Introduction to Virology practice flashcards

Introduction to Virology and Viral Definition

(Presented by Fathi AlBallali MD, PhD, Professor of Dermatology at the Faculty of Medicine, Benghazi University)

• The word "Virus" is derived from a Latin word meaning "Poison." • A virus is defined as a microscopic infectious agent that is prolific and found in virtually all life forms. • Viruses are obligate intracellular parasites, meaning they are incapable of replication outside of a living host cell. • The formal study of viruses is known as Virology. • Viruses are significantly more difficult to treat than bacteria. • Functional Capabilities: Viruses infect host cells for the purpose of reproduction and can adapt over time to various immune responses.

Viral Size and Morphology

• Size Range: Viruses are extremely small, with dimensions typically ranging from $25\,nm$ to $300\,nm$ . • Visualization: Due to their minute size, they can be visualized only through an Electron Microscope (EM). • Growth Requirements: They cannot be grown on artificial culture media; they require living cells (human, animal, bird, plant, insect, or bacteria) to survive and proliferate.

Structural Components of the Virion

• Virion Definition: The virion is the complete infectious form of a virus when it exists extracellularly (outside the host cell). It consists of the nucleic acid genome enclosed in a protein shell. • Nucleo-capsid (or Nucleoprotein): The combined structure of the viral nucleic acid and its capsid. • Essential Components found in all viruses: • Nucleic Acid Core: The genetic material, which can be either DNA or RNA, but never both. • Capsid: A protein sheath or coat that protects the genetic material. It is composed of individual protein subunits called capsomeres (sometimes referred to as the capsule). • Protein: This is the fundamental chemical component found in every viral species. • Optional or Variable Components: • Envelope: A lipoprotein layer found in some viruses. It consists of material derived from both the virus and the host cell membrane. • Enzymes: Some viruses carry specific enzymes (e.g., those found in Influenza) stored within the capsule. • Matrix Protein. • Spikes: Surface proteins used for attachment to host cell membranes.

Detailed Genomics of Viruses

• Genetic material configuration varies widely among families: • DNA or RNA: Can be single-stranded ( $ss$ ) or double-stranded ( $ds$ ). • Structure: Can be intact (a single piece) or fragmented (segmented). • Shape: Can be linear or circular.

Bacteriophages: Bacterial Parasites

• Definition: Bacteriophages are viruses specifically capable of infecting and destroying bacteria. • Parasitic Nature: They are considered bacterial parasites. Each specific phage type usually depends on a single specific strain of bacteria to serve as its host.

Specific Virus Classification and Dimensions

• DNA Viruses: • HBV (Hepadna-viridae): Circular $dsDNA$ , Diameter: $42-46\,nm$ . • HPV (Papilloma-viridae): Circular $dsDNA$ , Diameter: $65-120\,nm$ . • HCMV (Herpes-viridae): Linear $dsDNA$ , Diameter: $150-200\,nm$ . • HSV & VZV: Linear $dsDNA$ , Diameter: $209-239\,nm$ . • RNA Viruses (also known as Retroviruses in certain categories): • HCV (Flavi-viridae): Linear $(+)ssRNA$ , Diameter: $150-200\,nm$ . • RSV (Respiratory Syncytial Virus / Paramyxo-viridae): Linear $(-)ssRNA$ , Diameter: $45-86\,nm$ . • Influenza (Orthomyxo-viridae): Linear $(-)ssRNA$ , Diameter: $100-1000\,nm$ . • HIV (Retro-viridae): Linear $(+)ssRNA$ , Diameter: $84-170\,nm$ .

Viral Life Cycle and Replication Process

• All viruses share a general six-step replication framework:

Attachment: Viral proteins (spikes) located on the capsid or envelope interact with specific receptors on the target host cell membrane.
Penetration: Attachment triggers conformational changes that facilitate fusion or engulfment, allowing the virion to enter the host cell.
Uncoating: The viral capsid is removed and degraded by viral or host enzymes, stripping away the protein coat to release the viral components inside the cell.
Replication: The virus "takes over" the host machinery to initiate transcription and translation of the viral genome, creating many copies of itself.
Assembly (Maturation): New viral proteins are packaged with newly replicated genomes to form complete new virions.
Release: The new virus particles exit the host cell to infect subsequent targets. There are two primary release mechanisms: • Lysis: The virus causes the death of the host cell by rupturing it. These are "Cytolytic" viruses (e.g., Variola Major/Smallpox). • Budding: The virus exits the cell without killing it, often taking a portion of the host membrane to form an envelope. These are "Cytopathic" viruses (e.g., Influenza).

Transmission Routes and Zoonotic Reservoirs

• Important Transfer Routes: • Skin Contact: HPV (warts). • Respiratory: Influenza, measles, mumps, rubella. • Oral: Polyoma virus, Coxsackie, Hepatitis A, Rotavirus. • Milk: HIV, CMV, HTLV (Human T-Lymphotropic virus). • Trans-placenta: Rubella, CMV, HIV. • Sexually: Herpes 1 and 2, HIV, HPV, Hepatitis B. • Animal Bite: Rabies Virus. • Animal Reservoirs (Zoonosis): • Influenza: Birds (Avian flu), pigs (Swine flu), horses, and seals. • Rabies: Dogs and foxes. • Hanta-RNA virus: Rodents. • Ebola and Marburg: Monkeys. • HIV: Monkeys, Chimpanzees (HIV-1 groups M, N, P, O), Gorillas, and Sooty mangabeys (HIV-2).

Clinical Outcomes and Vaccination

• Clinical Outcomes: • Acute Infection: Can result in recovery, progression to chronic disease, or death. • Chronic Infection: Can be a lifelong subclinical infection, potentially leading to reactivation, relapsing/exacerbation cycles, or even Cancer. • Vaccines: • Function: Stimulate the immune system to recognize and attack viruses outside of host cells. They prepare the system to block future attacks by real viruses. • Composition: Usually consist of an attenuated (weakened) copy of the virus. • Limitation: Vaccines are for prevention; they cannot treat patients who are already infected. • Challenges: Viral mutation (as seen in Influenza) requires vaccines to be updated annually.

Questions & Discussion

• Question 1: Viruses range in size from? • Answer: $25-300\,nm$ (Option b). • Question 2: A structural component found in all viruses is? • Answer: Capsid (Option c). • Question 3: A chemical component found in all viruses is? • Answer: Protein (Option a). • Question 4: Viruses that can remain latent (usually in neurons) for many years are most likely? • Answer: Herpesviruses (Option b). • Question 5: What types of viruses contain the enzyme lysozyme to aid in their infection? • Answer: Bacteriophage (Option a). • Question 6: Which of the following is not an RNA virus? • Answer: Adenovirus (Option d).