Microbiology Exam 3: Virus

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Viruses

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virus - "poison" First identified – TMV as filterable disease agent. First virus detected was found affecting a plant (tobacco plants). The only way they could deal with it was to actually take the entire crop and burn it. Tobacco Mosaic Disease
Bacteria are too large to go through a filter. Most bacterial infections are caused by fungi thus both of them are unfilterable (bacteria & fungi)

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Viruses are host specific

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the tail fibers have to match up perfectly with the receptors of specific bacteria to successfully attach itself. Virus-host specificity

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Filterable disease agent

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Unknown organism able to be filtered out of a solution

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Viruses

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Viruses are incredible tiny. This is why they are filterable

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Viruses

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An obligatory intracellular parasite consisting of a piece of nucleic acid surrounded by a protein coat

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Viruses

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Most are smaller than bacteria. The largest are the size of rickettsias and chlamydias

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Viruses

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All viruses lack independent metabolism. They do not have organelles or ribosomes. They multiply only inside living cells, using the host cell's machinery. (compare to rickettsias and chlamydias)

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Viruses

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Are they living or non-living entities? Good question! Generally described as non-living entities.
Living things cannot be crystalized, but viruses can be crystalized. Plus viruses are able to replicate themselves (they reproduce such as other living things)
"Active" and "inactive" are often used in place of "live" or "dead."

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viral genome core

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contains either DNA or RNA, never both, 3 genes (smallest virus) or several hundred.

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capsid

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protein shell surrounding the genome. May be a very simple structure or complex.

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envelope

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some viruses have a lipid and carbohydrate coating in addition to the capsid, usually derived from the host cell (not present in all viruses)- Toga viraday

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naked viruses

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no envelope

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spikes

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protein projections associated with the envelope, may mutate frequently

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virion

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the extracellular state, also called viral particle. Used interchangeably with "virus."

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Groupings

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: There are virus families, but there are general groupings based on the following

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Host type

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animal viruses, plant viruses, or bacteriophages (viruses that infect bacteria).
o T2,T4,T6- even phages (the common bacteriophages discovered)

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Nucleic acid content

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DNA or RNA viruses. RNA viruses may be positive or negative. Negative RNA viruses carry a special enzyme - RNA polymerase.
o DNA Viruses
o RNA Viruses
o DNA
 Single Strand
 Double Strand
o RNA
 Single Strand
 Double Strand

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Culturing viruses

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• in bacteria (obviously for bacteriophages only)
• in living animals such as mice, rats, etc…..
• in embryonated poultry eggs
• in cell cultures

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primary cell cultures

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normal cells, tend to die after a few generations. (die no matter what you) Only grow in a monolayer

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continuous cell cultures

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abnormal cells (usually taken from a tumor source-cancer), immortal, pile up on each other (past monolayer), replicate indefinitely, live indefinitely. (HeLa cells – Wikipedia- first discovered continuous cells) HeLa cells were used to cure the first polio case.

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Viral Replication

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(Most of what we know is based on research with E. coli and T even phages) Only 2 ways

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Lytic Cycle- Viral Replication

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1. Adsorption - random collision and adherence to complementary host receptor sites, highly host cell specific. The point where virus-host specificity occurs
2. Penetration - nucleic acid core enters host cells, protein coating remains outside. Virus inserts viral DNA
3. Replication/Biosynthesis - vDNA takes over the host cell, directs production of viral components. Eclipse period - no visible sign of viruses present.
4. Assembly/Maturation - viral components assemble into whole viruses, few hundred to several thousand.
5. Release/Burst – lysis (breakdown or explsion) of host cell, releasing mature virions, each capable of infecting another cell.

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burst size (Lytic Cycle)

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number of new viruses released, 50-200

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burst time (Lytic Cycle)

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time interval between attachment and burst, as little as 20 minutes (in vitro).

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Lysogeny- Viral Replication

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Viruses insert vDNA in host DNA. vDNA splices into host DNA and remains dormant until induced (UV light, radiation, chemicals, ?) to undergo the lytic cycle.

Virus may be referred to as a prophage or provirus when it inserts into the host DNA. This latency period "hides" the virus from the body's immune system.

1. Adsorption
2. Penetration
3. Insertion into host DNA
4. “Something” will trigger the lytic cycle (exposure to ultraviolet light triggers lysogenic viruses to undergo lytic cycle)

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Transduction - Viral Replication

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Viruses may carry new genes from host to host. A host cell may exhibit new characteristics, used as a tool by researchers to splice genes – genetic recombination.
Also implicated in causing some forms of cancer.

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Eukaryotic Cell Viral Replication - how we know it differs from bacteriophages

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• Require specific receptor sites on eukaryote membranes.
• Non-enveloped viruses bind to potential host cell membrane and induce endocytosis.
• Enveloped viruses fuse with host cell membrane and only core and proteins (enzymes) enter the host cell.
• Replication and assembly much more complex. May occur in the cytoplasm or nucleus or both. Core must "deal" with nuclear membrane. Not well understood. Virus may use host cell's machinery or create its own.
• Release is by cell burst (8-40 hours) or budding (enveloped viruses). Viruses may be released slowly for long time, not killing the host cell.
• Eukaryotic host cells may become transformed, possibly become immortal (linked to cancer) or induce immune reactions.
• RNA viruses may use vRNA immediately or need to synthesize another strand of RNA to be used to create viral components. Retroviruses carry an enzyme, reverse transcriptase, to synthesize a strand of DNA.

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Cancer

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- estimated that 20% of all cancers are activated by viruses. Virus inserts at the site of an oncogene (parts of the host genome).
• Host cells are transformed into abnormal or immortal cancer cells. May produce abnormal products or abnormal amounts of normal cell products.
• Exhibit a loss of contact inhibition - cells don’t know when to stop dividing.
• Cancer cells also undergo dedifferentiation - revert back to an embryonic-like form
• May metastasize - spread to other body parts.
• Overall, cancer cells interfere with normal organ processes and rob vital nutrients.

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Viral Detection

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• In some cell cultures, see cytopathic (abnormal appearance) effect
• hemagglutination - clumping of red blood cells
• Some cells show characteristic changes - rabies causes Negri bodies in brain cells
• Some viral infections create specific signs in the human body

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Koch's Postulates

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cannot be applied to viral diseases because the virus cannot be cultivated in pure culture or there may not be a suitable animal model.

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Viral Treatment

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- antibiotics don't work. Antibiotics used for bacteria often target unique biochemical systems in the bacteria. Because viruses depend heavily on host functions for their multiplication, it has been difficult to find and target unique biochemical systems in viruses.

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Antiviral agents - Viral Treatment

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work by blocking polymerase functions in viruses or base analogs that create non-functional viral DNA, thus prevent viral replication.

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Interferon - Viral Treatment

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different types produced by different host cells. A non-specific action. Viral infection causes host cell to produce interferon. Interferon exposure to surrounding non-infected cells induces them to produce anti-viral protein. Anti-viral protein acts as a barrier thus providing protection for the uninfected cells. Bad side effects often result from interferon administration. Used as last ditch effort to treat some forms of cancer.

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Viral inactivation

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Usually by chemicals such as chlorine, iodine, phenol, detergents, heavy metals and toxic gases. Also heat, UV light. Totally dependent on the individual virus

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Virus-Like Agents- Viriods

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- tiny fragments of RNA, plant pathogens, must be able to penetrate cellulose cell walls, may multiply inside insects, may be passed to pollen or seeds, not much known. Suspected to be associated with human diseases – no direct evidence yet

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Virus-Like Agents- Prions

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proteinaceous infectious particle, current theory is that these are normal proteins that become infectious when folded into abnormal shapes. Invisible to the immune system. Tough enough to survive harsh solvents and extreme temperatures. Bovine spongiform encephalopathy (BSE or Mad Cow Disease), Chronic Wasting Disease (CWD) in deer and elk, Scrapie in sheep. Resistant to high heat, ionizing radiation, UV light, and common disinfectants. Biggest human risk is related to consumption of infected beef products. Incubation 2-20 yrs. The European Union totally banned (MBM) meat and bone meal cattle feed in 2001.