex capsids include vaccine, T phages, tailed phages, and Mimivirus. Figure 6.5 Are the capsomers at the vertices of an adenovirus pentamers or hexamers? What is the difference between a pentamer and hexamer? Looking at figure 6.5b, the light blue balls at the vertices represent the capsomers. It can be clearly seen that they contact five other coat proteins, seen as yellow balls, thus they are pentamers. Since viral protein coats are called capsids, the individual units of capsids are called capsomers. Capsomers are made of protomers, which consist of one or more viral coat proteins. A capsomer made of five protomers is called a pentamer or penton, while a capsomer made of six protomers is called a hexamer or hexon. Figure 6.7 Why is T4 said to have binal symmetry? Binal means two types of symmetry in the coat. For T4, the head has icosahedral symmetry and the tail has helical. Figure 6.10 Which of these mechanisms involves the production of a vesicle in the host cell? Which involves the interaction between viral proteins and host cell membrane proteins? Entry into the host cell via endocytosis by enveloped and nonenveloped viruses involve the formation of a vesicle or endosome within the host cell. All three types of viral entry illustrated involve the interaction of viral proteins with host cell membrane proteins (receptors). Figure 6.13 Why do the empty capsids remain attached to the cell after the viral genome enters the host cell? Empty viral capsids remain attached to the host bacterial cell because attachment was mediated by a specific receptor-ligand interaction. Figure 6.15 Why is a lysogen considered a new or different strain of a given bacteria species? A lysogen is considered a new or different strain of a given bacterial species because it contains phage DNA (containing viral genes) that confers new properties on the lysogenic bacteria (by expressing those viral genes), with one new common characteristic being that the cell cannot be reinfected by the same virus. Figure 6.19 What would happen to a plate with individual plaques if it were returned to an incubator so that the viral infection could continue? Plates that have individual viral plaques if reincubated will have continuation of the lytic cycle and the virus will continue to infect host cells and lyse them upon maturation. The plaques will continue to get larger with the spread of virus until they coalesce and all cells have been lysed- you will eventually get a clear plate because all bacteria will be lysed. Thus, the plaque assay is time dependent.
2 © 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
GUIDELINES FOR ANSWERING THE COMPARE, HYPOTHESIZE, INVENT QUESTIONS 1. Many classification schemes are used to identify bacteria. These start with Gram staining, progress to morphology and arrangement characteristics, and include a battery of metabolic tests. Build an analogous scheme that could be used to identify viruses. You might start by considering the host, or you might start with viruses found in a particular environment, such as a marine filtrate. Students determine which of the characteristics of viruses are general enough to split them into two or three large groups, and then further distinguish them. Students might first determine if the nucleic acid content were DNA or RNA. Then they could consider the geometry of the capsid, enveloped or not, host cell, natural environment, etc. Note they could not propose biochemical tests since viruses are metabolically inactive. Nor could they propose aerobic versus anaerobic, or motility. 2. The origin and evolution of viruses is controversial. Discuss whether you think viruses evolved before the first cell or whether they have coevolved and are perhaps still coevolving with their hosts. While there is no doubt that viruses coevolve with their hosts and continue to today, the question of viral origin is an open question, so students are free to propose ideas. The simplicity of nucleic acids surrounded by a self-assembling protein capsid sounds primitive enough to have come together independent of cellular organization, arguing viruses originated before cells. On the other hand, since viral replication requires a cellular host for propagation, evolution of viruses independent of host cells seems improbable. This question encourages students to consider the concept that “right and wrong” in science are not as important as models that are “more or less” consistent with observations and data. They may propose that viruses are some ‘escaped genes’ from cells, or some descendant of a primitive, ancestral life form. 3. Consider the separate stages of an animal virus life cycle. Assemble a short list of structures and processes that are unique to the virus and would make good drug targets for an antiviral agent. Explain your rationale for each choice. Refer to figure 6.9. Attachment-drug could block ligands on virus surface that interact with host receptors. Penetration-for viruses whose entire nucleocapsid enter the cell, drugs can be targeted to the nucleocapsid to block release of nucleic acid (process called uncoating). Synthesis-for RNA viruses, block unique enzymes used for RNA replication or production of mRNA (such as drugs targeting reverse transcriptase in HIV). Release- block viral proteins involved in the exiting of the virus from the cell (as some anti-influenza drugs do). The general concept students should grasp is that an critical step in the life cycle can potentially be interfered with by a small molecule, thus making it an antiviral drug (as long as it does not also block an essential function in the cell). 4. Chronic wasting disease (CWD) is a neurodegenerative disease of cervid animals (e.g., deer, elk, moose) caused by prions. Unlike other prion diseases, CWD is readily transmitted from one animal to another in a herd. How this occurs in not understood. However, evidence suggests the mode of transmission is by animal excreta: saliva, urine, and feces. In this mechanism, a healthy animal would be exposed to the prion present in excreta from a diseased animal. Since many cervids congregate, the prion would easily spread from one animal to another. One problem facing scientists studying CWD is measuring the amount of infectious prions in excreta and determining their source (i.e., whether they are produced solely in nervous system tissue or in organs of excretion). Why is CWD of concern to fish and wildlife biologists and game managers in states where CWD is prevalent (e.g., Colorado, Wyoming, Wisconsin)? Why would the presence of CWD prions in tissues other than those of the central nervous system be important to know? (Hint: Learn about the variant Creutzfeldt-Jakob epidemic that occurred in the 1990s.) 3 © 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition Read the original article: Haley, N. J., et al. 2011. Detection of chronic wasting disease prions in salivary, urinary, and intestinal tissues of deer: potential mechanisms of prion shedding and transmission. J. Virol. 85: 6309. http://jvi.asm.org/content/85/13/6309.full CWD is of concern because numbers of cervid animals could be depleted by this disease. There is also some concern if the prions can be transmitted to and may affect humans, which is not entirely known. Presence of prions in non-neural tissue, such as mouth and intestine, would promote transmission by shedding prions into excreta. Also, muscle tissue is what is eaten by human hunters, and carnivores in nature.
4 © 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.