Virus Cultivation: Outcomes of Infection Flashcards
Viruses cause a variety of diseases in humans, ranging from relatively mild illnesses such as the common cold to potentially fatal illnesses like rabies virus.
How may we combat these infections?
Some virus infections are treatable with anti-viral drugs or administration of anti-virus antibodies (see Passive Immunity in the Immunology Readings).
Vaccines can induce protective immunity to prevent infection by many viruses.
Viruses can be cultivated in vitro; this technology allows scientists to better understand the pathogenesis of disease caused by the virus, the mode of action of anti-viral drugs, and the development of vaccines.
How are eggs used in virology research?
Since viruses require living host cells in order to replicate, any study that follows the virus replication cycle requires the growth of susceptible and permissive host cells.
Some viruses can replicate in animal cells grown in an in vitro culture.
Others can only replicate in embryonated chicken egg, or in vivo (i.e., a living animal host)
The cells within chicken eggs are used to culture different types of viruses.
Viruses can be replicated in various locations within the egg,
How are cells for culture prepared?
Cells for culture are prepared by separating them from their tissue matrix.
(a) Primary cell cultures grow attached to the surface of the culture container. Contact inhibition slows the growth of the cells once they become too dense and begin touching each other. At this point, growth can only be sustained by making a secondary culture.
(b) Continuous cell cultures are not affected by contact inhibition.
How do cells grow in cell culture?
In cell culture, the cells grow in a plastic dish containing a complex culture medium that includes sugars, amino acids, vitamins, minerals and various growth factors.
Unlike bacteria, eukaryotic cells cannot grow in the absence of important signalling or stimulating molecules called growth factors.
Eukaryotic cells are also very sensitive to changes in pH and are therefore grown in incubators with CO2 to buffer the culture medium.
What is a primary cell culture?
Cell cultures can be either a primary cell culture or a tumour (transformed) cell line.
A primary cell culture is where the tissue is removed from an animal and treated with trypsin to dissociate the cells.
These cells will divide every few days and attach to the bottom of the plastic dish to form a monolayer.
However, the culture is not immortal - after only a few rounds of cell division, the cultures will eventually die.
What is a transformed cell line?
Tumour cells can grow indefinitely in cell culture because they have been transformed and are no longer sensitive to the constraints of primary cells.
These cells will also divide every few days to form a monolayer in the cell culture dish but they may also continue to grow beyond a monolayer.
Transformed cell lines can also be generated by inserting selected gene components of specific virus into a normal cell’s genome.
What is the cytopathic effect?
Animal cells infected with viruses may exhibit morphological changes. These changes are characteristic for a particular virus and are referred to as the cytopathic effect or “cell injuries.”
These cytopathic effects result from the changes induced by the virus either directly or indirectly.
Some viruses do not immediately kill the host cell. These changes can also be used to diagnose a particular virus from a patient sample.
What are some abnormalities that may be observed in cells experiencing the cytopathic effect?
- the “rounding” of the cells,
- detachment from the surfaces of the culture vessel,
- shrinkage of the nucleus,
- appearance of vacuoles in the cytoplasm,
- increase in membrane permeability,
- formation of inclusion bodies in the cytoplasm or nucleus (aggregates of viral protein that appear as granules within the cell),
- formation of syncytia (masses of fused cells containing more than one nucleus),
- or complete cell lysis.
What are inclusion bodies?
Aggregates of viral protein that appear as granules within the cell
Define: syncytia
masses of fused cells containing more than one nucleus
What is pathogenicity?
The capacity of one organism to cause disease in another, and that disease is an abnormal situation in which an infection has resulted in some damage to the host and/or an alteration in normal host functions.
When considering diseases caused by viruses, what two components should we consider?
The effects of the virus’ replication cycle and the effects of the immune response to the infection.
How do viruses enter the host organism?
Through the respiratory system, gastrointestinal system, genitourinary system, and abrasions in the skin or by direct inoculation (i.e., needles, insect bites).
- After infecting the host, the virus must reach a site where it undergoes its primary replication.*
- This site is usually close to the portal of entry.*
What influences the capacity of viruses to replicate in the cells of specific tissues or organs?
Influenced both by the virus’ attachment proteins and the host cell receptors (for infection) as well as the intracellular factors necessary for virus replication.
After the virus has replicated in its host, it must leave the host and enter the environment. What must it be able to do?
The virus must be stable enough to survive in the environment until a new susceptible host is infected.
How may viruses be transmitted?
In many cases, the virus is shed in respiratory secretions (i.e., coughs or sneezes), enteric or genitourinary secretions, or by ingestion of a blood meal from a viremic host by an insect.
Viruses may also be transmitted to a susceptible host by transfusion of blood or blood products or by organ transplant.
In viral diseases that produce skin lesion, the viruses found in the skin lesions could be transferred to new susceptible hosts by biting insects or carried by fomites (inanimate objects like dust particles) or by direct skin-to-skin contact.
What are the four possible outcomes of a viral infection in eukaryotic cells?
- lytic/acute infection
- persistent (chronic) infection
- latent infection
- oncogenic infection
What is lytic infection?
Lytic or acute infection - this is where the host cell is killed, usually during the release of the progeny virus particles.
What is chronic infection?
Persistent (chronic) infection - this is where there is a slow release of virus over a long time with only the death of a few of the infected cells.
What is latent infection?
An extreme case of a persistent infection.
The virus genome remains present in the cell (sometimes for the life of the individual), but it is not replicated and the virus does not harm the cell.
There are no disease symptoms and no free viruses detected in the body during latency.
The provirus may express some genes to ensure survival of the genome and allow for the reactivation of the replication cycle. The provirus can be re-activated and result in the production of virus particles at some later time.
What is oncogenic infection?
This is where there is the integration of viral genome into the host chromosome.
The viral genome is replicated along with host DNA and passed onto progeny cells.
Some viral genes are expressed, and often alter the cells and cause them to become tumour cells (cells that have lost normal growth control and divide continuously).
The process of a normal cell changing into tumour cell is called transformation.
What is the process of a normal cell changing into tumour cell called?
Transformation
What virus causes Burkitt’s lymphoma (tumour of jaw and abdomen)?
Epstein-Barr virus
Describe the various effects that viral infections may have on cells.
Some viral infections cause neoplastic transformations.
Some viral infections result in the formation of multi- nucleate giant cells (syncytium).
Some viral infections result in the formation of inclusion bodies (aggregates of viral protein) in cytoplasm and/or nucleus.
Some viral infections cause areas of rounded and dead or dying cells and cell lysis in the culture.
