1. Applied Microbiology
5th lecture: Chapter 6

2. Viruses

3. General Structure of Viruses
-> Size range –
most <0.2 μm; requires electron microscope
-> Virion – fully formed virus able to establish an infection

4. Form and Function of Viruses

5. General Structure of Viruses
Capsids
All viruses have capsids - protein coats that enclose and
protect their nucleic acid.
Each capsid is constructed from identical subunits called
capsomers made of protein.
The capsid together with the nucleic acid are
nucleoscapsid.
Some viruses have an external covering called envelope;
those lacking an envelope are naked

6. General Structure of Viruses
Capsids - Envelope

7. General Structure of Viruses
Two structural types:
-> helical - continuous helix of capsomers forming a
cylindrical nucleocapsid
-> icosahedral - 20-sided with 12 corners
- vary in the number of capsomers
- Each capsomer may be made of 1 or several proteins.
- Some are enveloped.

8. General Structure of Viruses
helical
Influenza virus

9. General Structure of Viruses
Icosahedral
Herpes simplex virus
Adenovirus

10. General Structure of Viruses
Viral envelope
-> mostly animal viruses
-> acquired when the virus leaves the host cell
-> exposed proteins on the outside of the envelope, called
spikes, essential for attachment of the virus to the host cell

11. Function of Capsid/Envelope
Viruses
Function of Capsid/Envelope
-> Protects the nucleic acid when the virion is
outside the host cell
-> Helps to bind the virion to a cell surface and
assists the penetration of the viral DNA or
RNA into a suitable host cell

12. Viruses
Viral Morphology

13. General Structure of Viruses
Complex viruses: atypical viruses
-> Poxviruses lack a typical capsid and are covered by a
dense layer of lipoproteins.
-> Some bacteriophages have a polyhedral nucleocapsid
along with a helical tail and attachment fibers.

14. General Structure of Viruses
Nucleic Acids
Viral genome – either DNA or RNA but never both
-> Carries genes necessary to invade host cell and redirect
cell’s activity to make new viruses
-> Number of genes is low - varies for each type of virus – few
to hundreds

15. General Structure of Viruses
DNA viruses
-> usually double stranded (ds) but may be single
stranded (ss)
-> circular or linear
RNA viruses
-> usually single stranded, may be double stranded,
may be segmented into separate RNA pieces
-> ssRNA genomes ready for immediate translation are
positive-sense RNA.
-> ssRNA genomes that must be converted into proper
form are negative-sense RNA.

16. General Structure of Viruses
Pre-formed enzymes may be present.
-> polymerases – DNA or RNA
-> replicases – copy DNA
-> reverse transcriptase –synthesis of DNA from RNA
(AIDS virus)

17. How Viruses are classified
-> Main criteria presently used are structure, chemical composition, and genetic makeup.
-> No taxa above Family (no kingdom, phylum, etc.)
-> Currently recognized: 3 orders, 63 families, and 263 genera
of viruses
-> Family name ends in -viridae, i.e.Herpesviridae
-> Genus name ends in -virus, Simplexvirus
-> Herpes simplex virus I (HSV-I)

18. How Viruses are classified

19. How Viruses are classified

20. Modes of Viral Multiplication
Viruses
Modes of Viral Multiplication
General phases in animal virus multiplication cycle:
-> Adsorption - binding of virus to specific molecule on host
cell
-> Penetration - genome enters host cell
-> Uncoating – the viral nucleic acid is released from the capsid
-> Synthesis – viral components are produced
-> Assembly – new viral particles are constructed
-> Release – assembled viruses are released by budding (exocytosis) or cell lysis

21. Modes of Viral Multiplication
Viruses
Modes of Viral Multiplication

22. Adsorption and Host Range
Viruses
Adsorption and Host Range
-> Virus coincidentally collides with a susceptible host cell
and adsorbs specifically to receptor sites on the cell
membrane
-> Spectrum of cells a virus can infect – host range
-> hepatitis B – human liver cells
-> poliovirus – primate intestinal and nerve cells
-> rabies – various cells of many mammals

23. Viruses
Adsorption

24. Viruses
Penetration
Flexible cell membrane is penetrated by the whole virus or its nucleic acid by:
-> endocytosis – entire virus is engulfed and enclosed in a
vacuole or vesicle
-> fusion – envelope merges directly with membrane
resulting in nucleocapsid’s entry into cytoplasm

25. Viruses
Penetration
Endocytosis
Fusion

26. Replication and Protein Production
Viruses
Replication and Protein Production
-> Varies depending on whether the virus is a DNA or RNA virus
-> DNA viruses generally are replicated and assembled in the
nucleus.
-> RNA viruses generally are replicated and assembled in the
cytoplasm.
-> Positive-sense RNA contain the message for
translation.
-> Negative-sense RNA must be converted into positive-sense
message.

27. Viruses Release Assembled viruses leave host cell in one of two ways:
-> budding – exocytosis; nucleocapsid binds to membrane which
pinches off and sheds the viruses gradually; cell is not
immediately destroyed
-> lysis – nonenveloped and complex viruses released when cell
dies and ruptures
Number of viruses released is variable
-> 3,000-4,000 released by poxvirus
-> >100,000 released by poliovirus

28. Viruses
Release
Budding
Budding of HIV from T-cells

29. Viruses Damage to Host Cells
Cytopathic effects - virus-induced damage to cells:
1. Changes in size & shape
2. Cytoplasmic inclusion bodies
3. Nuclear inclusion bodies
4. Cells fuse to form multinucleated cells.
5. Cell lysis
6. Alter DNA
7. Transform cells into cancerous cells

30. Viruses Damage to Host Cells Cytopathic effects
Herpes simplex virus -> multinucleated giant cells (skin cells)
Cytosolic inclusion bodies

31. Viruses
Damage to Host Cells

32. Persistent Infections -> Cells can maintain a carrier relationship
Viruses
Persistent Infections -> Cells can maintain a carrier relationship
-> Persistent infections - cell harbors the virus and is not
immediately lysed
-> Can last weeks or host’s lifetime; several can periodically
reactivate – chronic latent state
- measles virus – may remain hidden in brain cells for many years
- herpes simplex virus – cold sores and genital herpes
- herpes zoster virus – chickenpox and shingles

33. Persistent Infections -> Cells can maintain a carrier relationship
Viruses
Persistent Infections -> Cells can maintain a carrier relationship
-> Some animal viruses enter host cell and permanently alter its
genetic material resulting in cancer – transformation of the cell.
-> Transformed cells have increased rate of growth, alterations in
chromosomes, and capacity to divide for indefinite time periods
resulting in tumors.
-> Mammalian viruses capable of initiating tumors are called
oncoviruses.
- Papillomavirus – cervical cancer
- Epstein-Barr virus – Burkitt’s lymphoma

34. Viruses Bacteriophages
-> Bacteriophages – bacterial viruses (phages)
-> Most widely studied are those that infect Escherichia coli –
complex structure, DNA
-> Multiplication goes through similar stages as animal viruses.
-> Only the nucleic acid enters the cytoplasm - uncoating is not
necessary.
-> Release is a result of cell lysis induced by viral enzymes and
accumulation of viruses - lytic cycle.

35. Bacteriophages - Replication
Viruses
Bacteriophages - Replication
1. Adsorption – binding of virus to specific molecule on host cell
2. Penetration –genome enters host cell
3. Replication – viral components produced
4. Assembly - viral components assembled
5. Maturation – completion of viral formation
6. Release – viruses leave cell to infect other cells

36. Bacteriophages – Life Cycles
Viruses
Bacteriophages – Life Cycles

37. Bacteriophages Lysogeny: The Silent Virus Infection
Viruses
Bacteriophages Lysogeny: The Silent Virus Infection
-> Not all phages complete the lytic cycle.
-> Some DNA phages, called temperate phages, undergo adsorption
and penetration but don’t replicate.
-> The viral genome inserts into bacterial genome and becomes an
inactive prophage - the cell is not lysed.
-> Prophage is retained and copied during normal cell division
resulting in the transfer of temperate phage genome to all host
cell progeny – lysogeny.
-> Induction can occur resulting in activation of lysogenic prophage
followed by viral replication and cell lysis.

38. Bacteriophages Lysogeny: The Silent Virus Infection
Viruses
Bacteriophages Lysogeny: The Silent Virus Infection
-> Lysogeny results in the spread of the virus without killing the
host cell.
-> Phage genes in the bacterial chromosome can cause the
production of toxins or enzymes that cause pathology – lysogenic
conversion.
- Corynebacterium diphtheriae
- Vibrio cholerae
- Clostridium botulinum

39. Viruses
Bacteriophages

40. Viruses Prions Prions - misfolded proteins, contain no nucleic acid
-> cause transmissible spongiform encephalopathies - neurodegenerative diseases (fibrillation of proteins)
-> common in animals:
- scrapie in sheep & goats
- bovine spongiform encephalopathies (BSE),
mad cow disease
- humans – Creutzfeldt-Jakob Syndrome (CJS)
!!! Extremely resistant to usual sterilization techniques !!!

41. Detection and Treatment of Animal Viral Infections
Viruses
Detection and Treatment of Animal Viral Infections
-> More difficult than other agents
-> Consider overall clinical picture
-> Take appropriate sample
- Infect cell culture- look for characteristic cytopathic effects
- Screen for parts of the virus
Screen for immune response to virus (antibodies)
-> Antiviral drugs can cause serious side effects