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TORTORA • FUNKE
• CASE
Microbiology
AN INTRODUCTION
EIGHTH EDITION
B.E Pruitt & Jane J. Stein
Chapter 13
Viruses, Viroids, and Prions
PowerPoint® Lecture Slide Presentation prepared by Christine L. Case
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Differentiate between a
virus and a bacterium.
Viruses may be regarded
as exceptionally
complex aggregations of
nonliving chemicals OR
exceptionally simple
living microbes.
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Viruses
• Viruses contain DNA or RNA
• And a protein coat
• Some are enclosed by an envelope (lipids,
proteins, and carbohydrates)
• Some viruses have spikes
• Most viruses infect only specific types of cells
in one host
• Host range is determined by specific host
attachment sites and cellular factors
• Obligatory intracellular parasites, causing
synthesis of specialized elements that transfer
viral nucleic acid to other cells.
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Viruses (range from 20 to 1000 nm)
nm = 10-9 m
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Figure 13.1
Nonenveloped Polyhedral Viruses
Describe the chemical composition and physical structure of an enveloped and
a nonenveloped virus.
Virion = complete, fully developed viral particle of nucleic acid
surrounded by a coat
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Figure 13.2a, b
Enveloped Helical Virus
Viruses contain either DNA or RNA, but never both. Nucleic acid may
be single or double stranded, linear or circular, or divided into several
separate
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Helical Viruses
Helical viruses look like long or coiled threads.
Their capsids are hollow cylinders surrounding the DNA/RNA.
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Figure 13.4a, b
Complex Viruses
• Capsid – protein
coat surrounding
nucleic acid
• Composed of
capsomeres, single
or multiple proteins
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Figure 13.5a
Viral Taxonomy
Define viral species.
• Classification based on type of nucleic acid, replication,
and morphology.
• Family names end in -viridae
• Genus names end in -virus
• Viral species: A group of viruses sharing the same
genetic information and ecological niche (host).
Common names are used for species
• Subspecies are designated by a number
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Viral Taxonomy
Give an example of a family, genus, and common name for a virus.
• Herpesviridae
• Retroviridae
• Herpesvirus
• Lentivirus
• Human herpes
virus 1, HHV 2,
HHV 3
• Human
Immunodeficiency Virus
1, HIV 2
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Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
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Growing Viruses
Describe how bacteriophages are cultured.
• Viruses must be
grown in living
cells.
• Bacteriophages
form plaques
(clearings) on a
lawn of bacteria.
• Easiest to grow
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Figure 13.6
Growing Viruses
Describe how animal viruses are cultured.
• Viruses must be
grown in living
host cells.
• Animal viruses
may be grown in
living animals or
in embryonated
eggs.
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Figure 13.7
Growing Viruses
• Animal and plants viruses may be grown in cell
culture.
• Continuous cell lines may be maintained
indefinitely.
• Cytopathic effects due to viral growth
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Figure 13.8
Cytopathic effect of viruses
Uninfected mouse cells form monolayer (left). Infected cells 24
hours later pile up and round up (right).
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Figure 13.9
Virus Identification
List three techniques that are used to identify viruses.
• Serological tests
• Detect antibodies against viruses in a patient
• Use antibodies to identify viruses in neutralization
tests, viral hemagglutination, and Western blot
• Nucleic acids
• RFLPs – restriction fragment length polymorphisms
• PCR – polymerase chain reaction (used to identify
West Nile virus in U.S. in 1999)
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Multiplication of Bacteriophages (Lytic Cycle)
• Attachment
Phage attaches by tail fibers to
host cell
• Penetration
Phage lysozyme opens cell wall,
tail sheath contracts to force tail
core and DNA into cell
• Biosynthesis
Production of phage DNA
and proteins
• Maturation
Assembly of phage particles
• Release
Phage lysozyme breaks cell wall
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Lytic cycle of T-even bacteriophage
Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber
1 Attachment:
Base plate
Pin
Cell wall
Phage attaches
to host cell.
Tail
Plasma membrane
2 Penetration:
Phage penetrates
host cell and
injects its DNA.
Sheath contracted
Tail core
3 Merozoites released
into bloodstream
from liver may
infect new red
blood cells
Describe the lytic cycle of T-even bacteriophages.
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Figure 13.10.1
Lytic cycle of T-even bacteriophage
Burst time is generally about 20 – 40 minutes after phage absorption.
Burst size ranges from 50 to 200 new phage cells.
Tail
DNA
4 Maturation:
Viral components
are assembled into
virions.
Capsid
5 Release:
Host cell lyses and
new virions are
released.
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Tail fibers
Figure 13.10.2
One-step Growth Curve for bacteriophage
During biosynthesis and maturation, separate components of DNA
and protein may be detected in the host cell.
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Figure 13.11
• Lytic cycle
Phage causes lysis and death of
host cell
• Lysogenic cycle
Prophage DNA incorporated in
host DNA
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The Lysogenic Cycle – bacteriophage lambda in E.coli
Describe the lysogenic cycle of bacteriophage lambda.
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Figure 13.12
Specialized Transduction
Prophage
gal gene
Bacterial DNA
1 Prophage exists in galactose-using host
(containing the gal gene).
Galactose-positive
donor cell
gal gene
2 Phage genome excises, carrying
with it the adjacent gal gene from
the host.
gal gene
3 Phage matures and cell lyses, releasing
phage carrying gal gene.
4 Phage infects a cell that cannot utilize
galactose (lacking gal gene).
Galactose-negative
recipient cell
5 Along with the prophage, the bacterial gal
gene becomes integrated into the new
host’s DNA.
6 Lysogenic cell can now metabolize
galactose.
Galactose-positive recombinant cell
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Figure 13.13
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Multiplication of Animal viruses
• Attachment
Viruses attaches to cell membrane
• Penetration
By endocytosis or fusion
• Uncoating
By viral or host enzymes
• Biosynthesis
Production of nucleic acid and proteins
• Maturation
Nucleic acid and capsid proteins
assemble
• Release
By budding (enveloped viruses) or
rupture
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Attachment, Penetration, and Uncoating
Entry of herpes simplex virus into an animal cell.
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Figure 13.14
Compare and contrast the multiplication cycle of DNA- and RNA-containing
animal viruses.
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Multiplication of Papovarius, a DNA-containing Virus
Papovavirus
1 Virion attaches to host cell
7 Virions are released
Host cell
DNA
Capsid
DNA
2 Virion penetrates
cell and its DNA is
uncoated
Cytoplasm
6 Virions mature
Capsid proteins
mRNA
5 Late translation;
capsid proteins
are synthesized
4 Late transcription;
DNA is replicated
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3 Early transcription and
translation; enzymes are
synthesized
Figure 13.15
DNA-containing animal viruses: individual capsomeres visible
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DNA-containing animal viruses: envelop around this
herpes simplex virus broken (fried egg appearance)
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Pathways of Multiplication for RNA-Containing Viruses
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Figure 13.17
RNA-containing animal viruses: rubella (left), mouse mammary
tumor virus (right).
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Multiplication & Inheritance in a Retrovirus
Capsid
Reverse
transcriptase
DNA
Virus
Two identical + stands of RNA
1 Retrovirus penetrates
host cell.
Host
cell
DNA of one of the host
cell’s chromosomes
5 Mature
retrovirus
leaves host
cell, acquiring
an envelope as
it buds out.
Reverse
transcriptase
Viral RNA
Identical
strands of
RNA
2 Virion penetrates
cell and its DNA is
uncoated
4 Transcription of the
Viral proteins
RNA
provirus may also occur,
producing RNA for new
retrovirus genomes and
RNA that codes for the
retrovirus capsid and
envelope proteins.
Provirus
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3 The new viral DNA is
transported into the host cell’s
nucleus and integrated as a
provirus. The provirus may
divide indefinitely with the
host cell DNA.
Figure 13.19
Release of an enveloped virus by budding
Most enveloped viruses take
part of host’s plasma
membrane for their envelope.
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Figure 13.20
Cancer
Define oncogene and transformed cell.
• Activated oncogenes transform normal cells into
cancerous cells. (malignant transformation)
• Transformed cells have increased growth, loss of
contact inhibition, tumor specific transplant and T
antigens, chromosome abnormalities, can produce
tumors when injected into susceptible animals.
• Several DNA viruses and retroviruses are oncogenic.
• The genetic material of oncogenic viruses becomes
integrated into the host cell's DNA.
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Oncogenic Viruses
Discuss the relationship of DNA- and RNA-containing viruses to cancer.
• Oncogenic DNA Viruses
• Adenoviridae
• Oncogenic RNA viruses
• Retroviridae
• Papovaviridae
• Viral RNA is
transcribed to DNA
which can integrate
into host DNA
• Hepadnaviridae
• HTLV 1
• Heresviridae
• Poxviridae
• HTLV 2
•Retroviruses carry reverse transcriptase which
allows RNA to DNA, permitting oncogenic properties
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Provide an example of a latent viral infection.
• Latent Viral Infections
• Virus remains in asymptomatic host cell for long
periods
• Cold sores, shingles
• Persistent Viral Infections
• Disease processes occurs over a long period,
generally fatal
• Subacute sclerosing panencephalitis (measles
virus)
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Differentiate between persistant viral infections and latent viral infections.
•Persistent viral infections are caused by conventional
viruses, occur over a long period, generally fatal.
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Prions
Discuss how a protein can be infectious.
• Infectious proteins first discovered in 1980’s
• Inherited and transmissible by ingestion, transplant, &
surgical instruments
• Spongiform encephalopathies: Sheep scrapie,
Creutzfeldt-Jakob disease, Gerstmann-SträusslerScheinker syndrome, fatal familial insomnia, mad cow
disease
• PrPC, normal cellular prion protein, on cell surface
• PrPSc, scrapie protein, accumulate in brain cells
forming plaques
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Prions
How a protein can be infectious: if an abnormal prion protein
enters cell, it changes a normal prion to PrPSc, which changes
another normal PrP (accumulation of abnormal PrPSc)
PrPSc
PrPc
1
2
3
4
Lysosome
Endosome
5
6
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7
8
Figure 13.21
Some Plant Viruses
Name a virus that causes a plant disease.
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Table 13.6
Linear and circular potato spindle tuber viroid
Differentiate between virus, viroid, and prion.
• Plant Viruses
• Plant viruses
enter through
wounds or via
insects
• Viroids
• Viroids are
infectious RNA;
potato spindle
tuber disease
• Prion = infectious
protein
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Figure 13.22
Virus Families
• Single-stranded DNA,
nonenveloped viruses
• Parvoviridae
• Human parvovirus
• Fifth disease
• Anemia in
immunocompromised
patients
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Double-stranded DNA, nonenveloped viruses
• Mastadenovirus
• Respiratory
infections in
humans
• Tumors in animals
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Double-stranded DNA, nonenveloped viruses
• Papillomavirus
(human wart virus)
• Polyomavirus
• Cause tumors,
some cause
cancer
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Double-stranded DNA, nonenveloped viruses
• Orthopoxvirus
(vaccinia and
smallpox viruses)
• Molluscipoxvirus
• Smallpox,
molluscum
contagiosum,
cowpox
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Double-stranded DNA, nonenveloped viruses
• Simplexvirus (HHV1 and HHV
2)
• Varicellavirus (HHV 3)
• Lymphocryptovirus (HHV 4)
• Cytomegalovirus (HHV 5)
• Roseolovirus (HHV 6)
• HHV 7
• Kaposi's sarcoma (HHV 8)
• Some herpesviruses can
remain latent in host cells
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Double-stranded DNA, nonenveloped viruses
• Hepadnavirus
(Hepatitis B virus)
• Use reverse
transcriptase to
produce DNA from
mRNA
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Single-stranded RNA, + strand, nonenveloped
• Enterovirus
• Enteroviruses
include poliovirus
and coxsackievirus
• Rhinovirus
• Hepatitis A virus
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Single-stranded RNA, + strand, nonenveloped
• Hepatitis E virus
• Norovirus (Norwalk
agent) causes
gastroenteritis
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Single-stranded RNA, + strand, nonenveloped
• Alphavirus
• Alphaviruses are
transmitted by
arthropods;
include EEE, WEE
• Rubivirus (rubella
virus)
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Single-stranded RNA, + strand, nonenveloped
• Arboviruses can replicate
in arthropods; include
yellow fever, dengue, SLE,
and West Nile viruses
• Hepatitis C virus
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Single-stranded RNA, + strand, nonenveloped
• Coronavirus
• Upper respiratory
infections
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Single-stranded RNA, – strand, one RNA strand
• Vesiculovirus
• Lyssavirus (rabies
virus)
• Cause numerous
animal diseases
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Single-stranded RNA, – strand, one RNA strand
• Filovirus
• Enveloped,
helical viruses
• Ebola and
Marburg viruses
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Single-stranded RNA, – strand, one RNA strand
• Paramyxovirus
• Morbillivirus
• Paramyxovirus
causes
parainfluenza,
mumps and
Newcastle disease
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Single-stranded RNA, – strand, one RNA strand
• Hepatitis D virus
• Depends on
coinfection with
Hepadnavirus
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Single-stranded RNA, –
strand, multiple RNA strands
• Influenzavirus
(Influenza viruses A
and B)
• Influenza C virus
• Envelope spikes
can agglutinate
RBCs
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Single-stranded RNA, –
strand, multiple RNA strands
• Bunyavirus (CE virus)
• Hantavirus
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Single-stranded RNA, –
strand, multiple RNA strands
• Arenavirus
• Helical capsids
contain RNAcontaining
granules
• Lymphocytic
choriomeningitis
• VEE and Lassa
Fever
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Single-stranded RNA, two RNA strands,
produce DNA
• Lentivirus (HIV)
• Oncogenic viruses
• Use reverse
transcriptase to
produce DNA from
viral genome
• Includes all RNA
tumor viruses
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Double-stranded RNA, nonenveloped
• Reovirus (Respiratory
Enteric Orphan)
• Rotavirus
• Mild respiratory
infections and
gastroenteritis
• Colorado tick fever
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