Concepts in microbiology - viruses and prions Flashcards
Genetic material found in viruses
DNA or RNA
Which infectious agent has no nucleic acid?
Prions
What is meant by viruses being ‘metabolically inert’?
They do not possess ribosomes so cannot undergo protein synthesis. Some have enzymes
How are the swaps used to sample viruses different to microbiological samples?
Swabs need to be placed in a viral transport medium
How are viruses grown?
Using cells (unlike bacteria which can grow on agar plates)
Place in size order: bacteria, virus, eukaryotic cell
eukaryotic > bacterium > virus
Term used to describe a complete virus particle
Virion
Components making up a virion
envelope, capsid, nucleic acid genome
What makes up the envelope of some viruses?
Lipid membrane
Function of envelope in some viruses
protect the capsid
What is the capsid in viruses?
A protein shell around the nucleic acid genome (RNA or DNA) made of capsomers
Name of the protein units that make up the capsid in viruses
Capsomers
6 key stages of viral replication
Attachment, penetration, uncoating, synthesis of viral components, assembly, release
Possible effects of viruses on cells
Cell death, transformation, latent infection
Term used to describe the change in cell morphology caused by an infecting virus
cytopathic effects
Example of a virus that causes cell death
Herpes simplex virus spreads rapidly between epithelial cells causing them to lyse. Forms an ulcer
What is the transformation effect of a virus on a host cell?
The cell is not killed but changed to a cancerous cell
Example of a virus that causes transformation of cells
Human papilloma virus can cause oral cancer and oropharyngeal cancer
What is the latent infection effect of viruses on cells?
The remnants of virus and nucleic acids remain in the cell with no obvious effect on cell function
Example of a virus that causes latent infection of host cells
Herpes simplex virus causes recurrent Herpes labialis
Why do viruses infect specific cells?
antigen is only specific to particular glycoprotein receptors on cells
How may replicated virus particles be protected from the host’s immune system?
As the viral particles emerge from the host cell, the plasma membrane may form an envelope around the virus (now has self-antigens)
Name of drugs that prevent replication of virus
anti-viral drugs
Modes of viral transmission
inhalation via respiratory tract, ingestion via GIT, inoculation, congenital (mother to foetus), sexual transmission
Example of a virus passed on by inhalation via respiratory tract
coronavirus, rubella
Example of a virus transmitted by ingestion via GIT
Norovirus (spreads through contaminated food, water, surfaces)
How can viruses be passed on by inoculation?
Through skin abrasions, mucous membranes, transfusions, transplants, injections, bites (introduces infectious material into body)
Example of virus passed on by inoculation
Hepatitis B
Example of virus passed on congenitally
Rubella - infected pregnant mother has 90% chance of passing on Rubella which may lead to child having Congenital Rubella Syndrome. Also transmitted by inhalation.
Example of virus transmitted sexually
HIV
Example of virus affecting the nervous system
Rabies
Example of viruses affecting the respiratory tract
Adenovirus, influenza
Examples of viruses that cause localised diseases of skin and mucous membranes
Herpes simplex virus
Example of virus that affects the eyes
adenovirus (conjunctivitis)
Example of virus that affects liver
hepatitis C
Example of virus that affects the salivary glands
Mumps (parotid gland swelling)
Example of virus that affects the gastrointestinal tract
norovirus
Reasons for using viral diagnositics
Stop antibiotics, find most effective treatment, evaluate anti-viral drug susceptibility, public health and IP&C reasons (knowing stage of infection in individual and community for intervention), epidemiology / surveillance.
What is epidemiology?
the study of the distribution of disease in populations and the factors which determine this distribution
How to begin diagnosing a viral infection
Review the clinical history, examine patient, use a systems based approach for differential diagnosis (e.g. surgical sieve), make a provisional diagnosis, plan special investigations
What is the surgical sieve?
Method for coming up with multiple diagnoses for patient using mnemonic (MEDIC HAT PIN: metabolic, endocrine, degenerative, infective, congenital…causes)
What specimens are used for nucleic acid detection e.g. HSV?
swabs, faeces, aspirates, plasma, CSF, vesicle fluids, urine
Which specimen is monitored by PCR e.g. to count the HIV load?
plasma (from blood culture)
Which specimen is used in serology e.g. counting Hep B antibodies titres?
Serum (from blood culture)
Techniques used in viral diagnostics
nucleic acid detection / sequencing, antigen detection e.g. hep B surface antigen in blood serum
How does nucleic acid detection / sequencing work?
Based on PCR principle - small amount of nucleic acid is amplified so there is enough for detection.
When sequencing a virus containing RNA, what step must occur before PCR?
Reverse transcription to convert RNA to DNA. Primer added to RNA and a complementary DNA strand is produced. Primer added to complementary DNA…
What is the first class of antibody produced during acute infection phase?
IgM
Which antibody indicates more time has passed / a later stage of infection?
IgG (change in antibody class from IgM -> IgG
What is the genetic material in influenza viruses?
RNA
How is influenza transmitted?
Airborne droplets enter the respiratory tract
How does a virus (e.g. influenza) enter the host cell?
Antigens attach to receptors on host cell triggering receptor mediated endocytosis. Virus is internalised in a membrane bound capture vesicle
What are kinesins?
The host proteins that transport the virus (within capture vesicle) along microtubules.
What does the uncoating process involve in viral replication? Influenza example
Matrix proteins detach from proteins covering RNA genome due to influx of H+ from fusion with lysosome. Hemagglutinin inserts itself into vesicle membrane causing it to fuse with the viral membrane allowing the release of RNA.
How are viral components synthesised?
RNA enters the nucleus via nuclear pores. The RNA is transcribed to mRNA by RNA polymerase. mRNA is translated into viral proteins by ribosomes.
How are progeny viruses released from the host cell?
By budding
How are virions assembled within the host cell?
Viral proteins and viral RNA exit the nucleus and travel to the plasma membrane where they are packaged into viral envelope proteins when they bud from the host cell.
Examples of viruses that vaccines are ineffective against
common cold, HIV
Name of 2 glycoproteins found on the capsid of influenza virus
Haemagglutinin (HA) and neuraminidase (NA)
Function of haemagglutinin?
Bind to sialic acid on host cell surface receptors
How many types of hemagglutinin spikes are there in influenza?
14 H types
Function of neuraminidase
Neuraminidase cleaves the sialic acid molecule on the host receptors which the hemagglutinin is bound to, allowing the virion to be released from the host cell.
How many types of neuraminidase are there in influenza?
9 N types
2 ways that the antigens on influenza virus (HA and NA) can change
Antigenic drift, antigenic shift
What is antigenic drift and how is it caused?
Minor, gradual changes in the antigens due to point mutations in the genome of the virus each time it is replicated.
Why can antigenic drift render flu vaccines ineffective?
Changes in antigens accumulate over time so that antibodies are no longer complementary to the antigen (antigens not recognised by immune system)
Which change in antigen structure can cause an epidemic (regional outbreak)?
Antigenic drift
Which types of influenza are affected by antigenic drift?
A, B, C all drift (all 3 types)
What is antigenic shift?
A major change in antigenic structure of a virus caused by recombination
How can recombination occur in antigenic shift?
Pig cells have both human and bird virus receptors. The human and bird virus genes can randomly combine within the pig cell to generate a new flu virus.
Which change in antigenic structure can result in a pandemic (international outbreak)?
Antigenic shift
Which influenza type can undergo antigenic shift?
Only type A
How are influenza viruses classified?
Using the type of hemagglutinin and neuraminidase receptors found on the capsid e.g. H2N2 influenza virus has HA type 2, NA type 2
What is the chain of infection?
- infectious agent
- reservoir
- portal of exit
- mode of transmission
- portal of entry
- susceptible host
What are prion diseases referred to as?
Transmissible Spongiform Encephalopathies
What does Transmissible spongiform encephalopathies mean?
The prion disease can be passed on and mainly affects the brain causing a spongy appearance
What are prion diseases / transmissible spongiform encephalopathies?
Fatal neurological diseases caused by the accumulation of prion protein (a cell membrane protein) within the CNS.
Who discovered prions?
Prusiner - won 1982 Nobel prize
Where is the name prion derived from?
Proteinaceous Infectious particle
What is a prion?
Abnormal form of a protein found in the brain responsible for causing transmissible spongiform encephalopathies
Name of transmissible spongiform encephalopathy that affects sheep (studied by Prusiner)
Scrapie
Name of CJD-like disease in Papua New Guinea tribe in 1950s/60s
Kuru
What feature of prions may account for its resistance to sterilisation and disinfectants?
No nucleic acid
3 forms of prion disease
Infectious (transmitted via contaminated blood/instruments), genetic (families), sporadic (random 1 in 1 million develops sCJD)
Cause of prion disease
Accumulation of an abnormal form of a natural protein in the brain
Feature of normal prion proteins
Contain alpha helices that are pliable and no the protein is more readily broken down by the body
Feature of abnormal prion proteins
Contain beta-pleated sheets which are more rigid so prion is more difficult to be broken down
Where do normal prion proteins exist?
On the exterior surface of cell membranes (intercellular communication function)
Which infectious agent does not trigger an immune response?
Prions
What characteristic of prions determines where the pathology occurs in the brain and hence the symptoms?
Shape of prions e.g. structure, carbohydrate side chains.
What is PrPc?
Prion protein cellular - normal form of prion protein
What is PrPSc
Prion protein scrapie - abnormal form of prion protein
How are normal prions (PrPc) produced?
Prion protein gene is transcribed into mRNA which is translated at ribosomes in PrPc.
How do abnormal prion proteins (PrPSc) reproduce?
An abnormal prion (PrPSc) transforms a normal prion (PrPc) into a PrPSc by a chain reaction.
At which stage does the speed of abnormal prion protein (PrPSc) production increase?
Once the initial conversion of one PrPc to PrPSc is achieved, there is a more rapid aggregation of abnormal PrPSc.
Types of human CJD
Sporadic (sCJD)
Familial (fCJD)
Iatrogenic (iCJD)
Variant (vCJD)
How may familial CJD be transmitted?
Inherited mutations in prion gene
What is iatrogenic CJD?
The infectious form of CJD
How may iatrogenic CJD be transmitted?
Surgical instruments, contaminated hormones/dura mater graphs, Kuru (cannibalistic rituals)
Cause of variant CJD?
Consumption of food contaminated with bovine spongiform encephalopathy
Difference in age of onset between sporadic and variant CJD
sCJD approximately age 60-70 whereas vCJD has a much younger onset
Why were people more concerned about vCJD compared to sCJD?
vCJD has a much higher infectious dose and spreads outside the NS to peripheral tissues (appendix, tonsils, spleen)
Since the peak of vCJD cases in 2000, how was the epidemic overcome?
Interventions to restrict spread of disease e.g. food precautions, control of blood transfusion
Which staining technique was used to detect abnormal prions (vCJD) in the trigeminal ganglion?
Immunohistochemistry
Which technique was used to detect prions in vCJD in dental tissues (alveolar nerve and tongue)?
Immunocytochemistry
What are the concerning features of CJD?
Abnormal prions are difficult to clean from instruments, can survive steam sterilisation, decades long incubation period, asymptomatic / pre-symptomatic carriage of iatrogenic CJD
Example of action taken by health department to reduce risk of vCJD transmission
Single use of matrix bands and endodontic files (tested positive for residual blood contamination after sterilisation)
What infection prevention measure can be used when treating a patient with / increased risk of CJD?
Reprocess instruments according to best practice and return to use
