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Foundations in
Microbiology
Seventh Edition
Talaro
Chapter 6
An Introduction
to Viruses
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6.1 The Search for the Elusive Virus
• Louis Pasteur postulated that rabies was
caused by a virus (1884)
• Ivanovski and Beijerinck showed a disease
in tobacco was caused by a virus (1890s)
• 1950s virology was a multifaceted
discipline
– Viruses: noncellular particles with a definite
size, shape, and chemical composition
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6.2 The Position of Viruses in the
Biological Spectrum
• There is no universal agreement on how and
when viruses originated
• Viruses are considered the most abundant
microbes on earth
• Viruses played a role in evolution of
Bacteria, Archaea, and Eukarya
• Viruses are obligate intracellular parasites
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6.3 General Structure of Viruses
• Size range –
– most <0.2 μm; requires electron microscope
5
Viral Components: Capsids, Nucleic
Acids, and Envelopes
• Viruses bear no resemblance to cells
– Lack protein-synthesizing machinery
• Viruses contain only the parts needed to
invade and control a host cell
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General Structure of Viruses
• Capsids
– All viruses have capsids - protein coats that enclose
and protect their nucleic acid
– The capsid together with the nucleic acid are
nucleocapsid
– Some viruses have an external covering called
envelope; those lacking an envelope are naked
– Each capsid is constructed from identical subunits
called capsomers made of protein
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Figure 6.4 Structure of Virus
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General Structure of Viruses
• Two structural types:
– Helical - continuous helix of capsomers
forming a cylindrical nucleocapsid
– Icosahedral - 20-sided with 12 corners
– Vary in the number of capsomers
– Each capsomer may be made of 1 or several proteins
– Some are enveloped
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Helical Nucleocapsids
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Figure 6.7
Figure 6.8
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General Structure of Viruses
• Viral envelope
– Mostly animal viruses
– Acquired when the virus leaves the host cell
– Exposed proteins on the outside of the envelope,
called spikes, essential for attachment of the virus
to the host cell
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Functions of Capsid/Envelope
• Protects the nucleic acid when the virus is
outside the host cell
• Helps the virus to bind to a cell surface and
assists the penetration of the viral DNA or
RNA into a suitable host cell
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General Structure of Viruses
• Complex viruses: atypical viruses
– Poxviruses lack a typical capsid and are
covered by a dense layer of lipoproteins
– Some bacteriophages have a polyhedral
nucleocapsid along with a helical tail and
attachment fibers
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Figure 6.9
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Nucleic Acids
• Viral genome – either DNA or RNA but
never both
• Carries genes necessary to invade host cell
and redirect cell’s activity to make new
viruses
• Number of genes varies for each type of
virus – few to hundreds
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Nucleic Acids
• DNA viruses
– Usually double stranded (ds) but may be single
stranded (ss)
– Circular or linear
• RNA viruses
– Usually single stranded, may be double stranded,
may be segmented into separate RNA pieces
– ssRNA genomes ready for immediate translation are
positive-sense RNA
– ssRNA genomes that must be converted into proper
form are negative-sense RNA
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General Structure
• Pre-formed enzymes may be present
– Polymerases – DNA or RNA
– Replicases – copy RNA
– Reverse transcriptase – synthesis of DNA from
RNA (AIDS virus)
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6.4 How Viruses Are Classified
• Main criteria presently used are structure,
chemical composition, and genetic makeup
• Currently recognized: 3 orders, 63 families, and
263 genera of viruses
• Family name ends in -viridae, i.e.Herpesviridae
• Genus name ends in -virus, Simplexvirus
• Herpes simplex virus I (HSV-I)
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6.5 Modes of Viral Multiplication
General phases in animal virus multiplication
cycle:
1. Adsorption – binding of virus to specific
molecule on host cell
2. Penetration – genome enters host cell
3. Uncoating – the viral nucleic acid is released
from the capsid
4. Synthesis – viral components are produced
5. Assembly – new viral particles are constructed
6. Release – assembled viruses are released by
budding (exocytosis) or cell lysis
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Adsorption and Host Range
• Virus coincidentally collides with a susceptible
host cell and adsorbs specifically to receptor
sites on the cell membrane
• Spectrum of cells a virus can infect – host range
– Hepatitis B – human liver cells
– Poliovirus – primate intestinal and nerve cells
– Rabies – various cells of many mammals
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Figure 6.12
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Penetration/Uncoating
• Flexible cell membrane is penetrated by the
whole virus or its nucleic acid by:
– Endocytosis – entire virus is engulfed and
enclosed in a vacuole or vesicle
– Fusion – envelope merges directly with
membrane resulting in nucleocapsid’s entry
into cytoplasm
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Figure 6.13
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Replication and Protein Production
• Varies depending on whether the virus is a
DNA or RNA virus
• DNA viruses generally are replicated and
assembled in the nucleus
• RNA viruses generally are replicated and
assembled in the cytoplasm
– Positive-sense RNA contain the message for
translation
– Negative-sense RNA must be converted into
positive-sense message
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Release
• Assembled viruses leave host cell in one of two
ways:
– Budding – exocytosis; nucleocapsid binds to
membrane which pinches off and sheds the viruses
gradually; cell is not immediately destroyed
– Lysis – nonenveloped and complex viruses released
when cell dies and ruptures
• Number of viruses released is variable
– 3,000-4,000 released by poxvirus
– >100,000 released by poliovirus
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Figure 6.15
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Damage to Host Cell
Cytopathic effects - virus-induced damage to
cells
1. Changes in size and shape
2. Cytoplasmic inclusion bodies
3. Inclusion bodies
4. Cells fuse to form multinucleated cells
5. Cell lysis
6. Alter DNA
7. Transform cells into cancerous cells
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Figure 6.16
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Persistent Infections
• Persistent infections - cell harbors the virus and
is not immediately lysed
• Can last weeks or host’s lifetime; several can
periodically reactivate – chronic latent state
– Measles virus – may remain hidden in brain cells for
many years
– Herpes simplex virus – cold sores and genital herpes
– Herpes zoster virus – chickenpox and shingles
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• Some animal viruses enter host cell and permanently
alter its genetic material resulting in cancer –
transformation of the cell
• Transformed cells have increased rate of growth,
alterations in chromosomes, and capacity to divide for
indefinite time periods resulting in tumors
• Mammalian viruses capable of initiating tumors are
called oncoviruses
– Papillomavirus – cervical cancer
– Epstein-Barr virus – Burkitt’s lymphoma
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Multiplication Cycle in
Bacteriophages
• Bacteriophages – bacterial viruses (phages)
• Most widely studied are those that infect
Escherichia coli – complex structure, DNA
• Multiplication goes through similar stages as
animal viruses
• Only the nucleic acid enters the cytoplasm uncoating is not necessary
• Release is a result of cell lysis induced by viral
enzymes and accumulation of viruses - lytic cycle
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6 Steps in Phage Replication
1. Adsorption – binding of virus to specific
molecule on host cell
2. Penetration – genome enters host cell
3. Replication – viral components produced
4. Assembly – viral components assembled
5. Maturation – completion of viral formation
6. Release – viruses leave cell to infect other cells
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Figure 6.17
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Figure 6.18
Figure 6.19
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Lysogeny: The Silent Virus Infection
• Not all phages complete the lytic cycle
• Some DNA phages, called temperate phages,
undergo adsorption and penetration but don’t replicate
• The viral genome inserts into bacterial genome and
becomes an inactive prophage – the cell is not lysed
• Prophage is retained and copied during normal cell
division resulting in the transfer of temperate phage
genome to all host cell progeny – lysogeny
• Induction can occur resulting in activation of
lysogenic prophage followed by viral replication and
cell lysis
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Figure 6.17
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Lysogeny
• Lysogeny results in the spread of the virus
without killing the host cell
• Phage genes in the bacterial chromosome can
cause the production of toxins or enzymes that
cause pathology – lysogenic conversion
– Corynebacterium diphtheriae
– Vibrio cholerae
– Clostridium botulinum
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6.6 Techniques in Cultivating and
Identifying Animal Viruses
• Obligate intracellular parasites that require
appropriate cells to replicate
• Methods used:
– Cell (tissue) cultures – cultured cells grow in sheets
that support viral replication and permit observation
for cytopathic effect
– Bird embryos – incubating egg is an ideal system;
virus is injected through the shell
– Live animal inoculation – occasionally used when
necessary
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Figure 6.20
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Figure 6.21
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6.7 Medical Importance of Viruses
• Viruses are the most common cause of
acute infections
• Several billion viral infections per year
• Some viruses have high mortality rates
• Possible connection of viruses to chronic
afflictions of unknown cause
• Viruses are major participants in the earth’s
ecosystem
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6.8 Prions and Other Infectious
Particles
Prions - misfolded proteins, contain no nucleic
acid
–
–
Cause transmissible spongiform encephalopathies –
fatal neurodegenerative diseases
Common in animals:
•
•
•
•
•
Scrapie in sheep and goats
Bovine spongiform encephalopathies (BSE), a.k.a. mad cow disease
Wasting disease in elk
Humans – Creutzfeldt-Jakob Syndrome (CJS)
Extremely resistant to usual sterilization
techniques
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Figure 6.22
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Other Noncellular Infectious Agents
• Satellite viruses – dependent on other viruses
for replication
– Adeno-associated virus – replicates only in cells
infected with adenovirus
– Delta agent – naked strand of RNA expressed
only in the presence of hepatitis B virus
• Viroids – short pieces of RNA, no protein
coat; only been identified in plants
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6.9 Detection and Treatment of
Animal Viral Infections
• More difficult than other agents
• Consider overall clinical picture
• Take appropriate sample
– Infect cell culture – look for characteristic cytopathic
effects
– Screen for parts of the virus
– Screen for immune response to virus (antibodies)
• Antiviral drugs can cause serious side effects
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