Viruses are non-living agents that infect all life forms and are obligate intracellular parasites. They contain either DNA or RNA and have a protein coat called a capsid that is sometimes enclosed in an envelope. Viruses have a specific host range determined by attachment sites and cellular factors, and they replicate either through a lytic cycle that ends in host cell lysis or a lysogenic cycle where the host cell remains alive. Bacteriophages in particular can undergo lytic cycles involving attachment, penetration, biosynthesis, maturation and release steps or lysogenic cycles where the phage DNA is incorporated into the host genome.

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