Presentation comprises of introductory information on virus, related terminology, its composition and structure, classification, nomenclature and taxonomy for under graduate students.

1. INTRODUCTION TO
VIRUSES
Dr. Vinod Kumar Singh
Department of Vet. Microbiology
DUVASU, Mathura

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Terminology
1. Virus (latin : slimy liquid or poison) is a broad general
term for any aspect of the infectious agent and includes:
• the infectious or inactivated virus particle
• viral nucleic acid and protein in the infected cell
2. Virion is the physical particle in the extra-cellular phase
which is able to spread to new host cells; complete
intact virus particle

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Sub-viral agents
3. Satellites
• Contain nucleic acid
• Depend on co-infection with a helper virus
• May be encapsidated (satellite virus)
• Mostly in plants, can be human e.g. hepatitis delta virus
• If nucleic acid only = virusoid
4. Viroids
• Unencapsidated, small circular ssRNA molecules that replicate autonomously
• Only in plants, e.g. potato spindle tuber viroid
• Depend on host cell polII for replication, no protein or mRNA
5. Prions
• No nucleic acid
• Infectious protein e.g. BSE

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Bacteriophage
• Bacteriophage is any one of viruses that infect
bacteria
• They do this by injecting genetic material, which
they carry enclosed in an outer protein capsid.
• The genetic material can be ssRNA, dsRNA,
ssDNA, or dsDNA along with either circular or
linear arrangement

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Bacteriophage T4
20 Sided polyhedron
Double-stranded
Locate bacterium
Anchor to bacterium
Needle which injects DNA into bacteria

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Limits of
resolution
Electron Microscope
0.003 m
Light
Microscope
0.2 m
Unaided
Human eye
40 m
Microscopic
protozoa & fungi
4 - 40 m
Classes of
organisms
Bacteria
0.1 - 10 m
Viruses
0.03 - 0.3 m
0.001 0.01 0.1 1 10 100
Size (m)
Relative size of Microorganisms

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Relative size of Microorganisms

8. • China 1000 B. C.
– Prevention without knowledge of the agent, based on
recognition that survivors of smallpox were
subsequently protected against disease
– Variolation: Inoculation of healthy individuals with
dry material from smallpox pustules
• Roman Empire was weakened by viral
epidemics of measles and smallpox between 160
A.D. through 266 A.D
• Smallpox helped destroy the Aztec Empire
during the conquest of Mexico by Spain
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History

9. • 1718 - Lady Marry Wortley Montagu dicovered the
Ottoman Empire concept of variolation and brought
the idea to Britain
• 1796 - E. Jenner performed his vaccination studies
showing smallpox infection could be stopped by prior
infection with cowpox
• 1885 - L. Pasteur experimented with rabies
vaccination, using the term virus to describe the agent
– Although Pasteur did not discriminate between
viruses and other infectious agents, he originated the
terms virus and vaccination in honor of Jenner
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History

10. • 1879 - Adolf Mayer
• named tobacco mosaic disease based on the dark and light spots on infected
leaves
• Experiment on transmission of disease by inoculating extracted juice from
infected leaves
• Unable to culture the agent
• 1892 – Dimitri Ivanowski
• repeated Mayer’s observation
• additionally filter the sap of leaves using Chamberland filter
• 1898 - Martinus Beijerinck
• Showed that sap of infected tobacco plant retain its infectivity after filteration
by chamberland porcelin filter
• Extented these studies by showing that filtered sap diluted regain its strength
after replication in living growing tissue of plant
• Called this filterable agent as “Contagium vivum fluidum” -contagious
living liquid
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History

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• 1898 - Freidrich Loeffler and Paul Frosch: applied
filteration criteria to a disease transmission in cattle which
later k/a Foot-and-mouth disease
• 1900 -Walter Reed demonstrated that yellow fever is
spread by mosquitoes
• 1908 - Karl Landsteiner and Erwin Popper proved that
poliomyelitis is caused by a virus. Landsteiner and Popper
were the first to prove that viruses could infect humans as
well as animals
• 1911 - Francis Peyton Rous: discovered 1st tumor causing
virus
History

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• 1915 - Frederick Twort: detected existance of filterable
agents that could kill bacteria
• 1917 - Felix d’ Herelle:
• also noted killing of bacteria by agent he called
Bacteriophage
• defined Plaque Assay for quantification of bacteriphage
• 1931 - Woodruff and Goodpasture used ECE for virus
culture
• 1935 - Wendell Stanley established the chemical nature of
viruses when he crystallized TMV and found it to be mostly
protein
History

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• 1939 - Emory Ellis and Max Delbruck: one step growth curve
of bacteriophage`
• 1957 - Isaacs and Lindemann discover Interferon (INF)
• 1979 - WHO declared smallpox globally eradicated
History

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Virus Properties
1. Ultramicroscopic
2. Filterable agents
3. Obligate intracellular parasite: cannot be grown on artificial media
(but not all obligate intracellular agents are virus eg. Anaplasma,
Ehrlichia, Rickettsia)
4. Do not possess cellular organelles viz. mitochondria, golgi, ER
associated ribosome's)
5. Multiplication not by binary fission but resembles an assembly line
in which different parts of virus come together from different sites in
host cell to form new virus particle
6. Genome either DNA or RNA

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Property Bacteria Mycoplasma
Rickettsia/
Chlamydiae
Viruses Fungi
Size
(> 300 nm
diameter)
+ + + < 300 nm +
Culture on
Artificial
media
+ + _ _ +
Obligate
intracellular
_ _ + + _
DNA and
RNA
+ + +
DNA or
RNA
+
Functional
ribosomes
+ + + _ +
Characteristics of Infectious Agents

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• Composition varies in individual virus family
• Simplest virus consist of a core of nucleic acid surrounded by a
protein coat (Capsid) and +/- Envelope
• Components of virus particle include:
1. Nucleic acid- DNA or RNA
• single stranded / double stranded
• Segmented / non segmented
• linear / circular
Composition of Virus

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2. Proteins
• Structural -capsid made of capsomeres
• serve as antigens which elicit an immune
response
• Enzymes
• differ from host cell
• targets of antiviral therapy
3. Envelope
• lipoprotein envelope containing viral and host
cell components
Composition of Virus

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Structure of Virus

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• Viruses varies in shape, size and symmetry
1. Shape: Spherical, Rod, Elongated/Filamentous, Bullet, Brick,
cone, etc.
2. Size: usually ranges from 20 nm to 300 nm
3. Symmetry: refers to the way capsomere are arranged in the virus
capsid
• Virus particles exhibit 3 types of capsid symmetry
1. Icosahedral: isometric or cubic
2. Helical: tubular
3. Complex
Structure of Virus

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Icosahedral capsids
1
2
6
7
9
5
4
3
11 12
8
10
• 12 vertex
• 20 faces
• 60 sides
• 3 axis of rotation

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Icosahedral: Axis of symmetry
2-FOLD
5-FOLD
3-FOLD

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Icosahedral: Axis of symmetry

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Helical Symmetry
Tobacco mosaic virus
Amplitute : daimeter
Pitch: distance covered by each complete turn

24. • Virus Classification I - the Baltimore classification
– genetic content and replication strategy
• Virus classification II - the Classical system
– based on three principles
1. classifying virus itself not host
2. nature of nucleic acid
3. shared physical properties of the infectious agent (e.g
capsid symmetry, dimensions, lipid envelope)
• Virus classification III - the genomic system
– based on DNA/RNA sequence
– By the year 2014 there were 3180 viruses of plants, animals and bacteria - in
7 order, 104 families, 23 subfamilies and 505 genera
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Classification of virus

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Baltimore classification of virus
• David Baltimore (Nobel prize 1975) proposed a classification
based on combination of
• type of viral genome (genetic content)
• information about genome synthesis (replication strategy)
• focus on synthesis of mRNA
• (+) strand can be directly translated
• (-) strand cannot be translated
• Seven distinct virus groups

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Baltimore Classification of Virus
• Baltimore classification: seven classes of viruses :
1. ds DNA viruses (herpes, papova, adeno, pox)
2. ss DNA viruses (parvo)
3. ds RNA viruses (reo, rota)
4. (+) sense ss RNA viruses ((picorna, toga, flavi, corona)
5. (-) sense ss RNA viruses (arena, bunya)
6. RNA reverse transcribing viruses (retro, lenti)
7. DNA reverse transcribing viruses (hepedna)

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• Initially grouped based on disease and
symptoms viz.
• Hepatitis virus: viruses causing liver
pathology
• Mosaic viruses: viruses causing mottling
in plants
• 1966 – International Committee on
Nomenclature of Viruses (ICNV)
• International Committee on Taxonomy of
Viruses (ICTV)
Virus Taxonomy and Nomenclature

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• Virus Classification: polythetic system
• Order: highest taxa
• Family: practical used highest taxa
• Subfamily
• Genus
• Species
• Viruses with similar structural, genomic and replication properties
are grouped into families (suffix: viridae) e.g. Herpesviridae
• Families subdivided into genera (suffix: virus) e.g. Herpes
simplex virus, Cytomegalovirus, Varicella zoster virus
• Subtypes based on nucleotide sequence and antigenic reactivities
e.g. Herpes simplex virus type 1, Herpes simplex virus type 2
Virus Taxonomy

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• Order: suffix: virales
• Family: suffix: viridae
• Subfamily: suffix: virinae
• Genus: suffix: virus
• Species : can contain more than one word
• have no specific ending
• for species having more than one word ,
1st word plus any proper nouns are
capitalised
Virus Nomenclature
• must be single word written in
continuation without any space
•1st alphabet should be capitalized
• letter should be italics

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• Eg. Newcastle disease virus
• Order: Mononegavirales
• Family: Paramyxoviridae
• Subfamily: Paramyxovirinae
• Genus: Avulavirus
Virus Nomenclature