virology

1. Tortora Ch#13
Viruses

2. FOUNDATIONS OF VIROLOGY
 Non-living agents that infect all life forms
 Viral cultivation differs from bacterial cultivation
 1,500 known viruses
(estimates:  400,000 exist)
 Advent of electron microscope allowed for
visualization of viruses

4. GENERAL
CHARACTERISTICS OF
VIRUSES
 Virus = Latin for poison
 Obligatory intracellular parasites-
 Filterable
 Contain DNA or RNA
 Contain a protein coat = capsid made up of capsomeres.
 Some are enclosed by an envelope (naked vs. enveloped)
 Some viruses have spikes (COH/protein)
 Most viruses are tissue specific
 Multiply inside living cells using the synthesizing machinery of cells

5. Nucleic acid
 either DNA or RNA- never both
 single stranded or double stranded
 linear or circular
 can be present in form of segments
 amount of nucleic acid- few thousand nucleotides to 25000
nucleotides
 percentage of nucleic acid to protein- 1 % for influenza
- 50% bacteriophages

6.  Nucleic acid is protected by protein coat- capsid
 Capsid is composed of small protein subunits- capsomeres
 Capsomeres protein may be single or several type
 Arrangement of capsomere is characteristic of particular type of virus
 Envelope is combination of lipid, protein and carbohydrates
 May be formed of host cell membrane or viral nucleic acid
 Covered by spikes in some viruses
 Spikes- carbohydrate-protein complexes that project from envelope
 can be used for identification (heamagglutination)- influenzae

7. HSV envelopment and release

8. Morphology of an enveloped helical virus

9.  Different morphological types on basis of their capsid architecture
Helical viruses
 Long rods that may be rigid or flexible
 Nucleic acid found within a hollow cylindrical capsid
 Example- rabies virus, Ebola virus
Polyhedral viruses
 polyhedral-many sided
 Most polyhedral viruses are in shape of icosahedron (20 triangular aces and 12
corner)
 Example- poliovirus, adenovirus

11. Enveloped viruses
 spherical
 Enveloped helical virus - helical viruses covered by envelope
 Enveloped polyhedral virus - polyhedral viruses covered by envelope
 Example- influenza virus, herpes virus
Complex viruses
 complicated structure-bacteriophages
 additional structures are attached with capsid
 capsid-polyhedral
 tail sheath- helical
 Tail fibre
 Plate
 pin

13.  Host range is determined by
- specific host attachment sites
- Host cellular factors for multiplication
 For virus to infect cell- hydrogen bonding between outer surface of virus with host
cell surface receptors
 Receptor site- may be part of cell wall of host, fimbriae or flagella
- or plasma membrane for animal viruses
 Combination of many attachment site-strong bond

14. HOST RANGE AND
SPECIFICITY
Virus / host cell interaction usually very specific (narrow host range) – due to?
Tissue tropism
Tissue tropism is the cells and tissues of a host that support growth of a particular virus or
bacterium. Some bacteria and viruses have a broad tissue tropism and can infect many types
of cells and tissues. Other viruses may infect primarily a single tissue.

15. TAXONOMY OF VIRUSES
 No evidence for common viral ancestor.
 Classification based on type of nucleic acid, strategy for
replication, and morphology.
 Family names end in –viridae
 Genus and species names end in -virus.
 Viral species: A group of viruses sharing the same genetic
information and ecological niche (host). Common names
are used for species.
 Subspecies are designated by a number.

16.  International Committee on taxonomy of viruses (ICNV)
Taxonomic factors include
genome,
protein,
lipid,
carbohydrate,
antigenic and biological properties
(6 orders, 87 families, 19 subfamilies and 348 genera)
 Baltimore Classification
Composition of virus genome
Replication strategy
(7 classes were defined by this system)

17.  Herpesviridae
 Herpesvirus
 Human herpes virus HHV-1,
HHV-2, HHV-3
 Retroviridae
 Lentivirus
 Human immunodeficiency
virus HIV-1, HIV-2

18. VIRAL REPLICATION
 Obligate intracellular parasites using host cell
machinery
 Very limited number of genes encode proteins for
 Capsid formation
 Viral nucleic acid replication
 Movement of virus into and out of cell
 Kill or live in harmony within the host cell – Outside the
cell, viruses are inert
 Multiplication-either lytic or lysogenic
 Lytic cycle- ends with lysis and death of host cell
 Lysogenic cycle- host cell remains alive

19. BACTERIOPHAGE: LYTIC
CYCLE
1. Attachment to cell surface receptors (chance encounter – no active
movement)
2. Penetration – only genome enters
3. Biosynthesis – Production of phage DNA and proteins
4. Maturation – assembly to form intact phage
5. Release due to phage induced lysozyme production

20. 1
2
3

21. 4

22. LYTIC AND LYSOGENIC
CYCLES

23. RESULTS OF MULTIPLICATION OF
BACTERIOPHAGES
 Lytic cycle
 Lytic or virulent phage
 Phage causes lysis and death of host cell
 Lysogenic cycle
 Lysogenic or temperate phage
 Phage DNA incorporated in host DNA  Prophage
 Phage conversion
 Specialized transduction