Download General Properties of virus

General Properties of
Viruses
Dr. Yagoub Hmadt Allah
Ass. Prof. medical microbiology
General Properties of virus
It is very smallest infectious agent (20 –350nm)
 Obligate intracellular parasites
 Contain only one type of nucleic acid, either
DNA or RNA
 Do not possess cellular organization
 Lacks enzymes necessary for protein & NA
synthesis
 Depends on host cell machinery for replication
 Causes a large no. of human diseases ranging
from minor ailments like common cold to
terrifying
diseases
such as rabies, HIV etc.
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Microbiology

General Properties
– size, structure, shape,
chemical properties, resistance
 Replication
 Hemagglutination
 Cultivation
 Viral assay
 Viral infections: virus-host interactions
 Morphology
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Morphology - Size

Much smaller than bacteria

“Filterable agents” – can pass through filters
that can hold back bacteria

Vary widely in size:
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Largest – poxvirus (300nm)
Smallest – parvovirus (20nm)
Virion – extracellular infectious virus particle
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Morphology - Structure of Virus
E
L
E
C
T
R
O
N
Nucleic acid
Capsid
Envelope
Peplomer
Size in nanometers
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M
I
C
R
O
S
C
O
P
E
Morphology – Structure &
Shape of a virus

Nucleic acid & capsid with or without envelope.

Capsid – the protein coat surrounding the
nucleic acid core. It
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protects nucleic acid from inactivation
helps to introduce viral genome into host cell

Capsomers - the repeating protein subunits that
make up the capsid

Protomers – the polypeptide chains which
make up the capsomers
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4. Mature
Capsid
1. Protomers
2. Capsomers
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3. ProCapsid
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Morphology – Structure & Shape
of a virus: Capsid
– symmetrically arranged to
form an impenetrable shell (capsid)
around the nucleic acid core.
 Capsomers
 This
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symmetry is of two types:
Icosahedral (cubical)
Helical
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Morphology – Structure & Shape
of a virus: Capsid

Icosahedron – a polygon with 12 corners
(vertices) & 20 sides (facets)
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Side – equilateral triangle
Two types of capsomers form the capsid
Pentagonal capsomers form the vertices
Hexagonal capsomers form the sides.

Helical – the capsomers & nucleic acid
are wound together to form a helical or
spiral tube.

The overall shape of virus is quite
variable, but mostly they are spherical.
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Morphology – Structure & Shape
of a virus: Envelope
May or may not be present
 Derived from the host cell membrane
 Lipoprotein in nature – lipid is of host cell origin
while protein is from virus.
 Protein subunits seen as projecting spikes on
the surface of envelope – called Peplomer.
 A virus may have more than one type of
peplomer e.g. influenza virus.
 Confers chemical, antigenic & biological
properties.
 Susceptible to lipid solvents

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Resistance

Very heat labile but stable at low temperatures

Inactivated within seconds at 56C.

Can be kept frozen at -70C for long term storage.

Inactivated by sunlight, UV rays & ionising radiations.

More resistant than bacteria to chemical disinfectants.

Most active antiviral agents (virucidal) – oxidising agents
like hydrogen peroxide, potassium permangnate,
hypochlorites
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Viral replication
 Viruses
have no metabolic activity of
their own. Therefore, they depend on
living cells for providing energy and
synthetic machinery for synthesis of :
 1- viral nucleic acid , 2- viral protein.
 The virus genome provides the host cell
with the genetic information needed for
its replication.
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The sequence of events for virus replication
Stages in virus replication begin when virions infect cells
Attachment/
Adsorption
Penetration
Uncoating
Biosynthesis
Maturation &
Assembly
Release
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•1- Attachment: virus and cell are brought into contact by random
collision, but attachment occurs if the cell membrane contains specific
receptors for the virus, e.g HIV binds to CD4 receptors on helper T cells.
•2- penetration and uncoating: non envoloped virions are taken into
animal cells by endocytosis where they are uncoated by lysosomal
enzymes.
Enveloped viruses penetrate the membrane by fusion between the virus
envelope and the cell membrane releasing the nucleocapsid into the
cell; uncoating may occur at the cell surface. Uncoating renders viral
nucleic acid accessible for transcription and replication.
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Pathways for Viral Entry into Host Cell
Surface
Fusion
Receptor-Mediated Endocytosis
Fusion in
Endosome
Lysis of
Endosome
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Viral Replication
3- Eclipse phase : it is the period after
penetration during which no infectious
virus componant can be detected inside
the host cell. During this phase the cell
rediracted toward synthesizing early
proteins (enzymes) which are essential for
viral replication. {from the stage of
penetration till the appearance of mature
daughter virions, the virions cannot be
detected inside the host cell}.
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4- Intracellular viral synthesis: it includes
synthesis of both viral nucleic acid and
proteins. The viral nucleic acid
(genome) replicates by using strand of
the parental nucleic acid as a template
for the production of progeny DNA or
RNA molecules.
The essential step in protein synthesis is
transcription of mRNA from viral nucleic
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acid.
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The mRNA is translated to viral proteins
(capsid and enzymes) using host cell
ribosomes. The mRNA transcription
varies depending on the nucleic acid type:
a. In double stranded (ds) DNA viruses,
mRNA is transcribed from DNA by
DNA dependent RNA polymerase (
transcriptase).
+ss DNA
ds DNA-ss DNA RNA polymerase
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mRNA
b. In single stranded RNA viruses of
positive polarity (+ sense) the ssRNA
itself acts as mRNA for translation into
proteins. + ssRNA = mRNA
c. The ssRNA viruses of negative polarity ( - sense) must be
transcribed by RNA dependent RNA
polymerase – which is present in the
virus into complementary (+ sense) m
RNA . – ssRNA RNA polymerase + ssRNA = mRNA
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d. The ssRNA of retroviruses (+ sense) is
transcribed by a unique virion associated
reverse transcriptase into complementary
ssDNA, which is converted into dsDNA,
which becomes integrated into the cellular
genome causing malignant transformation of
cells in vivo and in vitro. It may be transcribed into
mRNA.
+ssRNA Reverse transcriptase
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ssDNA
dsDNA
Integrated
Transcribed to mRNA
e. Assembly of viral nucleic acid and protein
coats to form mature virus particles occurs in
the cytoplasm ( e.g. poliovirus) or in the
nucleus e.g herpes viruses.
f. Release: mature virus particles will
accumulate in the cell in enonmous number
and are liberated by rupturing the cell i.e
cytolysis. The viruses may release by a slow
process of leaking or budding through the
cell membrane. Enveloped viruses will
acquired
lipoprotein
envelope
during
budding.
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Classification of medical important viruses:
 The
classification of viruses depends on
their structure, antigenic composition
and other properties. Viral famillies and
genera have been designated, though
differentiation into species is still
incomplete.
 Viruses are classified into two major
divisions depending on the type of
nucleic acid.
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 1-
deoxyriboviruses, which contain DNA.
 2- riboviruses, which contain RNA.
Both of these are further subdivided
mainly on the basis of size and shape of
the virion, symmetry of the nucleocapsid
and strandness of the nucleic acid.
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Dr yagoub hamadta Allah Microbiology
Major families of viruses are briefly are:
A- DNA viruses:
1. parvoviridae:
- Size
: 18 – 26 nm
- Symmetry
: icosahedral
- Envelope
: absent
- DNA
: single stranded
- Example
: parvovirus which
cause gastroenteritis and haemolytic
disease
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 2-
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papovaviridae:
Size
: 40 – 55 nm
Symmetry
: icosahedral
Envelope
: absent
DNA
: double stranded
Example
: i.papilloma virus which
causes cutaneous, genital and
laryngeal warts.
ii. Polyomavirus which cause
neurological diseases
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3- Adenoviridae:
- Size
: 70 – 90 nm
- Symmetry
: icosahedral
- Envelope
: absent
- DNA
: double stranded
- Example
: adenovirus.some can
cause respiratory disease and
conjunctivitis.
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4- herpesviridae:
- Size
: 100 – 200 nm
- Symmetry
: icosahedral
- Envelope
: present
- DNA
: double stranded
- Example
: i. herpes simplex virus
- ii. Varicella /zoster viruses.
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5- poxviridae:
- Size : 300 – 450 nmX170 -260nm(brick shaped)
- Symmetry
: unknown
- Envelope
: present
- DNA
: double stranded
- Example
: poxviruses –Variola,
cowpox, monkey pox…..
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6- hepadnairidae:
- Size
: 42 nm
- Symmetry
: unknown
- Envelope
: present
- DNA
: partially double stranded
- Example
: hepatitis – B virus
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Riboviruses(RNA viruses):
RNA viruses included:
1- Picornaviridae.
2- Reoviridae (double stranded).
3- Orthomyxoviridae.
4- Paramyxoviridae.
5- Rhabdoviridae.
6- Bunyaviridae.
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7- Coronaviridae.
8- Togaviridae.
9 Arenaviridae.
10- Retroviridae.
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Abnormal Replicative Cycles

Incomplete viruses - A proportion of daughter
virions are not infective, due to defective
assembly.

Defective viruses – genetically defective, unable
to give rise to fully formed progeny.

Abortive infection – defect in the type of cell
(non permissive cell), not in the parental
viruses.
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Viral Hemagglutination

Hemagglutination
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Reversal of hemagglutination – Elution
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Originally seen with the Influenza virus by Hirst in
1941.
A convenient method of detection & assay of
Influenza virus.
Due to the presence of Hemagglutinin spikes on the
surface.
Due to the presence of Neuraminidase enzyme,
Receptor Destroying Enzyme (RDE)
Destruction of receptor – reversal of
hemagglutination – release of virus from the red cell
surface
Found only in Myxoviuses.
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Virus Culture
Embryonated Egg
Chorioallantioc membrane (CAM)
Allantoic cavity
Amniotic cavity
Yolk Sac
Cell Lines/
Tissue cultures
Primary
Diploid/ Secondary
Continuous
Animal inoculation
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Suckling mice
Embryonated Hen’s Egg

Chorioallantoic membrane (CAM) – visible lesions
called pocks. Each infectious virus particle forms one
pock. e.g. Variola, Vaccinia virus
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Allantoic cavity – Influenza virus (vaccine production) &
paramyxoviruses
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Amniotic cavity – primary isolation of Influenza virus
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Yolk sac – Chlmyadia, Rickettsiae & some viruses
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Embryonated Hen’s Egg
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Viral Assay

Viral content of a specimen: Total no. of
1.
2.
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Assay of Infectivity: two types
1.
2.
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Virus particles – EM, HA
Infectious virions only
Quantitative assays – actual no. of infectious
particle in an inoculum
Quantal assays – indicate the presence or
absence of infectious viruses, carried out in
animals, eggs or tissue cultures
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Viral Assay
Assay of Infectivity:
Quantitative assays

Plaque assay in monolayer cell
cultures
Pock assay on CAM
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*Each plaque/ pock represents one
infectious virus.

Plaques are clear zones that
develop on lawns of host cells.

The virus plaque is analogous to
the bacterial colony.
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