Microbiology 2 Flashcards
What is the structure of a virion?
- Genome composed of RNA or DNA (not both)
- Surrounded by a delivery system - a protein shell (capsid/nucleocapsid)
- In some cases a lipid envelope
What is the function of a capsid (in virus)?
- Protects the genome
- Deliver genome into cell
> capsid proteins bind to cell receptors, trigger uncoating and ensure genome ends up in correct place within cell - Made of large number of identical protein subunits
Viruses can be classified according to:
- Nature of the nucleic acid (DNA or RNA) and genome type (ds/ss/sense)
- Presence of envelope
> there are a number of different criteria but these are most important.
What is an example of a non-enveloped DNA virus?
Canine parvovirus
What is an example of an enveloped DNA virus?
Feline herpesvirus
What is an example of a non-enveloped RNA virus?
FMD virus
What is an example of an enveloped RNA virus?
Influenza virus
What are some features of DNA viruses?
- DNA stable
- Most replicate in nucleus
- Viral DNA replication
- DNA viruses less prone to mutation
What are some features of RNA viruses?
- RNA is labile and transient (unstable)
- Most replicate in cytoplasm
- Cells cannot copy RNA from RNA.
RNA viruses must provide own RNA dependent RNA polymerase - RNA viruses prone to mutation
What are features of enveloped viruses?
- fragile in envt
- do not survive adverse conditions
- easily inactivated by disinfectants
- Exit cells by lysis or budding and syncytia
- Require both CELL-MEDIATED and HUMORAL immunity
e. g. Herpes virus
What are features of non-enveloped viruses?
- very stable
- more resistant to acid, heat, cold, proteases, detergents, drying
- Exit host cells only by lysis
- Antibody (acquired o passive) directed at virus usually protective
e. g. canine parvovirus (DNA), FMD virus (RNA)
What is meant by ‘viral tropism’?
Capacity to enter and interact with host to cause disease.
Discuss how mutation in the viral genome can alter the host range and virulence?
- Mutation is a structural alteration in the nucleic acid (genotype)
- Can have a -/neutral/+ effect on the fitness of the virus
- Mutation can have important effects on:
> Virulence of virus
> Host range of virus - Point mutation, Insertions, deletions
- Viral recombination = transfer of genetic material between different but closely related viruses infecting the same cell.
What is antigenic drift?
Gradual accumulation of point mutations (more common)
What is antigenic shift?
Acquisition of a new gene from another virus - particularly common in some viruses e.g. Influenza - due to recombination of reassortment (relatively rare).
Viruses can “co-infect” to exchange genomic material.
Describe the types of virus-cell interactions that occur using examples of viral diseases that have this type of interaction.
- Lytic infections (cytocidal) e.g. Parvovirus, FMD
- Persistent infection
- -> Productive e.g. rabies, FMD
- -> Latent e.g. herpes, retrovirus
- -> Non-productive transforming e.g. retroviruses, equine sarcoid
What are the effects of viral infection on cell morphology?
- Cytopathic effect: effects seen in cell monoculture
- Inclusion bodies: Morphological changes, Intranuclear (DNAv) or intracytoplasmic (RNAv). not all viral infections produce inclusion bodies (e.g. Calici v)
- Alterations in cell membranes: Increase permeability, ion exchange/membrane potential, proliferation and rearrangement of membranes, Insertion of viral glycoproteins
- Formation of syncytia: Viral proteins inserted into plasma membrane of one cell may lead to fusion between neighbouring cells
- Haemadsorption: Cell in culture infected with some viruses may acquire the ability to absorb RBC due to presence of viral glycoproteins on membrane
How do viruses damage cells?
- Cell lysis
- Inhibit host cell nucleic acid synthesis/ shutdown protein syntehsis
- Cytopathic Effects of toxic viral products
- Cyolysis by immunologic mechanisms
Outline the routes of viral entry and discuss host defence at the sites.
- Respiratory system: most common e.g. Influenza, FMDV and some herpesviruses.
Distance particles travel into the RT depends on their size
Nasal passages, Nasopharynx, Larynx, LRT - Alimentary system: hostile envt. for viruses (most common are non-enveloped). Peristalsis, Mucous, Bile, Proteases, Gastric Acid, Peyer’s patch
- Skin: keratinised layer. Dead cells continual shedding.
- Conjunctiva: FHV. Tear producing
- Urogenital tract: urinary flushing, anatomical barriers, low pH, secretory IgA
