Latin Word Meaning “Poison”
Definitions:
Viruses are obligate intracellular parasites with a
relatively simple life cycle
and
Viruses are entities whose genomes (nucleic acids)
replicate inside living cells using the cellular
biosynthetic machinery and cause the synthesis of
specialized elements (virions) that can transfer
the viral genome to other cells.
SOME HISTORY OF VIRUSES:
plant case
first to be discovered, early-1890’s (TMV)
first to be crystallized, 1935 (TMV)
first to be reconstituted, 1955 (TMV)
animal case
discovered, mid-1890’s (FMDV)
cell culture and plaque assays 1952
vaccines (polio), 1950’s; gene delivery…
bacterial case
discovered, 1915 (“intestinal phage”)
phage and the origin of
molecular biology, 1937 -first EM pictures, 1941
Characteristics of Life
• 1. Cells
• 2. Grow and maintain their structure by taking
up chemicals and energy from the
environment
• 3. Respond to their external environment
• 4. Reproduce and pass on their organization
to their offspring
• 5. Evolve and Adapt to their environment
Viruses are:
• 1. Acellular
• 2. Obligate intracellular parasites
Requires appropriate host cell for almost all
metabolism
• 3. No ATP generating system
• 4. No Ribosomes or means of Protein Synthesis
Host range
• Spectrum of host cells that a virus can infect
•
Host range limited by
– Type of host
– Type of cell
• Some viruses only infect:
–
–
–
–
–
plants
invertebrates
protists
fungi
bacteria (Bacteriophages)
• Egs.
•
– Measles, polio, smallpox; humans only
– West Nile virus; mosquitos, birds, humans
Most viruses have a narrow host range
Polio virus - nerve cells
Adenovirus - cells in upper Respiratory Tract (cold Virus)
Host range is determined by Viruses
ability to interact with its host cell
• Binding Sites match Receptor Sites
• Binding Sites - on viral capsid or envelope
• Receptor Sites - on host cell membrane
Viral Size
20 nm to 1,000 nm
.02 u to 1 u
Comparing the size of a virus, a bacterium, and an animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Viral Structure
SIMPLEST VIRUSES ARE JUST:
• 1. Nucleic Acid
– DNA or RNA (But never both)
•
•
•
•
ssDNA
ds DNA
ss RNA
ds RNA
2. Capsid:
 Protein coat enclosing viral genome
 Capsomeres: Protein subunits form into regular shapes
Rods, icosahedrons, polyhedral head with a tail)
• Some Viruses:
a protein shell “ capsid”
• A. Envelope:
cylindrical
spherical
– derived from the host cell with embedded viral proteins
•
or some combination of the two shapes
– Binding Sites
B. Enzymes
•
without or with a lipid bilayer envelope
Capsid Functions
• Packaging or Condensation
• Protects viral genome (Nucleic Acid)
from host endonucleases
• Transport Nucleic Acid From Cell to Cell
• Provides Specificity for Attachment
HELICAL
ICOSAHEDRAL
CAPSOMER ARRANGEMENT
Penton Capsomer
(Surrounded by 5)
Hexon Capsomer
(Surrounded by 6)
Electron Micrographs of
Helical Nucleocapsids
of RNA Viruses
A. Measles
B. Influenza
C. VSV
Viral Morphology
1. Helical
Viral Morphology
2. Polyhedral
icosahedral
Viral Morphology
3. Enveloped
A. Enveloped Helical
B. Enveloped Polyhedral
Viral Morphology
4. Complex
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
DNA
Tail
fiber
Glycoprotein
Glycoprotein
18  250 mm
70–90 nm (diameter)
20 nm
(a) Tobacco mosaic virus
50 nm
(b) Adenoviruses
80–200 nm (diameter)
50 nm
(c) Influenza viruses
80  225 nm
50 nm
(d) Bacteriophage T4
Growing Viruses
• 1. Bacteriophages
– Lawn of Bacteria on a Spread Plate
– Add Bacteriophages
– Infection will result in “Plaques”
• Clear zones on plate
Growing Viruses
• Animal Viruses
– A. Living Animals
• mice, rabbits, guinea pigs
– B. Chicken Embryos (Eggs)
• used to be most common method to grow viruses
• Still used to produce many vaccines (Flu Vaccine)
– C. Cell Cultures
• Most common method to grow viruses today
Viroids and Prions
• Viroids
– Naked RNA (no capsid)
– 300 – 400 nucleotides long
– Closed, folded, 3-dimensional shape (protect against
endonucleases ?)
– Plant pathogens
– Base sequence similar to introns
Prions
• Proteinaceous infectious particle
• 1982
• Diseases
–
–
–
–
Scrapie (sheep)
Creutzfeldt-Jacob disease (CJD)
Kuru (Tribes in New Guinea)
Bovine Spongiform Encephalopathy (BSE)
• Mad Cow Disease
DNA
VIRUS
GENOMES
Single Stranded
Double Stranded
RNA
+ or -
Segmented
Double Stranded Segmented
Circular
RNA Viruses
HELICAL
ICOSAHEDRAL
ENV.
ENV.
NON-ENV.
SS
SS
DS
SS
TOGAVIRIDAE
FLAVIVIRIDAE
(RETROVIRIDAE)
PICORNAVIRIDAE
CALICIVIRIDAE
REOVIRIDAE
RHABDOVIRIDAE
PARAMYXOVIRIDAE
BUNYAVIRIDAE
ARENAVIRIDAE
ORTHOMYXOVIRIDAE
CORONAVIRIDAE
(RETROVIRIDAE)
RNA VIRUSES
TOGAVIRIDAE
FLAVIVIRIDAE
BUNYAVIRIDAE
PICORNAVIRIDAE
REOVIRIDAE
RHABDOVIRIDAE
ARENAVIRIDAE
CORONAVIRIDAE
PARAMYXOVIRIDAE
ORTHOMYXOVIRIDAE
RETROVIRIDAE
DNA Viruses
ICOSAHEDRAL
ENV.
DS
HERPESVIRIDAE
HEPADNAVIRIDAE
COMPLEX
NON-ENV.
SS
PARVOVIRIDAE
ENV.
DS
PAPOVAVIRIDAE
ADENOVIRIDAE
DS
POXVIRIDAE
Virus attachment to the cell
Viral Replication (reproduction)
•
•
•
•
Genome enters host cell
Host enzymes and components used to make new viral parts
Viral genome and capsid spontaneously self-assemble
Completed, infectious viruses exit the cell
• Bacteriophage
– 1. Lytic Cycle
– 2. Lysogenic Cycle
One mechanism for viral entry into the cell: Receptor-mediated endocytosis
One Step Growth Cycle
VIRIONS PER CELL
1000
TOTAL VIRIONS
100
MULTIPLICITY
OF INFECTION
EXTRA
CELLULAR
or
RELEASED
VIRIONS
YIELD
(BURST
SIZE)
10
LATENT
PERIOD
1
ECLIPSE
VIRIONS PER
CELL
PERIOD
0.1
1
7
10
15
HOURS AFTER ADSORPTION
20
ATTACHMENT
HOST
FUNCTIONS
PENETRATION
UNCOATING
Transcription
Translation
VIRAL
LIFE
CYCLE
REPLICATION
ASSEMBLY
(MATURATION)
RELEASE
Simplified viral reproductive cycle
Entry into cell and
uncoating of DNA
DNA
VIRUS
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of new
virus particles and their
exit from cell
Lytic vs lysogenic
• Lytic cycle (virulent phage)
 Assembly (Maturation): viral particles are assembled
Release of virus burst and kills host cell (Lysis)
• Lysogenic cycle (temperate phage)
– Viral DNA integrates into host genome
• 1 phage protein prevents transcription of other phage genes
– Can be transmitted to daughter cells
– Can initiate lytic cycle in response to environmental
signal
Bacterial viruses
Bacteriophages (Phages)
• Viruses that infect bacteria
• Bacteriophages cannot reproduce and survive on
their own, must take over host cell
• Fundamentally important microbes
– Controlling bacteria populations and energy
cycling
– Gene transfer and shuffling in the environment
– Tools for molecular biology and recombinant
DNA technology
T4 bacteriophage infecting an E. coli cell
Applications of Phage Biology
• Need for alternative therapies for treating bacterial infections
– Resistance exists to every antibiotic we have
•
•
•
•
•
Phages are potent antibacterials
Self-replicating (smart drugs?)
Narrow specificity so don’t damage the normal flora
Resistance not as significant
Resurgent interest in the application of phages to agriculture
and human health
• Used for years in Eastern Europe and Russia
Classification of Bacteriophages
•
•
•
•
•
The most important criteria used for classification are phage
morphology and nucleic acid properties
Flexible tail (lambda)
Contractile tail (T4)
Morphology
–
Head shape
–
Contractile tails
–
Noncontractile tails
–
Tailless
–
Filamentous
dsDNA
ssDNA
Filamentous (fd)
ssRNA
dsRNA
Tailless (SSV-1)
Morphology: T4
Head / Capsid
DNA
Sheath (Tail)
Tail fibres
Base plate
dsDNA Phage Life Cycle
• Vast majority of phages are dsDNA
• Two life styles
– Lytic (e.g. T4)
– Lysogenic (e.g. Lambda)
Lytic Life Cycle - 1
•
Adsorption to the host cell and penetration
– Specificity of phage infection
• ~10 phages for every type of bacteria
– Phage attach to specific receptor sites on
bacterial surface
• Proteins
• Lipopolysaccharides
• Teichoic acids and cell wall components
• Carbohydrates
• Sex pilus
– Phages then inject DNA into the cell
• Tail contraction (T4)
• Injection (PRD1)
• Unknown mechanisms
Phage DNA
Bacterial DNA
Lytic Life Cycle - 2
•
Synthesis of phage nucleic acids and proteins
– mRNA molecules transcribed early in the infection are synthesized using host
RNA polymerase (1 min)
• Make viral enzymes required to take over the host cell
– Degradation of host DNA (3 min)
– Transcription of viral genes (5-9 min)
•
Phage DNA is replicated (5 min)
– Phage DNA sometimes modified protect the phage DNA from host enzymes
(restriction endonucleases) that would degrade the viral DNA
•
The assembly of phage particles
– Phage mRNA directs the synthesis of capsid proteins and other proteins involved
in assembly and release of the virus (12 min)
– DNA packaged into the head (13 min)
– Phage particles assembled (15 min)
Lytic Life Cycle - 3
• Release of phage particles (22 min – 300 new phage
particles)
– Many phages lyse their host by damaging the cell
membrane and cell wall
• Holin – enzyme which destabilizes the host cell
membrane (pokes holes)
• Lysin – phage enzyme which breaks host cell wall
(lyses host bacteria)
Lytic Life Cycle - Summary
•
•
•
•
•
Adsorption to the host cell and penetration
– Viruses attach to specific receptor sites (proteins, lipopolysaccharides, teichoic acids, etc.)
on the host cell
– Many viruses inject DNA into the host cell, leaving an empty capsid outside
Synthesis of phage nucleic acids and proteins
– mRNA molecules transcribed early in the infection (early mRNA) are synthesized using
host RNA polymerase;
•
make viral enzymes required to take over the host cell
– Transcription of viral genes follows
Phage DNA is replicated
– Phage DNA sometimes modified protect the phage DNA from host enzymes that would
degrade the viral DNA
The assembly of phage particles
– Phage mRNA directs the synthesis of capsid proteins and other proteins involved in
assembly and release of the virus
– Phage pieces assembled
– DNA packaged into the head
Release of phage particles
– Many phages lyse their host by damaging the cell wall or the cytoplasmic membrane
– A few phages (e.g., filamentous phages) are released without lysing the host cell –
secreted instead
The lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5 Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
2 Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4 Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
Head
Tails
Tail fibers
3 Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
OVERVIEW OF VIRUS
LIFE CYCLE
Adsorption
(cell Surface)
Penetration
Uncoating
(Cyto. or Nuc.)
Eclipse
Begins
Biosynthetic
Period
(Cyto. and/or Nuc.)
Genome
Replication
Assembly
(Cyto. or Nuc.)
Eclipse
Ends
Release
From
Cell
Infection of
Other Cells
The lytic and lysogenic cycles of phage , a temperate
phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
Lytic cycle
is induced
New phage DNA and
proteins are synthesized
and assembled into phages.
or
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Measuring Phage Number – Plaque Assays
• Plaque assay – method for enumerating the
number of phage particles in a sample; results
are given in plaque forming units (PFU)
agar plate
(1.5%)
Mix phage
and bacteria
The Polymerase Chain Reaction (PCR)
•
•
•
•
PCR is used to synthesize large quantities of a
specific DNA fragment in vitro (in a test tube)
Synthetic DNA molecules with sequences identical
the target sequence are created during the reaction
Replication is carried out in successive heatingcooling cycles using a heat-stable DNA polymerase
from a thermophilic bacteria
PCR has proven valuable in molecular biology,
medicine (e.g., PCR-based diagnostic tests) and in
biotechnology (e.g., use of DNA fingerprinting in
forensic science)
PCR
Human viruses
PICORNAVIRIDAE
IMPORTANT ASPECTS
1. Small RNA viruses
2. Cause variety of human
diseases, including poliomyelitis
and common cold
3. Exhibit post-translational
maturation of viral proteins
PICORNAVIRIDAE
GENERA
Characteristic Members
Enterovirus
Poliovirus, coxsackievirus
Cardiovirus
Encephalomyocarditis virus
Theiler’s virus
Rhinovirus
Rhinovirus
Aphthovirus
Non-human viruses
Hepatovirus
Hepatitis A virus
Picornaviruses
PICORNAVIRIDAE
SIZE (nm):
27
ENVELOPED:
No
CAPSID SYMM:
NUCLEIC ACID:
CLASS:
Icosahedral
RNA
IV
FORM: ss+; 3' poly(A)
5' protein (VPg)
SEG:
1
GENES:
GENOME SIZE:
6–8
7 – 8 kb
MEMBERS
poliovirus, rhinovirus, coxsackievirus, hepatitis A virus
(+) Strand Viral RNA
5
3
HN
2
POLYPROTEIN
P2
P1
1AB
1A
1C
1B
COOH
P3
1D
Production &
Maturation of
Polyprotein
REPLICATION OF GENOME
OF PLUS STRAND RNA VIRUS
(+)
5
(+)
5
3
Parental (+) Strand
3
Protein Synthesis
3
Minus Strand
Synthesis (Stage 1)
5
Plus Strand
Synthesis (Stage 2)
(+)
5
(-)
(-)
(-)
3
(+)
(+)
Progeny (+) Strands
Picornavirus
RNA Synthesis
Strategy
Enterovirus (poliovirus)
Morphogenesis
HUMAN POLIOMYELITIS
Small Intestine
Bloodstream
Day 2
Invasion
Multiplication
(Final excretion in feces
through large intestine)
CNS
EFFECT
PARALYSIS
Day 10
Invasion
Multiplication
Intraneural Spread
Day 6
Primary Viremia
Viral Multiplication
SALK
POLIO VACCINES
SABIN
Inactivated Virus
TYPE
Attenuated Live
Virus
Intramuscular
Injection
ADMINISTRATION
Oral
3 Univalent
Injections
Lifelong
DOSAGE
5 Multivalent Doses
IMMUNITY
Lifelong
IgG
DIFFERENCE
IgG, IgA
Free From Vaccine
Caused Infections
SAFETY
Small Number of
Infections Caused
By Vaccine
Hepatitis A Virus Transmission
• Close personal contact
(e.g., household contact, sex contact, child
day care centers)
• Contaminated food, water
(e.g., infected food handlers, raw shellfish)
• Blood exposure (rare)
(e.g., injecting drug use, transfusion)
Geographic Distribution of HAV Infection
Anti-HAV Prevalence
High
Intermediate
Low
Very Low
Caliciviruses
Togaviruses
TOGAVIRIDAE
SIZE (nm):
ENVELOPED:
CAPSID SYMM:
NUCLEIC ACID:
CLASS:
70
Yes
Icosahedral
RNA
IV
SEG:
ss+; 5' cap
3' poly(A)
1
GENES:
GENOME SIZE:
7-8
10 – 12 kb
FORM:
MEMBERS
Sindbis virus, Semliki Forest virus, WEE, EEE, rubella virus
Togavirus protein
expression
Togavirus
RNA Synthesis
Strategy
TOGAVIRUSES
EARLY
LATE
RNA Released from
nucleocapsid
42s RNA
(genomic)
26s RNA
(subgenomic)
NON-STRUCTURAL
PROTEINS
STRUCTURAL
(or VIRION)
PROTEINS
Flaviviruses
FLAVIVIRIDAE
SIZE (nm):
ENVELOPED:
CAPSID SYMM:
NUCLEIC ACID:
CLASS:
45 - 60
Yes
Icosahedral
RNA
IV
FORM: ss+
SEG:
1
GENES: 8 - 10
GENOME SIZE:
10 - 11 kb
MEMBERS
Yellow fever virus, Dengue virus, West Nile virus
St. Louis encephalitis virus, Japanese encephalitis virus
Flavivirus
RNA Synthesis
Strategy
Key Clinical Facts about West Nile Virus in North America
West Nile virus infection is a mosquito-borne infection with a rapidly
expanding geographic distribution.
One in 5 infected individuals develops mild febrile illness; 1 in 150
develops meningitis, encephalitis, or both.
Advanced age is by far the greatest risk factor for severe neurologic
disease, long-term morbidity, and death.
Presence of West Nile virus-infected birds, onset of meningitis or
encephalitis in late summer or early fall, and profound muscle weakness
provide important diagnostic clues.
IgM antibody capture ELISA testing of cerebrospinal fluid or serum is the
most efficient diagnostic method; testing is available through state and
local health departments; false-positive results may occur after other flavivirus infections or vaccinations.
Rapid reporting of possible cases to health departments is essential to guide
public health control efforts.
From: Petersen and Marfin (2002) Ann. Intern. Med. 137:173-179
Transmission cycle of West Nile virus
From: Petersen and Marfin (2002) Ann. Intern. Med. 137:173-179
Culex mosquito laying eggs
Symptoms of West Nile Virus Reported among Hospitalized Patients during
Outbreaks in New York State (1999), Romania (1996), and Israel (2000)
Petersen, L. R. et. al. Ann Intern Med 2002;137:173-179
Schematic of virologic and serologic tests in West Nile virus encephalitis
Gea-Banacloche, J. et. al. Ann Intern Med 2004;140:545-553
West Nile Virus Disease in the United States, 1999-2003
Gea-Banacloche, J. et. al. Ann Intern Med 2004;140:545-553
2003 West Nile Virus Activity in the United States
2004 West Nile Virus Activity in the United States
(reported to CDC as of October 26, 2004)
Electron micrograph of West Nile virus
isolated from brain tissue from a crow
found in New York
Magnification: 65,625 X
Features of Hepatitis C Virus Infection
Incubation period
Acute illness (jaundice)
Case fatality rate
Chronic infection
Chronic hepatitis
Cirrhosis
Mortality from CLD
Average 6-7 weeks
Range 2-26 weeks
Mild (<20%)
Low
75%-85%
70% (most asx)
10%-20%
1%-5%
Hepatitis C Virus Infection, United
States
New infections (cases)/year 1985-89
1998
242,000 (42,000)
40,000 (6,500)
Deaths from acute liver failure
Rare
Persons ever infected (1.8%)
3.9 million (3.1-4.8)*
Persons with chronic infection
2.7 million (2.4-3.0)*
Of chronic liver disease - HCV-related
40% - 60%
Deaths from chronic disease/year
8,000-10,000
.
%95*Confidence Interval
Transmission of HCV
• Percutaneous
– Injecting drug use
– Clotting factors before viral inactivation
– Transfusion, transplant from infected donor
– Therapeutic (contaminated equipment, unsafe
injection practices)
– Occupational (needlestick)
• Permucosal
– Perinatal
– Sexual
ARBOVIRUSES
Group of Viruses
Containing Several Families of
ARthropod-BOrne
And Related Viruses
ARBOVIRUS
Included Families
TRANSMITTED
By
MOSQUITOES
Or
TICKS
Togaviridae
Flaviviridae
Bunyaviridae
Reoviridae
CORONAVIRIDAE
SIZE (nm):
ENVELOPED:
80 - 120
Yes
CAPSID SYMM:
Helix
NUCLEIC ACID:
CLASS:
RNA
FORM:
SEG:
GENES:
GENOME SIZE:
IV
ss+; 5' cap
3' poly(A)
1
7
27 - 32 kb
MEMBERS
human respiratory coronavirus, SARS-CoV
mouse hepatitis virus, infectious bronchitis virus (avian)
Coronavirus Replication Scheme
Electron micrograph
of SARS-CoV virion
Enhanced electron micrograph of SARS-CoV virions
Updated Interim U.S. Case Definition for Severe Acute
Respiratory Syndrome (SARS)
CDC 7/03
Clinical Criteria
Asymptomatic or mild respiratory illness
Moderate respiratory illness
Temperature of >100.4°F (>38°C)*, and
One or more clinical findings of respiratory illness (e.g., cough, shortness of
breath, difficulty breathing, or hypoxia).
Severe respiratory illness
Temperature of >100.4°F (>38°C)*, and
One or more clinical findings of respiratory illness (e.g., cough, shortness of
breath, difficulty breathing, or hypoxia), and
radiographic evidence of pneumonia, or
respiratory distress syndrome, or
autopsy findings consistent with pneumonia or respiratory distress
syndrome without an identifiable cause
SARS Epidemiologic Criteria
Travel (including transit in an airport) within 10 days of onset of symptoms to an
area with current or previously documented or suspected community
transmission of SARS [China (Mainland); Hong Kong; Hanoi, Vietnam;
Singapore; Toronto, Canada; Taiwan],
or
§
Close contact within 10 days of onset of symptoms with a person known or
suspected to have SARS.
§ Close contact is defined as having cared for or lived with a person known to have
SARS or having a high likelihood of direct contact with respiratory secretions
and/or body fluids of a patient known to have SARS. Examples of close contact
include kissing or embracing, sharing eating or drinking utensils, close
conversation (<3 feet), physical examination, and any other direct physical
contact between persons. Close contact does not include activities such as
walking by a person or sitting across a waiting room or office for a brief period
of time.
SARS Laboratory Criteria
Confirmed
Detection of antibody to SARS-associated coronavirus (SARS-CoV) in a
serum sample, or
Detection of SARS-CoV RNA by RT-PCR confirmed by a second PCR assay,
by using a second aliquot of the specimen and a different set of PCR
primers, or
Isolation of SARS-CoV.
Negative
Absence of antibody to SARS-CoV in a convalescent–phase serum sample
obtained >28 days after symptom onset.**
Undetermined
Laboratory testing either not performed or incomplete.
SARS Cases in the United States,
Spring 2003
Type of Case
No.
CoV+*
CoV-*
Pending
Probable
74
8
38
28
Suspect
344
0
169
175
*Based on presence of absence of SARS antibody at > 28 days
Lessons Learned:
Key Epidemiologic Features
•
•
•
•
SARS can spread rapidly around the world
Healthcare facilities played central role
Most cases were spread person-to-person
Vast majority of febrile respiratory infections in
U.S. were not SARS
Positive strand RNA virus comparison
Virus family
Envelope
Polyproteins Subgenomic mRNAs
Picornaviruses
No
Yes
No
Caliciviruses
No
Yes
No
Togaviruses
Yes
Yes
Yes (1)
Flaviviruses
Yes
Yes
No
Coronaviruses
Yes
Yes
Yes (6 – 7)
Examples of
cytopathic effects
of viral infection
of animal cells
PLAQUE REDUCTION TEST
Used to determine Ab titer to a given virus in a test serum:
1. Use Dilution Of Virus Stock That Yields A Fixed # Of
Plaques On Susceptible Monolayers Of Cultured Cells.
2. Mix This Dilution of Virus Stock With Serial Dilutions Of
The Test Serum.
3. Plate Out Dilutions On Cell Monolayers.
4. If Ab Present, Will Neutralize Virus And Prevent Plaque
Formation.
5. Quantitate Ab Titer As The Highest Dilution That Will
Neutralize The Plaque Formation Of the Specific Virus
Dilution Being Used.
Particle-to-plaque-forming-unit ratios of some animal viruses
Plant Viruses
Infection by tobacco mosaic virus (TMV)