This lecture for microbiology student

1. Chapter Six
Viruses
Topic: Discovery; general characteristics; morphology; chemical composition
Outline:
• Discovery of virus
• General characteristics of virus
• Size of virus
• Morphology of virus
• Classification of virus
• Chemical composition of virus

2. Teaching and learning methods: Lectures, visual aid, interactive forms,
questions- answer session and group discussion.
At end of the lecture student should be able to answer the following
questions:
• Summarize the general characteristics of virus.
• Explain the structure of virus
• Classify of the virus.
• Define capsid, virion and prion.
Reference books:
1. Microbiology: An Introduction, 12th edition- Tortora GJ & Funke BR, Chapter
Thirteen; Page: 359

3. Discovery
• In 1886, the Dutch chemist Adolf Mayer showed that tobacco mosaic disease
(TMD) was transmissible from a diseased plant to a healthy plant.
• In 1892, in an attempt to isolate the cause of TMD, the Russian bacteriologist
Dimitri Iwanowski filtered the sap of diseased plants through a porcelain
filter that was designed to retain bacteria. He expected to find the microbe
trapped in the filter; instead, he found that the infectious agent had passed
through the minute pores of the filter. When he infected healthy plants with
the filtered fluid, they contracted TMD.
• The first human disease associated with a filterable agent was yellow fever.
Advances in molecular biological techniques in the 1980s and 1990s led to the
recognition of several new viruses, including human immunodeficiency virus
(HIV) and SARS (Severe Acute Respiratory Syndrome) -associated
coronavirus.

4. General Characteristics
Viruses have several distinctive features:
• They are ultra-microscopic entity, cannot observe in light microscope. To observe them
electron microscope is required.
• They are the smallest living organisms.
• They do not have a cellular structure.
• They contain only one kind of nucleic acid either RNA or DNA as their genome.
• The nucleic acid is encased in a protein shell, which may be surrounded by a lipid-
containing membrane.
• They are obligate intracellular organisms that mean they are inert in the extracellular
environment and can replicate only in living cells.
• They multiply inside living cells by using the synthesizing machinery of the cell.
• Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other
cells.
• The virus infection may have little or no effect on the host cell or may result in cell damage
or death.
• They are on the boundary between what we regard as living and non-living.

5. How do viruses differ from bacteria?
Property Viruses Bacteria
Type of nucleic acid DNA, RNA but not both DNA and RNA
Protein Few Many
Lipoprotein membrane In some enveloped In all
Ribosomes Absent Present
Enzymes None or few Many
Binary fission No Yes
Plasma membrane Absent Present
ATP-generating metabolism Absent Present
Sensitive to antibiotics No Yes
Sensitive to interferon Yes No
Crystallization Yes No

6. Size
• Viruses are the smallest living organisms, ranging in size from about 20-1400
nm; on average they are about 50 times smaller than bacteria. They cannot
be seen with the light microscope and they pass through filters which retain
bacteria.

7. Figure-1: Virus sizes. The sizes of several viruses (blue) and bacteria (tan) are compared
with a human red blood cell, shown to the right of the microbes. Dimensions are given
in nanometers (nm) and are either diameters or length by width.

8. Structure
Viruses have a very simple structure consisting of the
following:
• Core: the genetic material, either DNA or RNA. The DNA
or RNA may be single-stranded or double-stranded.
• Capsid: The protein shell, or coat that encloses the
nucleic acid genome.
• Nucleocapsid: The protein-nucleic acid complex
representing the packaged form of the viral genome.
• Envelope: A lipid-containing membrane that surrounds
some virus particles. It is acquired during viral
maturation by a budding process through a cellular
membrane. Virus-encoded glycoproteins are exposed on
the surface of the envelope. These projections are called
peplomers. HIV and influenza are enveloped viruses
• Capsomeres- capsids are often built up of identical
repeating subunits called capsomeres.
Component of animal virus
(Herpesvirus)

9. Figure-2: Schematic diagram
illustrating the components of
the complete virus particle
(the virion).
a. Enveloped virus with
icosahedral symmetry.
b. Virus with helical
symmetry.

10. • Defective virus: A virus particle that is functionally deficient in
some aspect of replication.
• Structural units: The basic protein building blocks of the coat.
They are usually a collection of more than one non-identical
protein subunit. The structural unit is often referred to as a
protomer.
• Subunit: A single folded viral polypeptide chain.
• Virion: The complete virus particle. In some instances (e.g.,
papillomaviruses, picornaviruses), the virion is identical with the
nucleocapsid. In more complex virions (herpesviruses,
orthomyxoviruses), this includes the nucleocapsid plus a
surrounding envelope. This structure, the virion, serves to transfer
the viral nucleic acid from one cell to another.

11. Classification of viruses
1. On the basis of structure-
• Helical virus, E.g. TMV
• Polyhedral virus, E.g. Adenovirus
• Enveloped virus, E.g. HIV
• Complex virus, T2 virus (Bacteriophage)
2. On the basis of host specificity-
• Plant virus, E.g. TMV, BMV
• Animal virus, E.g. Herpes simplex virus
• Human virus, HIV
• Bactericidal virus, E.g. Bacteriophages

12. Classification of viruses
3. On the basis of nucleic acid-
a. DNA virus:
(i) Single-stranded DNA; non-enveloped; E.g. Parvoviridae (Dependovirus)
(ii) Double-stranded DNA; non-enveloped; E.g. Adenoviridae (Adenovirus)
(iii) Double-stranded DNA; enveloped; E.g. Poxviridae (Smallpox virus);
Hepadnaviridae (Hepatitis B virus).
b. RNA virus:
(i) Single-stranded RNA; non-enveloped; E.g. Picornaviridae (Hepatitis A virus);
Caliciviridae (Hepatitis E virus)
(ii) Single-stranded RNA; enveloped; E.g. Togaviridae (Rubella virus)
(iii) Double-stranded RNA virus, non-enveloped; E.g. Reoviridae (Reovirus)

13. Viruses as agent of disease
• Viruses can also infect eukaryotic cells and, as in prokaryotic
cells, each has its own specific host. For example, TMV will attack
only tobacco plants. Between them, viruses cause a wide range of
diseases among plants, animals and fungi. Diseases of humans
caused by viruses include measles, German measles (rubella),
chickenpox, influenza, herpes and AIDS.
• Viruses cause many different diseases in almost every other kind
of organism.

14. General Morphology
• Viruses may be classified into several different morphological types on the
basis of their capsid architecture. The structure of these capsids has been
revealed by electron microscopy and a technique called X-ray crystallography.
 Helical Viruses
Helical viruses resemble long
rods that may be rigid or
flexible. The viral nucleic acid is
found within a hollow,
cylindrical capsid that has a
helical structure (Figure-3). The
viruses that cause rabies and
Ebola hemorrhagic fever (EHF)
are helical viruses. Figure-3: A helical virus (TMV)

15. Polyhedral Viruses
• Many animal, plant, and bacterial
viruses are polyhedral, or many
sided, viruses. The capsid of most
polyhedral viruses is in the shape of
an icosahedron, a regular
polyhedron with 20 triangular faces
and 12 corners. The capsomeres of
each face form an equilateral
triangle. An example of a polyhedral
virus in the shape of an icosahedron
is the adenovirus (genus
Mastadenovirus). Another
icosahedral virus is the poliovirus.
Figure-4: Polyhedral virus (Icosahedron)

16. Enveloped Viruses
• As noted earlier, the capsid of some
viruses is covered by an envelope.
Enveloped viruses are roughly
spherical. When helical or polyhedral
viruses are enclosed by envelopes,
they are called enveloped helical or
enveloped polyhedral viruses. An
example of an enveloped helical
virus is the influenza virus (genus
Influenzavirus). An example of an
enveloped polyhedral (icosahedral)
virus is the herpes simplex virus
(genus Simplexvirus).
Figure-5: Enveloped virus

17. Complex Viruses
• Some viruses, particularly bacterial
viruses, have complicated structures
and are called complex viruses. One
example of a complex virus is a
bacteriophage. Some bacteriophages
have capsids to which additional
structures are attached. In this figure,
notice that the capsid (head) is
polyhedral and the tail sheath is
helical. The head contains the nucleic
acid. Another example of complex
viruses are poxviruses, which do not
contain clearly identifiable capsids but
have several coats around the nucleic
acid.
Figure-6: Structure of T2 bacteriophage

18. B
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