1. Foundations in Microbiology Sixth Edition
Lecture PowerPoint to accompany
Foundations in Microbiology Sixth Edition
Talaro
Chapter 6
An Introduction to the Viruses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

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.

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. Functions 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. 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. 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 varies for each type of virus – few to hundreds

15. Nucleic Acids DNA viruses RNA 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 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. 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

20. 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

22. Penetration/Uncoating
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

24. 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.

25. 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

27. Damage to Host Cell Cytopathic effects - virus-induced damage to cells
Changes in size & shape
Cytoplasmic inclusion bodies
Nuclear inclusion bodies
Cells fuse to form multinucleated cells.
Cell lysis
Alter DNA
Transform cells into cancerous cells

28. Persistent Infections
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

29. Mammalian viruses capable of initiating tumors are called oncoviruses.
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

30. Multiplication Cycle in 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.

31. 6 Steps in Phage Replication
Adsorption – binding of virus to specific molecule on host cell
Penetration –genome enters host cell
Replication – viral components produced
Assembly - viral components assembled
Maturation – completion of viral formation
Release – viruses leave cell to infect other cells

33. 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.

35. Lysogeny
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

37. Techniques in Cultivating and Identifying Animal Viruses
Obligate intracellular parasites that require appropriate cells to replicate
Methods used:
cell (tissue) cultures – cultured cells grow in sheets that support viral replication and permit observation for cytopathic effect
bird embryos – incubating egg is an ideal system; virus is injected through the shell
live animal inoculation – occasionally used when necessary

38. Prions and Other Infectious Particles
Prions - misfolded proteins, contain no nucleic acid
cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases
common in animals:
scrapie in sheep & goats
bovine spongiform encephalopathies (BSE), aka mad cow disease
wasting disease
humans – Creutzfeldt-Jakob Syndrome (CJS)
Extremely resistant to usual sterilization techniques

39. Other noncellular infectious agents
Satellite viruses – dependent on other viruses for replication
adeno-associated virus – replicate only in cells infected with adenovirus
delta agent – naked strand of RNA expressed only in the presence of hepatitis B virus
Viroids - short pieces of RNA, no protein coat; only been identified in plants, so far

40. 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