Bio 220: Lecture 20 - Viral Classification Schemes

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Viral Classification Schemes
Classified based upon the disease that they produce

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a. meningitis virus
b. pneumonia virus
c. hepatitis virus

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Viral Classification Schemes
Classified based upon host range or host specificity

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TMV (tobacco mosaic virus) will only infect tobacco plants. The T-even phage will only replicated in E. coli.

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Viral Classification Schemes
Classified based upon tissues affinities

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a. dermotrophic
b. pneumotropic
c. neurotrophic
d. viscerotropic
e. oncogenic

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Viral Classification Schemes
Classified based upon morphological characteristics

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1. type of nucleic acid (DNA or RNA, single or double stranded)
2. enveloped or naked
3. capsid size and number of capsomeres
3. helical or polyhedral or complex

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Viral Classification Schemes
Classified based upon physical and chemical properties

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are the viruses susceptible to certain forms of radiation or chemical agents such as lipid solvents which might include ether or formaldehyde

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Viral Classification Schemes
Classified based upon the location of the replication site within a host cell

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DNA viruses are typically assembled within the nucleus of the host cell, while RNA viruses are typically assembled within the cytoplasm of the host cell.

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Viral Classification Schemes
Classified based upon the method of transmission

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arboviruses for example are transmitted by the bite of blood-sucking arthropods (insects and arachnids)

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Viral Classification Schemes
Classified based upon immunological properties

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the influenza viruses are categorized based upon the antigenic H and N spikes. Other antigenic markers located on the external surfaces of viruses are used in this fashion

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dermotrophic

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1. Herpes simplex 1 (oral) and 2 (genital) viruses
2. Varicella virus (chickenpox) Another name is varicella-zoster virus
3. Rubeola virus (measles)
4. Rubella virus (German measles)

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pneumotrophic

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1. common cold viruses (several viruses including rhinovirus, coronavirus and adenovirus)
2. pneumonia virus

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neurotropic

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1. Herpes zoster virus (shingles) Also called varicella-zoster virus
2. rabies virus
3. encephalitis virus
4. polio virus

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viscerotropic

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hepatitis virus

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oncogenic

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viruses found in cancerous tissue

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Varicella virus is ___ when it causes chickenpox and ___ when it causes shingles.

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dermotrophic; neurotrophic
The virus has remained within the body in a quiet or latent form and re-emerges later in life in another tissue (nervous)

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The Herpes family of viruses (Herpesviridae)

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All of the viruses in this family are double stranded DNA viruses that display an icosahedral (20 sided) symmetry and are enveloped. They are medium sized viruses that can cause an array of diseases in all stages of life.

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A fetus can suffer from ___ or ___.

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CMV or cytomegalovirus
This can cause certain congenital defects including metal retardation and hearing loss.
Note: CMV causes a severe eye infection in AIDS patients

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A child can suffer from the varicella virus in the form of ___.

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chickenpox

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An adolescent can suffer from the Epstein-Barr (EB) virus in the form of ___.

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infectious mononucleosis

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Adults can suffer from an array of Herpes viruses such as:

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1. Herpes simplex 1 - can cause oral herpes (fever blisters, cold sores)
2. Herpes simplex 2 causes genital lesions
3. Varicella-zoster can cause shingles in adults.

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Shingles

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is a painful condition characterized by the formation of vesicles distributed about the waist, face, upper chest and back. The virus invades cutaneous sensory nerves and is usually limited to one side of the body at a time.

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The EB virus is associated with ____.

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chronic fatigue syndrome

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The EB virus is implicated in causing two cancerous diseases:

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Burkitt's lymphoma and nasopharyngeal carcinoma

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Isolation and purification of viruses

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filtration and centrifugation

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define: filtration

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use a membrane (Millipore filter with a grid of 0.2µm) filter to hold back bacteria

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define: centrifugation

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use standard differential centrifugation to remove cells, cell debris and nuclei. Increase to RPM to remove mitochondria and bacteria. Follow this with ultracentrifugation to pellet out the viral particles.

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Cultivation of viruses

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1. Grow virus in whole animals and then recover.
2. Use chicken embryos (fertile, embryonated eggs) for cultivation. Inoculate a viral suspension onto the appropriate membrane. Viruses will replicate in these undifferentiated embryonic tissues. This is commonly used in vaccine preparation. Look for pocks which are areas of cell destruction on the membrane to signal viral replication (more on pocks later)
3. Tissue (cell) culture using animal cells. Tissue can be used from a fetal animal (under differentiated) to facilitate replication. Liver or lung tissue is commonly used. Tissue culture cells can be used as a suspension of cells or in a monolayer (single layer). Use of monolayer allows for visualization of tissue deterioration by the viruses (CPE).
4. Use bacterial host cells for culturing bacteriophages.

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What is a CPE?

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Cytopathic effect (CPE) is noticed as the monolayer cells deteriorate as a result of the viral infection. This destruction of the tissue cells in monolayer allows for easy monitoring and assessment of viral growth. This visualization is not apparent when cells are in suspension.

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What is a Primary Cell Line?

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Primary cell lines will divide and reproduce for a few generations and then will die. The viral culture will disappear along with these host cells at the time of death. This may be a problem for the researchers.

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What is a Diploid Cell Line?

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Diploid cell lines make use of embryonic tissues for the culturing of viruses. These cell lines can be maintained for approximately 100 generations. The rabies virus that is used for rabies vaccine is grown in diploid cell lines and is sometimes referred to as diploid culture vaccine.

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What is a Continuous Cell Line?

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Continuous cell lines use cancer cells for culturing viruses. These cell lines can go through an infinite number of generations and are sometimes referred to as immortal. This is ideal for maintaining a viral culture for purposes of research and study. A famous continuous cell line is called the HeLa cell line after Henrietta Lacks who provided these cancerous cells. She died in 1951.