Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

16 The Viruses: Introduction and General Characteristics

CHAPTER OVERVIEW

Viruses are generally small, acellular entities that possess only a single type of nucleic acid and that must use the metabolic machinery of a living host in order to reproduce. Viruses have been and continue to be of tremendous importance for a variety of reasons: 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 largely based on discoveries in the field of virology. This chapter focuses on the general properties of viruses, the development of the science of virology, and the methodology used in the study of virology.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o define viruses and discuss the implications of the concepts embodied in the definition

o discuss the various requirements for culturing viruses

o discuss the methodology employed for virus purification and enumeration

o discuss the composition and arrangement(s) of viral capsids

o discuss the variety found in viral genomes (DNA or RNA, single or double stranded, linear or circular, etc.)

o describe the way in which viruses are classified

CHAPTER OUTLINE

I. Early Development of Virology

A. Many epidemics of viral diseases occurred before anyone understood the nature of the causative agents of those diseases

B. 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

C. The word virus, which is Latin for poison, was used to describe diseases of unknown origin, and only later came to be used to describe a particular type of disease-causing entity

D. Dimitri Ivanowski (1892) demonstrated that the causative agent of tobacco mosaic disease would pass through filters designed to remove bacteria; however, he thought the agent was a nonreproducing toxin

E. Martinus Beijerinck (1898-1900), working independently of Ivanowski, showed that the causative agent of tobacco mosaic disease was still infectious after filtration (i.e., capable of reproduction); he referred to it as a filterable virus

F. Loeffler and Frosch (1898-1900) showed that hoof-and-mouth disease in cattle was also caused by a filterable virus

G. Walter Reed (1900) showed that yellow fever in humans was caused by a filterable virus and could be transmitted by a mosquito

H. Ellerman and Bang (1908) showed that leukemia in chickens was caused by a filterable virus

I. Peyton Rous (1911) showed that muscle tumors in chickens were caused by a filterable virus

J. Frederick Twort (1915) first isolated viruses that would infect bacteria, but did not follow up on these observations

K. Felix d'Herelle (1917) firmly established the existence of viruses that infect bacteria, and devised a method for enumerating them; he also demonstrated that these viruses could reproduce only in live bacteria

L. W. M. Stanley (1935) crystallized the tobacco mosaic virus and showed that it was mostly (or completely) composed of protein

M. F. C. Bawden and N. W. Pirie (1935) separated the tobacco mosaic virus particles into protein and nucleic acid components

II. General Properties of Viruses

A. 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

B. Both DNA and RNA do not exist together in the same virion

C. They are obligate intracellular parasites

III. The Cultivation of Viruses-requires inoculation of a living host

A. Animal viruses

1. Suitable host animals

2. Embryonated eggs

3. Tissue (cell) cultures-monolayers of animal cells

a. Cell destruction can be localized if infected cells are covered with a layer of agar; the areas of localized cell destruction are called plaques

b. 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

B. 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

C. Plant viruses can be cultivated in

1. Plant tissue cultures

2. Cultures of separated plant cells

3. Whole plants-may cause localized necrotic lesions or generalized symptoms of infection

4. Plant protoplast cultures

IV. Virus Purification and Assays

A. Virus purification

1. Differential centrifugation separates according to size

2. Gradient centrifugation separates according to density or to sedimentation rate (size and density), and is more sensitive to small differences between various viruses

3. Differential precipitation with ammonium sulfate or polyethylene glycol separates viruses from other components of the mixture

4. Denaturation and precipitation of contaminants with heat, pH, or even organic solvents can sometimes be used

5. 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

B. Virus assays

1. Particle count

a. Direct counts can be made with an electron microscope

b. Indirect counts can be made using methods such as hemagglutination (virus particles can cause red blood cells to clump together or agglutinate)

2. Infectious unit counts are based on the observation that many virion particles may not be infectious

a. 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)

b. 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)

V. The Structure of Viruses

A. Virion size ranges from 10 nm to 400 nm

B. Nucleocapsid-the nucleic acid plus the surrounding capsid; for some viruses this may be the whole virion; other viruses may possess additional structures as well; viral nucleocapsids are usually constructed without outside aid in a process called self-assembly

C. Capsid-protein coat that surrounds the genome, protects the viral genetic material, and aids in transfer between host cells

1. Helical-hollow tube with a protein wall shaped as a helix or spiral; may be either rigid or flexible

2. Icosahedral-regular polyhedron with 20 equilateral triangular faces and 12 vertices; appears spherical

D. Nucleic acids-genome

1. May be either RNA or DNA, single- or double-stranded, linear or circular

2. May have the common bases that occur in RNA or DNA, or genome may have one or more unusual bases (e.g., hydroxymethylcytosine instead of cytosine)

3. Viruses with single-stranded RNA (ssRNA) come in several arrangements:

a. Plus strand viruses have a genomic RNA with the same sequence as the viral mRNA; the genomic RNAs 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

b. Negative strand viruses have a genomic RNA complementary to the viral mRNA

c. Segmented genomes are those in which the virion contains more than one RNA molecule; each segment is unique and frequently encodes a single protein; in some viruses segments may be packaged into more than one virion structure

E. Viral envelopes and enzymes

1. Envelopes are membrane structures surrounding some (but not all) viruses

a. Lipids and carbohydrates are usually derived from the host membranes

b. Proteins are virus specific

c. Many have protruding glycoprotein spikes (peplomeres)

2. Enzymes-some viruses have capsid-specific enzymes; these may be required for virus attachment or entry into the host cell; many, however, are involved in viral nucleic acid replication

F. Viruses with capsids of complex symmetry

1. Poxviruses are large (200 to 400 nm) with an ovoid exterior shape

2. 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

VI. Principles of Virus Taxonomy-Grouped according to

A. Nature of the host-animal, plant, bacterial, insect, fungal

B. Nucleic acid type

1. DNA or RNA

2. Single or double stranded

3. Molecular weight

4. Segmentation and the number of pieces of RNA

C. Capsid symmetry

D. Presence or absence of an envelope and ether sensitivity

E. Diameter of capsid (or nucleocapsid)

F. Number of capsomeres in icosahedral viruses

G. Immunological properties

H. Gene number and genomic map

I. Intracellular location of virus replication

J. Presence or absence of a DNA intermediate (ssRNA viruses)

K. Type of virus release

L. Disease caused by the virus, its special clinical features, or its mode of transmission

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