Introduction to virology for MBBS students A virus is an obligate intracellular parasite containing genetic material surrounded by protein Virus particles can only be observed by an electron microscope Most viruses range in sizes from 20 – 250 nanometers Viruses are obligate intracellular parasites Viruses are non-living entities Viruses cannot make energy or proteins independent of a host cell (Depends on host cell for replication) Viral genome are either RNA or DNA but not both. Viruses have a naked capsid or envelope with attached proteins Do not possess cellular organization Viruses do not have the genetic capability to multiply by division. They are NOT cultiviable on ordinary media. Much smaller than bacteria “Filterable agents” – can pass through filters that can hold back bacteria Vary widely in size: Largest – poxvirus (300nm) Smallest – parvovirus (20nm)

1. INTRODUCTION TO
VIROLOGY

2. Introduction to Virology
• A virus is an obligate intracellular
parasite containing genetic material
surrounded by protein
• Virus particles can only be
observed by an electron
microscope
• Most viruses range in sizes from 20
– 250 nanometers 2

3. Viral Properties
• Viruses are obligate intracellular parasites
• Viruses are non-living entities
• Viruses cannot make energy or proteins independent
of a host cell (Depends on host cell for replication)
• Viral genome are either RNA or DNA but not both.
• Viruses have a naked capsid or envelope with
attached proteins
• Do not possess cellular organization
• Viruses do not have the genetic capability to multiply
by division.
• They are NOT cultiviable on ordinary media.

4. Cellular Acellular
Prokaryotes Eukaryotes Viruses
Bacteria and
archaea
Fungi,algae,protozoa,
helminths
Viruses and
bacteriophage
s
Types of Microbes
(b) VirusTypes
Bacterial virus
AIDS virus
Envelope
Capsid
Nucleic
acid
(a) Cell Types
Prokaryotic
Chromosome
Ribosomes
Flagellum
Cellwall Cell
membrane
Eukaryotic
Flagellum
Mitochondria
Ribosomes
Cell membrane
Nucleus
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5. 5
Morphology - Size
• Much smaller than bacteria
• “Filterable agents” – can pass through filters that can hold back
bacteria
• Vary widely in size:
– Largest – poxvirus (300nm)
– Smallest – parvovirus (20nm)

6. Viruses are Ultramicroscopic
Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997
6

7. Shape of Viruses
• Spherical
• Rod-shaped
• Brick-shaped
• Tadpole- shaped
• Bullet-shaped
• Filament-shaped

8. Shapes of Viruses
Spherical

9. Shapes of Viruses
Rod-shaped

10. Shapes of Viruses
Brick-shaped
.

11. Shapes of Viruses
Tadpole-shaped

12. Shapes of Viruses
Bullet-shaped

13. Shapes of Viruses
Filament-shaped

14. 14
VIRAL STRUCTURE – SOME
TERMINOLOGY
• virus particle = virion
• protein which coats the genome =
capsid
• capsid usually symmetrical
• capsid + genome = nucleocapsid
• may have an envelope

15. Virion
• The complete
infectious unit
of virus particle
15

16. Viral Structure - Overview
Fig 1. Schematic overview of the structure of viruses
** does not exist in all viruses
Nucleic acid
Nucleocapsid
Capsid
Viral envelope**
Envelope protein
Membrane protein
Spike protein

17. Basic virus structure
Capsid
protein
Nucleocapsid
Naked capsid
virus
DNA
RNA
or =
+
Nucleocapsid
Lipid membrane,
glycoproteins
Enveloped virus
+

18. Medic
White, DO and Fenner,FJ.
al Virology, 4th Ed. 1994
Virus particle = virion

19. Viral genome
• Viral genome comprises viral nucleic acid genome (DNA
or RNA). Controls viral heredity and variation and
responsible for the infectivity.
√ DNA: either double stranded (ds) or single stranded (ss);
: either linear or circular
√ RNA: either double stranded (ds) or single stranded (ss);
: either linear or circular
: ds RNA viruses: always linear
: ss RNA viruses: either segmented or non-
segmented
: ss RNA viruses: + sence(mRNA) or –sence (strand
complementary to mRNA)

20. HERPESVIRIDAE
HEPADNAVIRIDAE
ENVELOPED
PAPILLOMAVIRIDAE
POLYOMAVIRIDAE
(formerly grouped together as the
PAPOVAVIRIDAE)
CIRCULAR
ADENOVIRIDAE
LINEAR
NON-ENVELOPED
DOUBLE STRANDED
PARVOVIRIDAE
SINGLESTRANDED
NON-ENVELOPED
POXVIRIDAE
COMPLEX
ENVELOPED
DNA VIRUSES
All families shown are
icosahedral except for
poxviruses
Dr
.
T
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VM.RoadoifiMedDfromVolk et al., Essentials of Medical Microbiology, 4th Ed. 1
9
49
81

21. FLAVIVIRIDAE
TOGAVIRIDAE
RETROVIRIDAE
ICOSAHEDRAL
CORONAVIRIDAE
HELICAL
ENVELOPED
ICOSAHEDRAL
PICORNAVIRIDAE
CALICIVIRIDAE
ASTROVIRIDAE
NONENVELOPED
SINGLESTRANDED
positivesense
ORTHOMYXOVIRIDAE
PARAMYXOVIRIDAE
RHABDOVIRIDAE
FILOVIRIDAE
BUNYAVIRIDAE
ARENAVIRIDAE
SINGLESTRANDED
negativesense
REOVIRIDAE
DOUBLE STRANDED
RNA VIRUSES
ENVELOPED
HELICAL ICOSAHEDRAL
NONENVELOPED
Dr
.
T
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VM.RoadoifiMedDfromVolk et al., Essentials of Medical Microbiology, 4th Ed. 1
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22. Viral Capsid
• Capsid – the protein coat
surrounding the nucleic acid
core. It
– 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
1. Protomers
2. Capsomers
3. Pro-Capsid
4. Mature Capsid

23. Symmetry of
Nucleocapsid
– Cubic (Icosahedral)
• a polygon with 12 corners (vertices) & 20 sides
(facets), each an equilateral triangle.
• Eg. adenovirus
– Helical
• Protein binds around DNA/RNA in a helical fashion
• eg. Coronavirus
– Complex
• Is neither cubic nor helical eg. poxvirus
ICOSAHEDRAL SYMMETRY
HELICAL SYMMETRY
COMPLEX SYMMETRY
POXVIRUS FAMILY

24. Envelope
• A lipid-containing membrane
that surrounds capsid.
• It is acquired during viral
maturation by a budding
process through a cellular
membrane.
• viruses can be divided into
2 kinds: enveloped virus and
naked virus.
Enveloped icosahedral virus
Enveloped helical virus

25. Enveloped
Virus
Non-
Enveloped
Virus
Cubic Helical

26. 5 BASIC TYPES OF VIRAL STRUCTURE
HELICAL ENVELOPED HELICAL
ENVELOPED ICOSAHEDRAL
ICOSAHEDRAL
nucleocapsid
icosahedral nucleocapsid
helical nucleocapsid
lipid bilayer
COMPLEX
nucleocapsid
lipid bilayer
glycoprotein spikes
= peplomers
Adapted from Schaechter et al., Mechanisms ofMicD
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27. • ADSORPTION
• PENETRATION
• UNCOATING AND ECLIPSE
• SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN
• ASSEMBLY (maturation)
• RELEASE
BASIC STEPS OF VIRAL REPLICATION

28. VIRAL REPLICATION
Stages in virus replication begin when virion infects host cells
1 Virus attachment
and entry
2 Uncoating of virion
Migration of genome
nucleic acid to nucleus
4 Synthesis of viral nucleic
acid and protein.
5 Virion assembly
6 Release of new virus
particles

29. 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


 pH:


. stable between pH 5 and 9 Enteroviruses
– resistant to acidic conditions

30.  Radiation: UV and ionising radiation inactivates viruses
 Photodynamic Inactivation
 Vital dyes- Toluidene blue
 Disinfectants
 Most viruses are destroyed by oxidising 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 faecal material.

31. Transmission of Viruses
• Respiratory transmission
– Influenza A virus
• Faecal-oral transmission
– Enterovirus
• Blood-borne transmission
– Hepatitis B virus
• Sexual Transmission
– HIV
• Animal or insect vectors
– Rabies virus

32. Viruses enter the body of the host
in a variety of ways, for example...

33. The commonest forms of
transmission are via...
INHALED DROPLETS
in sneezing of coughing
for example the COMMON COLD
or INFLUENZA VIRUSES.

34. or by...
drinking water or
eating raw food, for example,
HEPATITIS A and POLIOVIRUS.

35. The commonest forms of
transmission are also via...
sexual intercourse for example
HIV and HEPATITIS B and...

36. also...
vertical transmission -
from mother to baby for example
HIV, HEPATITIS B and RUBELLA...

37. also...
•bites of vector arthropods such as
mosquitoes for example YELLOW
FEVER, and DENGUE.

38. Most viral infections...
do not lead to such serious
complications and the host...

39. get well after a period of sickness
to be immune for the rest of their lives.
Examples are MEASLES INFECTION, RUBELLA or German
measles,
MUMPS and many others...

40. Embryonated Egg
Cell Lines/ Tissue
cultures
Animal inoculation
Chorioallantioc membrane (CAM)
Allantoic cavity
Amniotic cavity
Yolk Sac
Primary
Diploid/ Secondary
Continuous
Suckling mice
Virus Culture