Microbiology 4 - Viral properties

Question 1

What is a virus?

Answer 1

Infectious OBLIGATE intracellular PARASITES

Question 2

Define the nature of viruses.

Answer 2

• Small size
• Dependence on a host
• Structural and genetic diversity

Question 3

What are the dufferent morphologies of viruses?

Answer 3

• May have a symmetrical protein capsid and no envelope
• May be enveloped using the lipid membrane from a host (pleiomorphic/typical shape)
• May have both a capsid and an envelope
• Bacteriophages are the ones that look like spaceships

Question 4

What is the central dogma for viral replication?

Answer 4

DNA -> RNA -> Protein

- Makes negative sense RNA (complmentary to the mRNA)

Question 5

Why do viruses have a high mutation rate?

Answer 5

They use their own polymerases which have no proof reading capacity.

Question 6

Why are RNA viral genomes limited in size?

Answer 6

RNA is more unstable than DNA

Question 7

How does viral DNA/RNA appear?

Answer 7

• It can be segmented - similar to human chromosomes, which allows a form of recombination called re-assortment to occur.
• Can have viral sex

Question 8

Describe the generic virus replication cycle.

Answer 8

• The protein coat binds to viral receptors on the cells (which have other functions)
• When inside, the virus falls apart and the genome is replicated
• Early regulatory proteins are synthesised
• Then late structural proteins
• There is then assembly of the late proteins with the viral genome

Question 9

How do retroviruses work?

Answer 9

They integrate their DNA with the host DNA.

Question 10

What is the cytopathic effect?

Answer 10

The virus lysing the cell - due to shut down of host protein synthesis or accumulation of proteins.

Question 11

What are viral plaques?

Answer 11

A visible structure formed within a cell culture, where viruses replicate and spread to generate regions of cell destruction.

Question 12

What are syncytia?

Answer 12

Multinucleated giant cells that form when viral proteins cause fusion of cells.

Question 13

How is immunostaining of viral cells performed?

Answer 13

Antibodies are generated in the lab that bind to viruses and indicate the infected cells

Question 14

Describe the growth phases of a virus.

Answer 14

1) Infection - an eclipse phase occurs where concentration of viruses outside the cells is low as the viruses all enter the cells
2) Logarithmic phase where the cells rapidly grow
3) Cell death - no cells are left so the graph will plateau

Question 15

What are the methods of viral diagnosis?

Answer 15

• Detecting the viral genome using PCR
• Detect viral antigen using IFA (immunofluorescence assay/ELISA)
• Detecting virus particels (EM)
• Detecting the cytopathic affect in cultured cells (isolation)
• Detecting antibodies to virus (serology)

Question 16

What does the method of viral diagnosis used depend on?

Answer 16

• Source of the specimen
• Purpose of test (therapy or surveillance)
• Who will use it and where
• What stage is the disease (active replication or antibody response)

Question 17

How are viruses attenuated?

Answer 17

• Cells are grown in the lab and as a result they acquire mutations that adapt them to a new host
• This means that are no longer functioning in a human
• Used to make vaccines

Question 18

How do norovirus cultures move to the target cells?

Answer 18

Enteric bacteria promote norovirus infection of B cells, which may allow the virus to cross the intestinal epithelium

Question 19

How are viruses manipulated?

Answer 19

• Viral genomes can be synthesised and introduced into permissive cells to direct synthesis of their new component - made de novo
• Allows reverse genetics (create viruses with engineered mutations)

Question 20

What are viruses named after?

Answer 20

• The disease
• The person who discovered it
• The place it was discovered
• The part of the body affected
• The way it was spread

Question 21

What is the size range of viruses?

Answer 21

1-10 nm