It describes the structure and general properties of viruses.

1. VIRUSES

2. Classification scheme
Microbial world
Organisms Infectious agents
(Living) (Nonliving)
Bacteria Archae Eukarya
(Prokaryotes)
Algae Protozoa Fungi Helminthes
Viroids Prions
VIRUSES

3. Beijerinck
1921
Virus (Latin- poison and other
noxious substances, first used in
English in 1892)
"Organisms at the edge of life”
Pasteur and Jenner developed
vaccine against virus.
Dmitri Ivanovsky Tobacco Mosaic
Virus (TMV) .
Martinus Beijerinck coined the
term “Virus”.
At the twilight zone separating the
“living” from the “nonliving”.
Introduction

4. Introduction
 Viruses – Smallest infectious agents (20-400 nm in
diameter) , Obligate intracellular infectious agents
containing only one kind of nucleic acid (DNA or RNA as
their genome)
 1000 nm = 1 mm
 Morphological definitions:
Capsid- Protein shell or coat that encloses the nucleic acid
genome.
Capsomers- Morphologic units composing the capsid;
Shapes - vary (spherical/wedge)

7. Introduction
 Envelope- A lipid containing membrane that surrounds
some virus particles and are acquired during viral
maturation by a budding process through a cellular
membrane of host cell.
 Peplomers- Virus encoded glycoprotein exposed on the
surface of the envelope (also known as spikes).
 Nucleocapsid- Capsid together with the enclosed Nucleic
Acid.

8. Introduction
 Virion – an entire virus particle, consisting of an
outer protein shell called a capsid and an inner
core of nucleic acid.
 Complete virus particle.
Herpes, Orthomyxo viruses  Nucleocapsid + Envelope
Papova viruses, Picorna viruses Nucleocapsid
Intactness of structure and the property of infectiousness

9. Origin of viruses
Regressive hypothesis (degeneracy
hypothesis)
Viruses may have once been small cells that parasitized larger cells. Over time,
genes not required by their parasitism were lost. The regressive, or reduction,
hypothesis asserts that viruses are remnants of cellular organisms.
Cellular origin hypothesis (vagrancy
hypothesis)
Some viruses may have evolved from bits of DNA / RNA that "escaped" from
the genes (plasmids or transposons) of a larger organism. The progressive, or
escape, hypothesis states that viruses arose from genetic elements that gained
the ability to move between cells.
Co-evolution hypothesis
Viruses may have evolved from complex molecules of protein and nucleic acid
at the same time as cells first appeared on earth.

10. Character Bacteria Virus
Cell type Prokaryotic cells Acellular
Number of cells Single-celled No cell
Size Larger than viruses (0.3-2 μm) Minute (20-400nm)
Microscopy Visible under Light Microscope. Visible only under an Electron Microscope.
Structure Organelles and genetic material within a cell wall
Genetic material within a capsid, some have
an envelope membrane.
Genome DNA and RNA DNA or RNA, nucleocapsid
Cell wall peptidoglycan and lipopolysaccharide. No cell wall. Protein coat presents instead
Cell organelles Presence of non-membrane-bound cell organelles
Absent. Uses host organelles; obligate
intracellular parasites
Living attributes Living organisms. Between living and non-living things.
Replication Binary fission (asexual)
It invades a host cell and takes over the cell
causing it to make copies of the viral
DNA/RNA. Destroys the host cell releasing
new viruses.
The need for host
cell
Able to reproduce by itself Need a living cell to reproduce
Duration of illness
A bacterial illness commonly will last longer than 10
days.
Most viral illnesses last 2 to 10 days
Treatment Antibiotics Antiviral drugs
Examples
E. coli, Salmonella spp., Listeria spp., Mycobacteria
spp., Staphylococcus spp., Bacillus anthracis, etc.
HIV, Hepatitis A virus, Rhino Virus, Ebola
virus, etc.

11. Morphological Properties
 Size:
 20-400 nm in diameter
 Picornaviridae/Parvoviridae (smallest)
 Poxviridae (Largest)
 Shape:
 Mostly roughly spherical
 Rabies virus – bullet shaped
 Pox virus – brick shaped
 TMV – rod shaped

12. Structure/ Chemical composition
 NUCLEIC ACID CORE
 VIRAL CAPSID
 ENVELOPE
 VIRAL PROTEINS

13. NUCLEIC ACID CORE (GENOME)
 Structure: contains either DNA or RNA
 Size: 3-3000 kbp
 Strandedness: either single-stranded (RNA viruses except
Reovirus) or double-stranded (DNA viruses except
Parvovirus)
 Polarity: RNA can be +ve sense (act as messenger RNA and
can directly translated into viral proteins), -ve sense or
ambisense
 Segmentation: Nucleic acid as single or several segments
(Influenza virus has 8 RNA molecules, each carrying different
genetic information. Such viruses are termed segmented
viruses)

14. VIRAL CAPSID
COMPOSITION
 Capsomers: morphological subunits
FUNCTIONS
 Protection from physical, chemical and enzymatic
damage
 Recognition of the host cell- (Viral attachment protein-
VAP and cellular receptor)

15. SYMMETRY
 Icosahedral: 20 faces (each and equilateral triangle), 12
vertices, and 5 fold, 3 fold and 2 fold axes of rotational
symmetry. There are exactly 60 identical subunits on
the surface of icosahedron.
 Helical: protein subunit bound in a periodic way to the
nucleic acid, winding into a helix.
 Complex: do not exhibit simple cubic or helical but are
more complicated. E.g., brick shaped, bullet shaped etc.

17. ENVELOPE
 Lipid bilayer (derived from host cell membrane) and
virus-coded glycoprotein (peplomers) as spikes
 Matrix proteins: internal virion proteins that links the
internal nucleocapsid
 Glycoproteins: Transmembrane proteins, anchored to
the membrane; also includes transport channels (ion-
channels)
 Influenza virus- Hemagglutinin and Neuraminidase

18. ENVELOPE FUNCTIONS
 Promote interaction with nucleocapsid proteins (final
stage of assembly)
 Act as VAPs to cellular receptors
 Major viral antigens
 Interaction (spikes) with neutralizing antibodies
Tegument: Herpesvirus nucleocapsid are enveloped but
have a featureless layer of tegument interposed between
the nucleocapsid and envelope.
Function: not known.

19. VIRAL PROTEINS
 Structural proteins: essential for the formation of new
viral particles.
 Some are associated with NA
 Some form protective layers (capsid and envelope)
 VAP
 Antigenicity
 Enzymes: essential for initiation of replication
 Polymerases, Proteases, Endonucleases

20. Chemical Properties - Resistance
 Temperature:
 Enveloped viruses are more heat-labile
 Inactivated at 560C-30’ (exception- HBV) and few sec at 1000C.
 Some may be preserved by lyophilization
 Salts:
 Stable at 1M salt solution e.g., MgCl2, MgSO4
 pH:
 Usually stable between pH 5 and 9
 Enteroviruses – resistant to acidic conditions

21. Chemical Properties - Resistance
 Radiation: UV and ionizing radiation inactivates viruses
 Photodynamic Inactivation
 Vital dyes- Toluidene blue; Visible light - Inactivation
 Disinfectants
 Most viruses are destroyed by oxidizing agents such as chlorine,
iodine and hydrogen peroxide.
 However, majority of viruses are resistant to phenol; chlorination
does not always inactivate enteroviruses or hepatitis viruses,
particularly if present with organic or fecal material.

22. Chemical Properties - Resistance
 Formaldehyde
 Destroys viral infectivity by reacting with nucleic acid.
 Used in the production of inactivated viral vaccines.
 Antibiotics and other antibacterial agents
 Antibiotics and sulfonamide have no effect on virus.
 Quaternary ammonium compounds have no effect.
 Organic iodine are ineffective.
 Isopropanol and ethanol are relatively ineffective against certain
viruses like picornaviruses.

23. Classification-Basis
Morphology
Genome properties
Physicochemical properties
Protein properties
Strategy of replication
Antigenic properties
Biologic properties
Host specificity