Viral properties and disease Flashcards
Define the nature of viruses, and summarise a generic life cycle of a virus, explaining its parasitic nature in relation to the host cell Explain the basis for the classification of viruses, and how viruses are detected, cultivated and manipulated Viral routes of infection: explain the different routes by which viruses cause infection, define the term tropism, and explain what determines the tropism of a virus Viral infection outcomes: list different outcomes of infection by viruses, acute in (52 cards)
What is the generic size of viruses?
Very small. 100nm. Can only be seen by electron microscopy.
How can it be proven that a virus causes a disease according to Koch’s Postulates? (x2)
Microorganism found in large numbers in all diseased animals, but not in healthy. The organism must be isolated, grown, purified and injected into healthy animals. Those healthy animals must produce the same disease, and those suspected microorganisms recovered and compared to the first microorganism as the same that was isolated. NOT ETHICAL.
What are viruses? How is genetic material contained?
Obligate (intracellular) parasites. DNA OR RNA.
How do viruses briefly work inside cells?
Viral genome replicated and directs synthesis of more viral components and genomes by manipulation of cellular systems.
What are the three virus morphology types?
Non-enveloped: have symmetrical protein CAPSID (protein shell of virus).
Enveloped: lipid envelope derived from host cell membrane of the last cell it replicated (virus leaves previous host cell and buds off using the cell’s membrane).
Some are a combination of both (capsid around genome and envelope around that).
How are viruses classified? What is the classification called?
How they carry their genes (DNA, RNA) and the mechanism of transcription. Baltimore classification.
Give three examples of different viral classes. (This is just for broader understanding).
DNA genome –> RNA –> Protein
Negative sense RNA –> DNA –> RNA –> protein (photo attached)
(Positive-sense m)RNA –> Protein
7 classes describing genetic material and transcription mechanism – some more complicated.
What are the consequences of virus having an RNA genome? (x2(x1)).
Have to use their own polymerases to replicate because body doesn’t replicate RNA. These lack proof-reading mechanisms leading to higher mutation rate. RNA genomes small because of instability. So tend to be simpler.
What are the consequences of virus having DNA genome? (x2(x1)).
Hold a lot of information. So, lots of room for accessory genes which can modify the host immune response. Can impose more difficult packaging strategies.
What is the generic life cycle of viruses? (x6 stages).
1) Find cell. Protein shell of virus binds to viral receptors on cell. But they are not actually virus-specific receptors – they are normal receptors for normal cell function which are exploited by viruses. 2) Virus uncoats (removes envelope/capsid) so genome has access to cell machinery. 3) (Synthesise mRNA) and exploit ribosomes in cell. 4) Proteins synthesised. Early-proteins: turn cell into into virus factory, OR late-proteins: form capsid or coat (protein components of virus). 5) Replicates genome by using its own or the cell’s polymerases. 6) Newly synthesised proteins and newly synthesised genomes join to create viruses which bud out of cell and spread.
How are viruses studied in the laboratory?
Too small, so study viruses by looking at effect on cell. Light microscope.
What do viruses do to appearance of cells in a tissue? What is this effect called? How can live/dead cells be distinguished?
Cells infected with virus: cells become round, dense, and then clump together. Called cytopathic effect (diseased state). Live cells will pick up stain, and dead cells (from cytopathic cell) don’t pick up stain.
What two mechanisms cause the cytopathic effect of viruses?
Cytopathic effect is the result of: mechanism of virus taking over the cell machinery; cells shut themselves down to save organism.
What do viruses leave to tissues?
Plaques – holes in the cell layer where virus has infected a cell and viruses have burst out and effected surrounding cells.
What do plaques tell us about our sample? How are the number of plaques determined in the laboratory?
Counting plaques tells us how many viruses were present in the sample that we put on the tissue in the first place. Titrate sample (e.g. urine) and make a series of 10-fold dilutions. Count plaques and extrapolate dilution back to determine amount of virus in clinical sample.
What is the alternative to plaques?
Some viruses may fuse cells together instead – forming a SYNCYTIA. Counting number of syncytium tells us number of viruses.
How can you determine where the virus is in a sample?
IMMUNOSTAINING. Generate antibodies that are unique to the virus proteins, add a fluorescent dye. You can use this to determine which cells are infected with the virus and what parts of the cell the virus proteins are located.
What is the nature of viral population growth?
ECLIPSE PHASE – start, where virus decreases in population – time where the virus goes INTO the cells and manipulates cell machinery.
Log growth curve.
Exponential because each infected cell = many more viruses synthesised.
Plateaus when all cells in petri dish used up and killed by virus.
How is a viral diagnosis done when i)we know what virus we are looking for (x2); ii)when virus is new (x2); iii)studying prevalence after infection?
i)Virus sample from patient in question is taken. Viral genome is extracted from sample and primers specific to part of the genome we are looking for mixed. If virus we are looking for is present, it will be amplified to a level we can detect. Sometimes, may use procedure that detects proteins and antigens of the virus (IFA (immune-fluorescence antibodies like explained in the immunostaining question), ELISA). ii)use electron microscopy (difficult, expensive, not sensitive). OR hemagglutination assay. iii)Diagnose after infection by taking blood of somebody who you think may have had that virus. Perform serology and look at antibodies in population. May help to determine if widespread vaccination needed.
What is needed for cultivation of virus?
Permissive cells and the environment needed for those cells to live. NB: Some viruses cannot be cultivated because they require more than just the host cell.
How are viruses manipulated in the lab?
Viruses are small so, we can make a virus from scratch. If we know the sequence of a virus genome, we can make that virus, introduce ONLY the genome into the cell (because when virus enters, it uncoats anyway), and many more viruses will grow (including the protein components). This is called reverse genetics – allows us to make viruses at will with mutations engineered into their genomes. This means that we don’t have to do long-winded attenuation in a different cell to evolve it into a vaccine.
What are the routes of transmission of viruses? (x8) RODZ By SMG
Respiratory (droplets), Faecal-oral (contaminated water), direct contact (saliva, fomites (contaminated surfaces)), Zoonoses (caught by animals and insect bites), blood (sex, transfusions), sexual contact, maternal-neonatal, germ line (can never get rid of them). RODZ By SMG
What does iatrogenic mean?
Transmission by healthcare worker.
What does Nosocomial mean?
Transmission type where infection caught in hospital.
