Viruses/pathogens

General Characteristics and Historical Context of Viruses

Viral Definition and Core Properties

• Size and Visibility: Viruses are characterized as being extremely small, falling into the sub-microscopic range. They cannot be seen with conventional light microscopy and require an Electron Microscope for visualization.
• Obligate Intracellular Parasites: Viruses are acellular particles that are incapable of survival or biological activity outside of a host cell.
• Metabolic Limitations: Viruses cannot undergo metabolism independently. They lack the necessary genetic information to produce the components required for independent replication.
• Host Hijacking: To reproduce, a virus must infect a host cell and hijack its metabolic machinery. The resulting viral particles produced by the host are known as virions.
• Host Range: Viruses can infect all types of cellular organisms, including humans, animals, plants, and bacteria. They typically possess a specific host range, often requiring vectors (animals that transmit pathogens between hosts).

Historical Discovery of Viruses

• Edward Jenner: Recognized as the \"Father of Immunization.\" Over $130$ years before the invention of the electron microscope, Jenner successfully inoculated patients with cowpox to prevent smallpox.
• Tobacco Mosaic Disease (TMD): The discovery of viruses as distinct entities began with the study of TMD.
• Dmitri Ivanovski (1892): He obtained an extract from TMD-infected plants and passed it through filters with extremely small pores (measured at $0.1\,\mu m$). He demonstrated that the causative agent was smaller than any known bacteria.
• Martinus Beijerinck (1899): Beijerinck proposed that the causative agent was a \"biological poison.\" He coined the term virus, which is Latin for \"poison.\"

The Status of Viruses as Living Entities

• Debate continues regarding whether viruses are \"alive.\" They do not possess enzymes for protein synthesis, replication, or the generation of Adenosine Triphosphate ($ATP$).
• They do not undergo traditional cell division or growth.
• Clinical Challenge: Because viruses utilize host cell functions to replicate, developing effective antiviral drugs is difficult. Most interventions that interfere with viral replication also inadvertently interfere with the health and function of the host cell.

Viral Structure and Taxonomy

Structural Components

• Nucleic Acid: The viral genome can be Double-Stranded ($ds$) or Single-Stranded ($ss$), and it may consist of either $RNA$ or $DNA$. Morphologically, the genome can be circular or linear.
• Capsid: A protein coat surrounding the nucleic acid. It is composed of protein subunits called capsomeres. The capsid provides protection and aids the virus in entering the host cell.
• Nucleocapsid: The combination of the nucleic acid and the capsid.
• Envelope: A phospholipid membrane obtained from the host cell's membrane. While only present in some viruses, it serves as both a protective layer and a virulence factor (recognition sites for binding).

Viral Morphology and Shapes

• Helical: Characterized by a rod-like shape (e.g., Tobacco Mosaic Virus).
• Polyhedral: Many-sided, typically icosahedral (e.g., Adenovirus).
• Spherical: Enveloped viruses that appear round (e.g., Influenza Virus).
• Complex: Possession of intricate structures such as a head, tail sheath, and tail fibers (e.g., Bacteriophages).

Classification of Pathogenic Viruses by Genome Architecture

• Double-Stranded DNA (dsDNA):     * Enveloped: Herpesvirus, Hepadnavirus.     * Nonenveloped: Adenovirus, Polyomavirus, Papillomavirus.
• Single-Stranded DNA (ssDNA):     * Nonenveloped: Parvovirus.
• Double-Stranded RNA (dsRNA):     * Nonenveloped: Reovirus.
• Single-Stranded RNA (ssRNA):     * Enveloped: Coronavirus, Paramyxovirus, Bunyavirus, Orthomyxovirus, Togavirus, Flavivirus, Retrovirus, Rhabdovirus, Filovirus.     * Nonenveloped: Picornavirus, Calicivirus.

Detailed Virion and Genome Metrics

Viral Genetics and Host Specificity

The Viral Genome

• Viral genomes are significantly smaller than cellular genomes. For comparison, a virus might have as few as $3$ genes, whereas bacteria like E. coli have approximately $4,000$ genes.
• The genome can be linear with multiple molecules (resembling eukaryotic cells) or circular and singular (resembling prokaryotic cells).

Host Ranges and Tropism

• Specific Host Range: Many viruses only target a specific cell type in a specific host. For example, HIV specifically targets human $CD4$ T-cells.
• Broad Host Range: Some viruses can infect multiple species. For example, West Nile Virus targets birds, mammals, and reptiles.
• Specificity is determined by the affinity between viral surface proteins (on the capsid or envelope) and host cell receptor proteins.

Bacteriophage vs. Bacterial Structure

• Bacterial Cell: Contains a cell wall, plasma membrane, $DNA$ genome, ribosomes for protein synthesis, and flagella for movement.
• Bacteriophage: Consists of a capsid "head" containing $DNA$ or $RNA$, a tail sheath, and tail fibers/spikes used for attachment and injection of genetic material.

Viral Life Cycles

The Bacteriophage Life Cycle

1. The Lytic Cycle

In the lytic cycle, the phage destroys the host cell to release new virions. The stages are:

1. Attachment: The phage attaches to the bacterial cell wall via receptors (e.g., $LPS$) using tail fibers.
2. Penetration: The phage tail sheath contracts, injecting viral $DNA$ into the host cell while the capsid remains outside.
3. Biosynthesis: The host's metabolic machinery is redirected to synthesize viral components (heads, sheaths, tail fibers, etc.).
4. Maturation: Viral components are assembled into complete virions.
5. Release (Lysis): The host cell wall is broken down, the cell lyses, and new virions are released.

2. The Lysogenic Cycle

In this cycle, the phage genome integrates into the host genome without immediate destruction.

• Integration: The phage $DNA$ becomes a prophage by incorporating into the bacterial chromosome.
• Reproduction: The host cell replicates the prophage along with its own $DNA$ during binary fission, passing it to daughter cells.
• Lysogenic Conversion: The presence of prophage $DNA$ can alter the bacterial phenotype and increase virulence. For example, Vibrio cholerae is significantly more virulent when it carries a toxin-producing phage.
• Induction: Under stressful conditions, the prophage DNA is excised from the chromosome and enters the lytic cycle.

3. Transduction

Transduction is the process where a virus transfers bacterial genetic material from one host to another. When the prophage is excised, it may take a short piece of bacterial $DNA$ with it. This package of viral and bacterial $DNA$ is then injected into a new host cell and incorporated via recombination.

The Animal Virus Life Cycle

Animal viruses follow a similar trajectory but utilize different mechanisms for entry and release.

• Attachment: Interaction with target cell receptors (e.g., influenza attaching to epithelial cells).
• Penetration: Accomplished via Endocytosis (the cell engulfs the virus) or membrane fusion.
• Uncoating: The viral $RNA$ or $DNA$ is released from the capsid.
• Biosynthesis: The nature of the genome determines replication. For example, Influenza $RNA$ enters the nucleus to be replicated by viral $RNA\,polymerase$, and viral $mRNA$ is used to produce proteins.
• Assembly: New viral particles are constructed.
• Release: This often occurs via Budding (for enveloped viruses), where the virus exits the cell without necessarily killing it immediately.

Latency, Persistence, and Chronic Infection

Viral Latency

• Definition: Some viruses remain hidden or dormant within host cells without causing symptoms. They may exist as extrachromosomal elements or integrate into the host genome.
• Example: Varicella-zoster: Causes chickenpox. Following the acute infection, the virus goes dormant in nerve-cell ganglia approximately $10-12$ days post-infection. If reactivated later, it causes shingles, a painful localized rash.

Persistent (Chronic) Infections

• Definition: Diseases with symptoms that are recurrent or persistent over a long duration.
• Example: HIV: Untreated patients can remain asymptomatic for years. During this time, the virus persistently interferes with the immune system (e.g., preventing antigen expression). This chronic damage eventually leads to AIDS (Acquired Immunodeficiency Syndrome).

Retroviruses and Reverse Transcription

• Retroviruses use $RNA$ as their primary genomic material.
• Infection Mechanism: Upon entry, the virus uses the enzyme Reverse Transcriptase to convert viral $RNA$ into $DNA$.
• Integration: The viral $DNA$ is transported into the nucleus and integrated into the host DNA, at which point it is called a provirus.
• Maturation: Host machinery produces new viral $RNA$ and proteins. The enzyme protease is required to release proteins that form the mature HIV virion.

Viruses, Cancer, and Medical Applications

Viruses and Oncogenesis

Specific viruses are correlated with the development of cancer:

• Human Papilloma Virus (HPV): Linked to cervical cancer.
• Hepatitis C: Linked to liver cancer.
• Merkel Cell Polyomavirus (MCV): Associated with Merkel cell carcinoma. This involves the integration of the viral genome and mutations that inactivate helicase or activate Large T antigens under immunosuppressed conditions (e.g., in the elderly or AIDS patients).

Gene Therapy

• Process: Transplanting healthy genes into a cell to replace defective or missing ones.
• Mechanism: Genetic tools (e.g., healthy genes, $RNAi$, or gene editing tools) are packaged into an Adeno-Associated Virus (AAV). The AAV is injected into the target tissue, enters the cell, and releases the material to correct the genetic defect.

Phage Therapy

• Definition: Using bacteriophages to treat bacterial infections.
• Pharmacodynamics: Limits include bacterial resistance to phages.
• Pharmacokinetics: Obstacles include the body's impact on the phage (metabolism/excretion) or the phage's inability to penetrate to the target bacterial site.

Prions

Definition and Characteristics

• Prions: Misfolded proteins that can transmit their misfolded shape to normal variants of the same protein. They are not viruses; they contain no nucleic acid.
• Structure: They involve a transition of amino acids from an alpha-helix configuration into a beta-sheet form.
• Pathology: Prions cause fatal neurodegenerative diseases characterized by spongiform pathology (holes in the brain) and rapid brain shrinkage.

Prion Diseases

• Creutzfeldt-Jakob Disease (CJD): A rapid, fatal disease causing loss of cognitive and motor function. It is resistant to standard sterilization methods for surgical equipment.
• Kuru: Known for its transmission via ritualistic cannibalism.
• Bovine Spongiform Encephalopathy (Mad Cow Disease): Affects cattle and can be transmitted to humans as vCJD.
• Outcome: Most prion diseases are fatal within one year of the onset of symptoms.