viruses general properties
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General Properties of Viruses
Kalpesh Zunjarrao
Viruses do not fall under any category of unicellular organisms because:
o Do not possess cellular organisation
o Contain only 1 type of N. A. (DNA / RNA)
o Obligate intracellular parasitesLack enz necessary for protein & n. a. synthesisFor replication depend on synthetic machinery of host cell
o Multiply by complex method
MORPHOLOGY
Size: Extracellular infectious viral particle is called ‘Virion’
Viruses are much smaller than bacteriaFor a time, they were known as ‘filterable agents’
Can not be seen under light microscope
Size range: 20-300 nm
Parvovirus: 20 nm Pox virus: 300 nm (can be seen under light microscope)
Estimation of Size: Earliest method:o Passing through membrane filter of graded pore sizeo Average pore size of finest filter that allows passage of
virion gave an estimate of size
Next method:o Ultracentrifuge: depending on rate of sedimentation,
particle size was calculated
Latest & direct method:o Electron microscope
Structure: Virion consists of nucleic acid core surrounded by protein
coat called ‘capsid’
Capsid: made up of subunits called ‘Capsomers’
Genome + capsid: nucleocapsid
Functions of capsid:o Protection of n.a. core from inactivation by nucleaseso Introduction of viral genome into host by adsorbing on the
host cell surfaceo Antigenic in nature
Types of symmetries:1. Icosahedral: Icosahedron: polygon with 12 vertices or
corners & 20 facetes or sides Each facete has shape of equilateral triangle It’s a rigid structure
2. Helical: N.A. & capsomers are wound together to form
helical tube Tube can be rigid or pliable
Some viruses show complex symmetry
Viruses can be enveloped or non-enveloped Envelope is lipoprotein in natureo Lipid derived from host cello Protein: virus codedo Protein subunits are seen as projecting spikes on surface of
envelope: called ‘Peplomers’
Overall shape of virus varies with different groups of viruses:
o Most animal viruses: spherical, some: irregularo Rabies virus: bullet shapedo Ebola virus: filamentouso Pox virus: brick shapedo TMV: rod shapedo Bacteriophage: complex morphology
Chemical properties: Nucleic acids:o Viruses contain only 1 type of n. a.o Single or double stranded RNA or DNAo N. A. can be extracted by treatment with certain chemicals Proteins:o Capsid & envelopeo Protects n.a. & determines antigenic properties Some viruses contain small amount of carbohydrates Most viruses don’t possess enzymes but some of them
may possess (neuraminidase, reverse transcriptase)
Viral hemmaglutination: Large number of viruses agglutinate erythrocytes of many
species Hemmaglutination by influenza virus is due to presence of
protein spikes ‘Hemagglutinin’ Hemagglutinin has ability to bind glycoprotein receptor
sites on erythrocytes Convenient method of detection of viruses Procedure:
RBCs are added to serial dilutions of viral suspension → highest dilution producing hemmaglutination is ‘titre’
Non agglutinated RBCs settle down at bottom in the form of ‘button’
Agglutinated RBCs spread into shield like pattern
ButtonTitre
Hemagglutination is inhibited by Antibodies to virus.This principle can be used in ‘Hemagglutination inhibition test’.This test is used for detecting antiviral antibodies
Some viruses carry surface enzymes (neuraminidase) which act on receptors on erythrocytes- They are called ‘Receptor destroying enzymes’ (RDE)- Destruction of receptor leads to reversal of hemagglutination. Called as ‘Elution’
MULTIPLICATION
Genetic information required for viral replication is present in viral NA but they lack enzymes
Viruses depend on synthetic machinery of host cell Viral multiplication cycle is divided into 6 sequential
phases:o Adsorptiono Penetrationo Uncoatingo Biosynthesiso Maturationo Release
Adsorption (Attachment): Contact between virion & host cell: by random collision Adsorption takes place only if there is affinity between
them Cell surface contains some receptors to which viruses can
attach In case of influenza viruses: hemagglutinin on virus
surface attaches to glycoprotein receptors sites on respiratory epithelium
Destruction of receptors by RDE prevents viral adsorption In HIV virus: attachment between CD4 receptors on host
cell & viral surface glycoprotein ‘gp120’ Susceptibility to viral infection depends on presence or
absence of receptors on cells
Penetration: Bacterial cells possess rigid cell wall. Thus, viruses can
not penetrate into the cell. Only nucleic acid is introduced Animal cells → no cell wall → whole virus can enter into
the cell Virus particle may be engulfed by process resembling
phagocytosis, called ‘Viropexis’ In case of enveloped viruses: viral envelop fuse with
plasma membrane of host cell → nucleocapsid released into the cytoplasm
Uncoating: Stripping the virus of its outer layer & capsid In most cases, uncoating is effected by action of lysosomal
enzymes In pox virus: Uncoating is 2 step process.
1st step in phagosome: outer coat removed by lysosomal enz2nd step in cytoplasm: viral uncoating enz removes protein covering
Biosynthesis: Synthesis of viral nucleic acid, protein capsid & various
enzymes required for synthesis, assembly & release Certain ‘regulator proteins’ are also synthesized Regulator proteins: shut down normal cellular metabolism &
stimulates production of viral components Site of viral synthesis depends on type of virus Most DNA viruses: synthesize n. a. in host cell nucleus
(exception: poxvirus which synthesizes all components in host cytoplasm)
Most RNA viruses: synthesize all components in cytoplasm(Exceptions: Orthomyxoviruses, some paramyxoviruses synthesized partly in nucleus)
Proteins: always synthesized in cytoplasm
Biosynthesis consists of following steps:1. Transcription of mRNA from viral nucleic acid2. Translation of mRNA into ‘early proteins’
Early/non-structural proteins are enzymes which initiate & maintain synthesis of virus componentsThey may also shut down production of host proteins
3. Replication of viral nucleic acid4. Synthesis of ‘late / structural proteins’ required for viral
capsid
Critical step in biosynthesis: transcription of mRNA from viral nucleic acid
Once this is achieved, host cell resources can be used for translating mRNA into viral components
Maturation (Assembly): Assembly of daughter virions follows the synthesis of
viral nucleic acid & proteins Assembly may take place in cytoplasm or nucleus Herpes & Adenoviruses are assembled in nucleus Picorna & Poxviruses are assembled in cytoplasm At this stage, non-enveloped viruses are present
intracellularly as fully developed virions but in case of enveloped viruses, only nucleocapsid is complete
Envelops are derived from host cell membrane during process of budding
Host cell membrane that becomes envelope is modified by addition of virus-specific antigens
Release: In case of bacteriophages, release takes place by lysis of
bacterium In animal viruses, release usually occurs without cell lysis Certain viruses are released by process of budding from the
cell membrane over period of time. Host cell is unaffected & may even divide, daughter cells continuing to release virions
Progeny virions released into surrounding medium may infect other cells
In case of some viruses, transmission occurs directly from cell to cell, very little free viruses being demonstrable extracellularly in the medium
Poliovirus causes profound damage to host cell & may be released by cell lysis
From the stage of penetration till appearance of mature daughter virions, virus can not be demonstrated inside host cell
This period during which virus seems to disappear or go ‘underground’ is called as ‘eclipse phase’
Single life cycle of replication takes 15-30 mins in bacteriophages & about 15-30 hours for animal viruses
Single infected cell releases large number of progeny viruses. This can be demonstrated in bacteriophages but difficult in case of animal viruses which are released over a prolonged period
CULTIVATION
Viruses: Obligate intracellular parasites
Can not be grown on inanimate culture medium
3 methods employed for cultivation of viruses:o Inoculation into animalso Embryonated eggso Tissue culture
Animal Inoculation: Earliest method for cultivation: human volunteers → high
risk involved → used only when virus is relatively harmless
Monkeys were used for isolation of poliovirus → limited application due to cost
Use of white mice: most widely employed in virology Guinea pigs, rabbits: used in some situations Growth of virus in animal can be indicated by death,
disease or visible lesion Disadvantage: immunity may interfere with viral growth
Embryonated eggs: Embryonated hen’s egg was 1st used for cultivation by
Goodpasture (1931) & method was further developed by Burnet
Embryonated eggs offer several sites for cultivation of viruses
Inoculation on chorioallantoic membrane (CAM)o produces visible lesions (pocks)o Different viruses: different pock morphologyo Each infectious viral particle can form 1 pock. Thus, pock
counting can be used for assay of pock-forming viruses like variola & vaccinia
Inoculation into allantoic cavity:o provides rich yield of influenza & paramyxoviruseso Used for growing influenza virus for vaccine production Inoculation into amniotic sac: used for primary isolation of
influenza virus Yolk sac inoculation:
for cultivation of some viruses, Chlamydiae & Rickettsiae
Tissue culture: Tissue & organ culture: used for study of morphogenesis
& wound healing 1st application of tissue culture in virology: for
maintaining vaccinia virus in fragments of rabbit cornea Major obstacle in using tissue culture: bacterial
contamination Antibiotics: prevention of contamination Every human virus can be grown in tissue culture
Types of tissue culture: Organ culture:o Small bits of organs can be maintained in vitro preserving
their architecture & functiono Useful for isolation of viruses which appear to be
specialised parasites of certain organso Tracheal ring organ culture: for coronavirus isolation
Explant culture:o Fragments of tissues can be grown as explants embedded
in plasma clots or in suspensiono Originally known as tissue cultureo Adenoid tissue explant culture: for Adenovirus isolation
Cell culture:o Routinely employed for growing viruseso Tissues dissociated into cells by proteolytic enzymes →
cells washed → counted → suspended in growth mediumo Cell culture medium components:
Amino acids, vitamins, salt, glucose, buffer (HCO3-), fetal
calf serum, antibiotics & phenol redo Cell suspension is dispensed in bottles/ petri plates →
incubated → cells adhere to glass surface → form monolayer of cells
o Bottles incubated at stationary condition or in roller drums for aeration
Cell cultures are classified into 3 types:1. Primary cell culture:o Normal cells freshly taken from bodyo Capable of only limited growth in cultureo Eg: monkey kidney, human embryonic kidney, Human
amnion
2. Diploid cell culture:o Cells of single cell type that retain original diploid
chromosome number & karyotypeo Can be subcultured for limited number of times (due to
senescence)o Eg: Human fibroblasts
3. Continuous cell culture:o Cells of single cell type derived from cancer cellso capable of continuous serial cultivationo Eg: HeLa, HEp-2, Vero cell lines
Detection of virus growth in cell cultures:
Cytopathic effects (CPE):o Morphological changes in cultured cells → can be
observed by microscopic examinationo Viruses causing CPE are called ‘cytopathogenic viruses’o Help in presumptive identification of viruseso Eg:
- Enteroviruses produce rapid CPE by crenation of cells- Measles virus produces syncytium- Adenovirus produces large granular clumps
Metabolic inhibition:o In normal cell cultures, medium becomes acidic due to
metabolismo When viruses grow → metabolism inhibited → no acid
Hemadsorption:o Hemagglutinating viruses can be identified by addition of
guinea pig erythrocyteso If viruses are multiplying, erythrocytes adsorb onto cell
surface
Interference:o Growth of non-cytopathogenic virus in cell culture can be
tested by subsequent challenge with known cytopathogenic virus
o Growth of first inhibits infection by second virus
Transformation:o Tumor forming viruses induce cell transformation → loss
of contact inhibition → piled-up growth ‘Microtumors’
Immunofluorescence:o Cells from virus infected cultures → stained by
fluorescent conjugated antiserum → examined under UV microscope
CLASSIFICATION&
NOMENCLATURE
Till 1950, little was known about viruses They were named haphazardly, based on the disease they
caused or site of isolation They were grouped according to tropism or affinity to
different organs Were classified as Dermotropic, Neurotropic,
pneumotropic & viscerotropic Bawden suggested that nomenclature & classification
should be based on properties of viruses & not the responses
Classification & nomenclature are now official responsibility of International committee on Taxonomy of Viruses (ICTV)
Viruses are classified into 2 main divisions:Riboviruses & Deoxyriboviruses
Further classification is based on properties such as strandedness of n.a., symmetry of nucleocapsid, presence of envelop, size & shape of virion & number of capsomers
DNA viruses: Poxviridae family:o Large, brick-shaped or ovoid (300 X 240 X 100 nm)o Complex structure, having lipid containing outer coat & core
carrying single linear ds-DNAo Multiplication & maturation: in cytoplasmo Several genera
Herpesviridae family:o Medium sized containing linear ds-DNAo Icosahedral nucleocapsid has 162 capsomers surrounded by
lipid containing envelopeo Multiplication: in nucleuso Maturation: by budding through nuclear membraneo Only 1 genus: Herpesvirus
Adenoviridae family:o Medium sized (70-90 nm) non-enveloped, icosahedral
viruses with 252 capsomerso 2 genera: Mastadenovirus & Aviadenovirus
Papovaviridae family:o Small (40-55 nm), non-enveloped, ds-DNA viruses with
72 capsomerso 2 genera: Papillomavirus & Polyomavirus
Parvoviridae family:o Very small (18-26 nm) non-enveloped, ss-DNA viruses
with 31 capsomerso 3 genera: Parvovirus, Adenosatellovirus, Densovirus
Hepadnaviridae family:o Spherical (42 nm) virus with core surrounded by envelope
having specific antigenso Human Hepatitis type B virus & related viruses of animals
RNA viruses: Picornaviridae family:o Small (20-30 nm), non-enveloped, icosahedral, ss-RNA
viruseso 3 genera: Enterovirus, Rhinovirus, Hepatovirus (HAV)
Orthomyxoviridae family:o Medium sized (80-120 nm), spherical or elongated,
enveloped viruses with hemagglutinin & neuraminidase peplomers
o Genome consists of ss-RNA in several pieceso 1 genus: Influenzavirus
Paramyxoviridae family:o Pleomorphic virions with lipid envelope having surface
projectionso Genome: un-segmented, linear ss-RNAo 3 genera: Paramyxovirus, Morbillivirus, Pneumovirus
Togaviridae family:o Spherical viruses (40-70 nm) with lipoprotein envelope &
ss-RNAo Multiply in arthropods & vertebrateso 3 genera: Alphavirus (Group A arboviruses), Rubivirus,
Pestivirus
Flaviviridae family:o Formerly grouped as group B arboviruses under togaviridae
Bunyaviridae family:o Spherical, enveloped virions (90-100 nm)o Arthropod-borne viruseso 5 genera: Bunyavirus, Hantavirus, Nairovirus, Phlebovirus,
Ukuvirus
Arenaviridae family:o Spherical or pleomorphic viruses (50-300 nm) with number of
electron dense particles giving sandy appearanceo Rodent parasites but can infect humans rarelyo 1 genus: Arenavirus
Rhabdoviridae family:o Bullet shaped viruses (130-300 nm long & 70 nm wide) with
lipoprotein envelope carrying peplomerso 2 genera: Vesiculovirus, Lyssavirus
Reoviridae family:o Icosahedral, non-enveloped viruses (60-80 nm) with double
layered capsido Genome: ds-RNA in 10-12 pieceso 3 genera: Reovirus, Orbivirus, Rotavirus
Coronaviridae familyo Pleomorphic, enveloped viruses (100 nm) with club-shaped
peplomers. Only 1 genus: Coronavirus
Retroviridae family:o Icosahedral viruses (100 nm) with lipoprotein envelopeo They have RNA dependent DNA polymerase (Reverse
transcriptase)o 3 subfamilies: Oncovirinae, Splumivirinae, Lentivirinae
Calciviridae family:o Naked spherical particles (35-39 nm) with 32 cup shaped
depressions arranged in symmetry
Filoviridae family:o Long, filamentous, enveloped viruses (80 nm in diameter &
14,000 nm long) with helical nucleocapsid & ss-RNA
Thank You
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