Introduction to Viruses

Viruses Introduction

• Viruses infect every type of organism
• Seawater can contain 100 million viruses per milliliter
• Louis Pasteur postulated that diseases were caused by “living things" smaller than bacteria and proposed the term virus

Discovery of Viruses

• Ivanovski and Beijerinck determined that a virus caused disease in tobacco
• Loeffler and Frosch discovered that an animal virus results in foot-and-mouth disease in cattle
• Filterable viruses are fluids from host organisms that can pass through porcelain filters designed to trap bacteria, with the filtrate remaining infectious.

Debate About Viruses

• Viruses are not living things because of their inability to replicate independently of a host cell, thus they are called infectious molecules
• Viruses do not exhibit most life processes of cells but they can direct them, so they are more than inert molecules
• Viruses are better described as active or inactive rather than alive or dead

Roles of Viruses

• Viruses infect cells and influence their genetic makeup
• They shape the way cells, tissues, bacteria, plants, and animals have evolved
• 10% of the human genome are sequences that come from viruses
• 10-20% of bacterial DNA contains viral sequences
• Viruses are obligate intracellular parasites that have to invade a specific host cell and instruct it to multiply

Properties of Viruses

• Viruses are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals
• They are ubiquitous and have had major impact on the development of biological life
• Viruses are ultramicroscopic in size, ranging from 20 nm up to 450 nm in diameter
• They are not cells, and their structure is compact and economical
• They do not independently fulfill the characteristics of life
• Viruses are inactive macromolecules outside the host cell but active inside host cells
• The basic structure is a protein shell (capsid) surrounding a nucleic acid core
• Their nucleic acid can be either DNA or RNA, but not both
• Nucleic acid can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA
• Molecules on the virus surface impart high specificity for attachment to host cell
• Viruses multiply by taking control of host cell's genetic material and regulating the synthesis and the assembly of new viruses
• They lack enzymes for most metabolic processes and the machinery for synthesizing proteins

Viral Classification

• Animal viruses were classified on the basis of their hosts and the diseases they cause
• New classification systems emphasize hosts and diseases; structure; chemical composition; and similarities in genetic makeup
• The International Committee on the Taxonomy of Viruses recognizes 3 orders of viruses

Viral Structure

• Viruses are the smallest infectious agents
• Parvoviruses are the smallest viruses, around 20 nm in diameter
• Mimiviruses are the largest viruses, around 450 nm in length
• Pandoraviruses were discovered in July 2013, and found to be almost 1000 nm

Viral Components

• Viruses bear little resemblance to cells as they lack ribosomes and cytoplasm
• Their structure is composed of regular, repeating subunits that give rise to their crystalline appearance
• The minimal components needed to invade and control a host cell are an external coating and a core containing one or more nucleic acid molecules of DNA or RNA and sometimes one or two enzymes
• A virus particle has a covering consisting of a capsid and an envelope (not in all viruses) covering a central core

Viral Components: Capsid

• Capsid is the protein shell that surrounds the nucleic acid
• The nucleocapsid is the capsid together with the nucleic acid
• Naked viruses consist only of a nucleocapsid

Viral Components: Envelope

• The envelope is an external covering of a nucleocapsid, usually a modified piece of the host's cell membrane

Viral Components: Spikes

• These can be found on naked or enveloped viruses
• They project from the nucleocapsid or the envelope and allow viruses to dock with host cells

Viral Capsid and Envelope

• The capsid is the most prominent feature of viruses
• It is composed of identical protein subunits (capsomers) that spontaneously self-assemble into the finished capsid
• The two main types are helical (spiral) and icosahedral (20-sided) and complex (irregular)

Enveloped Viruses

• Enveloped viruses take a bit of the cell membrane when they are released from a host cell
• The viral envelope differs significantly from a host's membranes because some or all of the regular membrane proteins are replaced with viral proteins
• Enveloped viruses are pleomorphic, have a variable shape, and range from spherical to filamentous

Nucleic acids

• Viruses contain either DNA or RNA but not both
• The number of viral genes is small compared with that of a cell
• Hepatitis B virus has 4 genes, and herpesviruses have hundreds
• Viruses possess only the genes needed to invade host cells and redirect their activity

Other Substances in the Virus Particle

• Polymerases that synthesize DNA and RNA
• Replicases that copy RNA
• Reverse transcriptase synthesizes DNA from RNA

Viral Nucleic Acid

• DNA can be ds or ss and linear or circular
• RNA is usually ss; can be (+), ready for translation, or (-), and must be converted first.
• Retroviruses convert RNA to DNA

Viral Replication

• Viruses are tiny parasites that seize control of the synthetic and genetic machinery of cells
• The viral replication cycle dictates the way the virus is transmitted, what it does to the host, responses of immune defenses, human measures to control viral infections

Multiplication Cycles in Animal Viruses

• General phases are adsorption, penetration, uncoating, synthesis, assembly, and release
• The length of the replication cycle varies from 8 hours in polioviruses to 36 hours
• A virus can infect a host cell through a specific interaction between molecules on the surface of the virus and the cell

Host range:

• This is the limited range of cells that a virus can infect
• Hepatitis B: liver cells of humans
• Poliovirus: intestinal and nerve cells of primates
• Rabies: various cells of all mammals
• Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus

Tropism

• Viruses have specificities for certain cells

Synthesis: Replication and Protein Production

• DNA viruses enter the host cell's nucleus, and are replicated and assembled there
• RNA viruses are replicated and assembled in the cytoplasm
• Retroviruses turn their RNA genomes into DNA

Life Cycle of dsDNA Viruses

• Viral DNA enters the nucleus where genes are transcribed into a messenger RNA (early phase)
• The RNA transcript moves into the cytoplasm to be translated into viral proteins needed to replicate viral DNA
• Parts of the viral genome are translated into proteins required to form the capsid (late phase)
• New viral genomes and capsids are assembled

Assembly

• New viruses are created by putting together the "parts" manufactured in the synthesis process

Release

• The number of viruses released by infected cells is variable, controlled by the size of the virus and the health of the host cell
• Poxvirus-infected cells release 3,000 to 4,000 virions, and poliovirus-infected cells can release 100,000 virions
• If even a small number of these virions happen to meet another susceptible cell and infect it, the potential for rapid viral proliferation is immense

Cytopathic Effects (CPEs)

• These are caused by gross changes in shape and size
• Development of intracellular changes
• Inclusion bodies: compacted masses of viruses or damaged cell organelles in the cell
• Syncytia: fusion of multiple host cells into single large cells containing multiple nuclei

Persistent Infections

• Cells harbor the virus, but are not immediately lysed
• Can last from a few weeks to the remainder of the host's life
• Can remain latent in the cytoplasm

Provirus

• A persistent infection in which the viral DNA is incorporated into the DNA of the host

Chronic Latent State

• Viruses go into a period of inactivation in cells and are periodically activated in response to various stimuli
• Examples are herpes simplex virus (cold sores and genital herpes) and herpes zoster virus (chickenpox and shingles)

Viruses and Cancer

• About 10% of human cancers are caused by oncogenic viruses or oncoviruses
• Transformation: conversion of a normal animal cell into a cancer cell
• A virus can carry genes that directly cause cancer
• Cancer-causing viruses can cause alteration of host genes that induce a loss of growth regulation in the cell

Bacteriophage

• Discovered in 1915 by Twort and d'Herelle
• The Greek word phage means "eating"
• Most contain double-stranded DNA, but some RNA types exist
• Every bacterial species is parasitized by various specific bacteriophages
• Scientists believe there are more bacteriophages than all other life in the biosphere combined

T-even Bacteriophage

• Viruses e.g., T2 & T4 infect Escherichia coli
• Complex structure with icosahedral capsid head containing DNA

T-even Bacteriophage Life Cycle

• It absorbs to host bacteria using specific receptors on the bacterial surface
• The nucleic acid penetrates after being injected through a rigid tube inserted through the cell wall and membrane.
• The entry of nucleic acid causes cessation of host cell DNA replication
• No uncoating is necessary
• Host machinery is used for viral replication and synthesis of viral proteins
• Parts spontaneously assemble into bacteriophages
• An average-size Escherichia coli cell can contain up to 200 new phage units
• Eventually, the cell becomes so packed with viruses that it lyses (splits open), releasing mature virions
• Upon release, virulent phages can spread to other susceptible bacterial cells and begin a new cycle of infection

Lysogeny

• Lysogeny is a silent Phage Infection
• Temperate phages undergo adsorption and penetration, but don't undergo immediate replication
• Viral DNA enters an inactive prophage state, inserted into a bacterial chromosome, and is copied during normal bacterial cell division
• Lysogeny defines the condition in which the host chromosome carries viral DNA

The Danger of Lysogeny in Human Disease

• Occasionally phage genes in the bacterial chromosome cause the production of toxins or enzymes that cause pathology in human's
• Lysogenic conversion is when a bacterium acquires a new trait from its temperate phage such as the diphtheria toxin, cholera toxin, or botulinum

Cultivating and Identifying Animal Viruses

• Viruses require living cells as their medium
• In vivo: uses laboratory animals and bird eggs
• In vitro: uses cell or tissue culture methods
• The primary purposes of viral cultivation are to isolate and identify viruses in clinical specimens, prepare viruses for vaccines, and do detailed research on viral culture

Live Animal Inoculation

• Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits are used
• Invertebrates such as insects or non-human primates are employed for this purpose
• Because viruses exhibit host specificity, for certain animals, they can propagate viruses more readily

Bird Embryos

• Bird eggs provide an intact and self-supporting unit, a sterile environment, and contain their own nourishment
• Chicken, duck, and turkey eggs are used for inoculation
• Virus is injected through the eggshell by drilling a small hole

Techniques for Cell (Tissue) Culture

• Isolated animal cells are grown in vitro, in cell or tissue culture
• Cell cultures are grown in sterile chambers with special media that contain nutrients for cells to survive
• Cells form a monolayer, or single, confluent sheet of cells that supports viral multiplication and allows for the close inspection of cultures for signs of infection

Viral Growth in Culture

• Plaques are areas where virus-infected cells have been lysed and show up as clear, well-defined patches in the cell sheet
• Plaques develop when grown in bacterial cell culture
• Viruses released by an infected cell radiate out to adjacent host cells
• Plaques appear as a macroscopic, round, clear space that corresponds to area of dead cells

Noncellular Infectious Agents: Spongiform Encephalopathies

• Smaller and simpler than viruses
• Brain tissue removed from infected animals resembles a sponge
• These have a long period of latency before clinical signs begin to appear
• These diseases are progressive and universally fatal and are caused by prions

Creutzfeldt-Jacob disease (CJD)

• This infects the human nervous system
• Causes gradual degeneration and death

Bovine Spongiform Encephalopathy (BSE)

• Also know as "mad cow disease"
• Humans can contract the disease by consuming contaminated beef
• Infected individuals develop a variant of (CJD)

Prions

• Composed of only protein
• Contain no nucleic acid
• Questions remain regarding propagation of agent, since all infectious agents require nucleic acids

Viroids

• Viruslike agents that parasitize plants
• About one-tenth the size of an average virus
• Composed of naked strands of RNA, lacking a capsid or any other type of coating
• Pathogens of tomatoes, potatoes, cucumbers, citrus trees, and chrysanthemums

Viruses and Human Health

• The number of viruses that occur on a worldwide basis numbers in the billions
• Acute infections: colds, hepatitis, chickenpox, influenza and herpes
• Viral infections are common in certain regions of the world: Dengue fever, Rift Valley fever, and yellow fever
• Viruses with high mortality rates include rabies, AIDS, and Ebola
• Viruses that cause long-term sickness are polio, neonatal, and rubella

Treating Viral Infections

• Antibiotics have no effect on viruses
• Antiviral drugs that block viral replication can have severe side effects
• Interferon naturally occurs in human cells and can treat and prevent viral infections
• Vaccines stimulate immunity and are available for a limited number of viral diseases