Viruses intro Flashcards
Explain the basic classification and characterisation of viruses.
- Type of Genetic Material
DNA or RNA: Viruses can have either DNA or RNA as their genetic material. This can be further classified into:
Single-stranded (ss) or double-stranded (ds).
Positive-sense (ssRNA), which can be directly translated into proteins, or negative-sense (ssRNA), which must be converted to a positive-sense strand first. - Shape and Structure
Capsid Shape: The protein coat, or capsid, can be:
Helical: Rod-shaped (e.g., Tobacco mosaic virus).
Icosahedral: Spherical (e.g., Adenovirus).
Complex: Other shapes that don’t fit the above categories (e.g., Bacteriophage).
Envelope: Some viruses have an outer lipid envelope derived from the host cell membrane, while others are non-enveloped (naked). - Size
Viruses vary significantly in size, typically ranging from about 20 nm to 300 nm. The size can influence their mode of transmission and infectivity. - Host Range
Viruses can be classified based on the types of organisms they infect:
Animal viruses
Plant viruses
Bacterial viruses (bacteriophages) - Mode of Transmission
Viruses can be transmitted through various routes:
Direct contact: Skin or mucosal surfaces.
Airborne: Respiratory droplets.
Vector-borne: Via insects (e.g., mosquitoes).
Fomites: Contaminated surfaces. - Pathogenicity
Viruses can be categorized based on their effects on the host:
Lytic viruses: Cause cell death and release new virions (e.g., many bacteriophages).
Lysogenic viruses: Integrate into the host genome and may remain dormant (e.g., certain temperate phages). - Classification Systems
The Baltimore Classification categorizes viruses into seven groups based on their type of nucleic acid and their method of replication:
dsDNA viruses
ssDNA viruses
dsRNA viruses
(+) ssRNA viruses
(−) ssRNA viruses
ssRNA retroviruses
dsDNA retroviruses
Outline the principles of viral culture.
- Selection of Host Cells
Cell Lines: Viruses require living cells to replicate. Researchers select appropriate host cells, which can be:
Primary cell cultures: Freshly isolated cells from tissues.
Established cell lines: Continuously dividing cells (e.g., HeLa, Vero).
The choice depends on the virus’s host range and specific growth requirements. - Inoculation
The selected cell culture is inoculated with the virus. This involves adding a viral suspension to the cells under conditions that facilitate viral entry and replication. - Incubation Conditions
Temperature: Typically 37°C for mammalian viruses.
Atmosphere: Appropriate gas mixture (e.g., 5% CO₂ for mammalian cell cultures).
Time: The duration depends on the virus’s replication cycle and characteristics. - Observation of Cytopathic Effects (CPE)
After inoculation, the cultures are monitored for signs of infection, such as:
Changes in cell morphology (e.g., rounding, detachment).
Cell lysis and death.
These observations indicate viral replication and help assess viral activity. - Harvesting
Once CPE is evident or at specific time points, the virus is harvested from the culture. This can involve:
Collecting the culture supernatant, which contains the released virus.
Detaching infected cells to recover intracellular viruses. - Titration
The viral titer is determined to quantify the virus present in the culture. Common methods include:
Plaque assay: Measures the number of infectious units.
TCID50 (Tissue Culture Infectious Dose): Determines the dilution at which 50% of cultures show infection.
Hemagglutination assay: Used for viruses that agglutinate red blood cells. - Storage
For long-term storage, harvested viruses can be:
Frozen at -70°C or in liquid nitrogen.
Lyophilized (freeze-dried) for stability. - Contamination Control
Maintaining aseptic conditions is crucial to prevent contamination by bacteria, fungi, or other viruses. This involves:
Using sterile techniques and materials.
Regular monitoring of cultures for signs of contamination. - Safety Measures
Working with viruses, especially pathogenic ones, requires adherence to biosafety protocols. This includes:
Utilizing appropriate biosafety levels (BSL-1 to BSL-4) based on the virus’s risk level.
Wearing personal protective equipment (PPE) and working in biosafety cabinets when necessary.
Describe examples of human virus infections of major importance.
- Influenza Virus
Overview: Influenza viruses (types A, B, C, and D) cause seasonal flu epidemics and can lead to pandemics.
Impact: High morbidity and mortality rates, particularly in vulnerable populations (e.g., the elderly, young children).
Prevention: Annual vaccination is crucial, as the virus frequently mutates. - Human Immunodeficiency Virus (HIV)
Overview: HIV is the virus that causes Acquired Immunodeficiency Syndrome (AIDS).
Impact: Affects the immune system, leading to increased susceptibility to opportunistic infections and certain cancers.
Prevention/Treatment: Antiretroviral therapy (ART) can manage the infection, and preventive measures like PrEP (pre-exposure prophylaxis) reduce transmission. - Hepatitis Viruses
Types: Hepatitis A (HAV), B (HBV), C (HCV), D (HDV), and E (HEV).
Impact:
HBV and HCV can lead to chronic liver disease, cirrhosis, and liver cancer.
HAV and HEV typically cause acute infections and are often transmitted via contaminated food and water.
Prevention/Treatment: Vaccines are available for HAV and HBV; antiviral treatments exist for HBV and HCV. - SARS-CoV-2 (COVID-19)
Overview: The novel coronavirus responsible for the COVID-19 pandemic.
Impact: Causes respiratory illness, with symptoms ranging from mild to severe. Long-term effects (long COVID) are also a concern.
Prevention: Vaccines have been developed and widely distributed; public health measures (masking, social distancing) were crucial in controlling spread. - Herpes Simplex Virus (HSV)
Types: HSV-1 (primarily oral herpes) and HSV-2 (primarily genital herpes).
Impact: Both types can cause painful lesions and are lifelong infections. They can also lead to severe complications, especially in immunocompromised individuals and newborns.
Management: Antiviral medications can help control outbreaks and reduce transmission. - Human Papillomavirus (HPV)
Overview: A group of over 200 related viruses, some of which can cause cervical and other cancers.
Impact: Certain high-risk HPV strains are responsible for the majority of cervical cancer cases.
Prevention: Vaccines are available that protect against the most dangerous strains of HPV. - Dengue Virus
Overview: Transmitted by Aedes mosquitoes, it causes dengue fever.
Impact: Characterized by high fever, severe headache, and joint/muscle pain; can progress to severe dengue, which is life-threatening.
Prevention: No specific antiviral treatment; prevention focuses on mosquito control and, more recently, vaccination. - Ebola Virus
Overview: Causes Ebola virus disease, which is highly fatal.
Impact: Outbreaks primarily occur in Africa and can lead to high mortality rates.
Prevention/Treatment: Vaccines have been developed, and supportive care is critical for treatment.
Describe the structure of viruses
Middle bit is nucleic acid with virion associated polymerase on it, then surrounded by a protein caspid. This is all surrounded by a lipid emvelope with spike projections.
- Genetic Material
Nucleic Acid: Viruses can contain either DNA or RNA as their genetic material. This nucleic acid can be:
Single-stranded (ss) or double-stranded (ds).
Linear or circular in shape.
Segmented in some cases, meaning the genome is divided into separate pieces. - Capsid
Protein Coat: The capsid is a protective protein shell that encases the viral genome. It is composed of protein subunits called capsomers.
Shapes:
Helical: Spiral-shaped (e.g., Tobacco mosaic virus).
Icosahedral: Spherical shape made of 20 triangular faces (e.g., Adenovirus).
Complex: Irregular shapes that don’t fit the helical or icosahedral classifications (e.g., Bacteriophages). - Envelope
Lipid Bilayer: Some viruses possess an outer lipid envelope derived from the host cell membrane. This envelope contains viral glycoproteins that are essential for infection.
Enveloped vs. Non-enveloped: Enveloped viruses (e.g., HIV, Influenza) are generally more sensitive to environmental conditions, while non-enveloped viruses (e.g., Adenovirus, Norovirus) tend to be more resilient. - Surface Proteins
Glycoproteins: These are proteins embedded in the viral envelope (if present) or associated with the capsid. They play crucial roles in:
Attachment: Binding to specific receptors on host cells.
Entry: Facilitating the entry of the virus into the host cell. - Additional Structures
Matrix Proteins: Some enveloped viruses have matrix proteins between the capsid and the envelope that provide structural support.
Enzymes: Certain viruses carry enzymes within their capsid or envelope, such as reverse transcriptase in retroviruses, which is necessary for their replication.
Describe how viruses replicate
Viruses replicate by hijacking the host cell’s machinery, as they cannot reproduce on their own. The replication process varies depending on the type of virus (DNA or RNA), but generally follows these key steps:
- Attachment
Receptor Binding: The virus attaches to a specific receptor on the surface of a susceptible host cell using its surface proteins (glycoproteins). This specificity determines the virus’s host range. - Entry
Penetration: After attachment, the virus enters the host cell through various mechanisms:
Endocytosis: The host cell engulfs the virus in a vesicle.
Membrane Fusion: For enveloped viruses, the viral envelope fuses with the host cell membrane, releasing the capsid into the cytoplasm. - Uncoating
The viral capsid is removed, releasing the viral genome into the host cell’s cytoplasm. This step may involve cellular enzymes or changes in pH. - Replication and Transcription
DNA Viruses: The viral DNA enters the nucleus and utilizes the host’s DNA-dependent RNA polymerase to transcribe mRNA, which is then translated into viral proteins.
RNA Viruses:
Positive-sense RNA: Can be directly translated into proteins by the host ribosomes.
Negative-sense RNA: Must first be transcribed into positive-sense RNA by viral RNA polymerase before translation.
Retroviruses (like HIV): Reverse transcriptase converts the viral RNA into DNA, which integrates into the host genome. - Assembly
Newly synthesized viral genomes and proteins are assembled into new virions (virus particles) in the cytoplasm or nucleus of the host cell. - Budding or Lysis
Enveloped Viruses: New virions acquire an envelope by budding off from the host cell membrane, taking part of the membrane with them.
Non-enveloped Viruses: Typically cause cell lysis, leading to the release of new virions upon cell death. - Release
The new virions are released into the extracellular space, ready to infect other cells and continue the cycle of infection.
Summary
The replication process of viruses is highly efficient and exploits the host cell’s resources. Understanding this process is crucial for developing antiviral therapies and vaccines to combat viral infections.
Explain the concept of host range in relation to viruses
Some viruses may only infect humans, e.g. smallpox, measles,
Some may also infect other animals / birds
Transmission of a novel virus to humans
Coinfection of human and animal or bird strains in one organism may lead to recombination and generation of a new strain
Describe the consequences of a viral infection
Clearance of virus (with no, short or long term immunity)
Measles (long term immunity)
Chronic infection
HIV, hepatitis B, hepatitis C
Latent infection: Herpes Virus
Transformation (long term infection with altered cellular gene expression)
Epstein-Barr Virus, Human Papillomavirus
Explain the concept of viral latency
Following primary infection, some viruses lie dormant in the cell.
The full viral genome is retained in the host cell, but its expression is restricted, such that few viral antigen and no viral particles are produced.
Reactivation of viral replication can occur
Reactivations may or may not cause apparent disease
Reactivation more likely to occur and more severe in immunocompromised
Examples:
Herpes Simplex Virus
Varicella Zoster Virus
Describe the link between viruses and cancer and explain the mechanisms through
which this results
A number of viral infections can lead to cancer (often requires some other event to also occur)
Mechanisms
Modulation of cell cycle control (driving cell proliferation)
Modulation of apoptosis (prevention of programmed cell death)
Reactive oxygen species mediated damage (some persistent viral infections can cause persistent inflammatory processes which lead to cancer via reactive oxygen species).
Examples include; Human T-cell Lymphotropic Virus (HTLV): Adult T-cell leukaemia/lymphoma
Human Papillomavirus (HPV): cervical, anal, oropharyngeal cancers
Hepatitis B and Hepatitis C: hepatocellular carcinoma.
Describe the indications for and principles of anti-viral therapy
All antiviral agents are virustatic, none are virucidal
As viruses utilise host cell enzymes in order to replicate, there are limited viral proteins that are potential targets for antiviral drugs
Toxicity to the host cell is not uncommon: side effects
Only used in a minority of viral infections.
Antivirals may be used for:
Prophylaxis (to prevent infection)
Pre-emptive therapy (when evidence of infection/replication detected, but before symptoms are apparent)
Overt disease
Suppressive therapy (to keep viral replication below the rate that causes tissue damage in an asymptomatic infected patient)
Describe how to prevent viral infections and explain how viruses can be eradicated
- Immunisation
- Vaccination
- Passive immunisation with immunoglobulin
- Prophylactic treatment post exposure
- Infection prevention and control measures
- Isolation of symptomatic patients
- Personal protective equipment
- Safe use and disposal of sharps
- Blood / tissue / organ screening
- Antenatal screening
Viruses with the following properties can potentially be eradicated:
- No animal reservoir or ability to amplify in the environment
- Clearly identifiable, with accurate diagnostic tool
- No chronic carrier state
- Efficient and practical intervention, e.g. vaccination
- Political / social support
> Examples: Smallpox (eradicated in 1979)
Measles
Polio
What are detection methods of viruses for a) part of an organism, b) whole organism or c) immune response
a) Antigen detection, DNA/RNA detection: Extraction of genetic material from sample, Amplification of region of target organism genome (if organism present in sample).
b) Microscopy, culture
c) Similar methods as those used for antigen detection
May be used to determine:
Acute / recent infection
Prior infection / response to vaccination.
