Viral properties Flashcards
What are Koch’s postulates?
- The microorganism must be found in large numbers in all diseased animals, but not in healthy ones
- The organism must be isolated from a diseased animal and grown outside the body in a pure culture
- When the isolated microorganism is injected into other healthy animals, it must produce the same disease
- The suspected microorganism must be recovered from the experimental hosts, isolated, compared to the first microorganism, and found to be identical
What is a virus?
An infectious obligate intracellular parasite
What is the average size of a virus?
100nm
What are the two types of virus morphology?
Non-enveloped = symmetrical protein capsid
Enveloped = Lipid envelope derived from host membrane
TRUE OR FALSE:
Some viruses can have a combination of both capsid and envelope
TRUE
E.g. Herpes virus
How are viruses classified?
Baltimore classification system
Made up of 7 classes, based on the genome of viruses
What are the different groups under the Baltimore classification system?
- DNA viruses (double stranded and single stranded)
- RNA viruses (double stranded, positive and negative)
- DNA and RNA viruses (retroviruses and double stranded DNA)
Why might viruses have evolved to use different nucleic acids in their genome?
- RNA viruses and retroviruses use their own polymerase to replicate = lack proof-reading capacity –> high mutation rate (may be advantageous)
- RNA viral genomes are limited in size due to inherent instability to RNA vs DNA
- RNA viruses often use complex coding strategies to make more proteins than expected from a small RNA genome
- DNA viruses have large genomes = plenty of room for accessory genes that can modify host immune response. Genes are often lost in passage in culture
- Segmented genomes allow an additional easy form of recombination = reassortment. BUT! This also impose more difficult packaging strategies
Outline a generic virus replication cycle
- Virus finds cell (proteins on envelope bind to receptors on surface of cell)
- Virus envelope fuses with cell membrane and the viral contents enter the cell
- Some copies of the virus genome gets replicated
- Some gets reverse transcribed to viral DNA, which is integrated into the host genome
- It is then transcribed and translated into proteins
- The proteins and copies of the genome then assemble to form new virus particles, which exit the cell
What is the cytopathic effect?
Death of a cell as a result of being infected by a virus
This could be due to shut down of host protein synthesis or accumulation of viral proteins
Give 3 methods of detecting viruses in a sample
- Plaque assay - viruses form plaques (of dead cells)
- Syncytia - viruses with surface proteins fuse cells together
- Immunostaining - antibodies generated in the lab to unique virus proteins indicate which cells are infected/where in the cell the virus proteins are located
Viruses form plaques in cell monolayers. How can these plaques be used to quantify the number of virus in a sample?
The plaque assay:
- Serial 10-fold dilutions and is then spread on a monolayer of susceptible cells
- A plaque will appear where an individual virus has killed some cells
- The number of plaques can be counted and scaled up to quantify the amount of virus in a sample
What are the 3 phases of virus growth?
- Eclipse phase
- Logarithmic phase
- Cell death
Give 5 methods of diagnosing a viral infection
- Detecting viral genome (PCR)
- Detecting viral antigen (Indirect Fluorescence Antibody, ELISA)
- Detecting virus particles (electron microscopy, haemaggluttination assay)
- Detecting virus cytopathic effect in cultured cells (virus isolation)
- Detecting antibodies to virus (serology)
How are viruses cultivated?
- Providing permissive cells (often continuous lines of transformed cell cultures)
- NOTE: they are different from the cells in our bodies
- Viruses may accumulate mutations that adapt them to the “new” host
- This can lead to attenuation (viruses no longer harms host) and was the basis for generation of vaccines in the past
