Viruses, Viroids, Prions, and Antiviral Drugs

General Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from bacteria and other microorganisms. They possess distinct structural and functional properties that define their role as obligate intracellular parasites.

• Obligatory intracellular parasites: Viruses can only replicate inside living host cells.

• Protein coat (capsid): Surrounds the viral nucleic acid, providing protection and aiding in host cell attachment.

• Nucleic acid: Viruses contain either DNA or RNA, but never both.

• No ribosomes: Viruses lack the machinery for protein synthesis.

• No ATP-generating mechanism: Viruses do not produce energy independently.

Host Range of Viruses

The host range refers to the spectrum of host cells a virus can infect. This is determined by specific interactions between viral surface proteins and host cell receptors.

• Most viruses infect only specific types of cells within one host species.

• Bacteriophages: Viruses that infect bacteria.

• Host range is limited by compatibility between viral attachment structures and host cell receptors.

Viral Sizes

Viruses are among the smallest infectious agents, with sizes ranging from 20 nm to 1000 nm.

• Much smaller than most bacteria.

• Size affects methods of detection and filtration.

Viral Structure

The complete, fully developed infectious viral particle is called a virion. Virions have several key structural components:

• Nucleic acid: DNA or RNA, single or double stranded.

• Capsid: Protein coat composed of subunits called capsomeres.

• Envelope: Some viruses possess a lipid membrane derived from the host cell.

• Spikes: Glycoprotein projections that aid in attachment to host cells.

Morphological Types of Viruses

Viruses are classified based on their shape and structure:

• Helical viruses: Hollow, cylindrical shapes (e.g., Tobacco mosaic virus).

• Polyhedral viruses: Many-sided, often icosahedral (e.g., Adenovirus).

• Enveloped viruses: Surrounded by a lipid envelope (e.g., Influenza virus).

• Complex viruses: Complicated structures, such as bacteriophages with tail fibers.

Taxonomy of Viruses

Viruses are classified into families, genera, and species based on genetic information, structure, and host range.

• Family names: End in -viridae (e.g., Herpesviridae).

• Genus names: End in -virus (e.g., Herpesvirus).

• Viral species: Group of viruses sharing the same genetic information, structure, and ecological niche.

Growing and Culturing Viruses

Viruses require living cells for replication and are cultured using specialized methods.

• Bacteriophages: Grown in bacterial cultures; form plaques on agar plates.

• Animal viruses: Cultured in living animals, embryonated eggs, or cell cultures.

• Cell cultures: Can be primary (normal cells) or continuous (transformed cells).

Viral Identification

Viruses are identified using several laboratory techniques:

• Cytopathic effects: Observable changes in host cells due to viral infection.

• Serological tests: Detection of antibodies against viruses (e.g., neutralization, hemagglutination, Western blot).

• Nucleic acid analysis: PCR and sequencing methods.

Viral Multiplication: One-Step Growth Curve

The viral replication cycle can be studied using a one-step growth curve, which tracks the number of infectious particles over time.

• Latent period: Time during which viruses are not detectable outside cells.

• Burst size: Number of virions released per infected cell.

Multiplication of Bacteriophages

Bacteriophages replicate via two main cycles:

• Lytic cycle: Phage causes lysis and death of the host cell, releasing new virions.

• Lysogenic cycle: Phage DNA incorporates into host genome as a prophage, replicating with the host cell.

• Phage conversion: Host cell exhibits new properties due to prophage genes.

• Transduction: Transfer of bacterial genes by phages (generalized or specialized).

Multiplication of Animal Viruses

Animal viruses follow a multi-step replication process:

• Attachment: Viruses bind to host cell membrane receptors.

• Entry: By endocytosis or membrane fusion.

• Uncoating: Viral nucleic acid is released by viral or host enzymes.

• Biosynthesis: Production of viral nucleic acid and proteins.

• Maturation: Assembly of viral components.

• Release: By budding (enveloped viruses) or cell lysis (nonenveloped viruses).

Comparison: Bacteriophage vs. Animal Virus Multiplication

Types of Viral Infections

Viral infections can be classified based on their duration and effects:

• Acute infection: Rapid onset, short duration (e.g., influenza).

• Latent infection: Virus remains dormant in host cells, can reactivate (e.g., herpes simplex).

• Persistent infection: Disease process occurs over a long period, often fatal (e.g., HIV, subacute sclerosing panencephalitis).

Viruses and Cancer

Some viruses are oncogenic, meaning they can cause cancer by transforming host cells.

• Oncogenes: Genes that can induce transformation of normal cells into cancerous cells.

• Oncogenic viruses: Include Retroviridae, Herpesviridae, Adenoviridae, Poxviridae, Hepadnaviridae.

• Examples: Human papillomavirus (HPV) causes cervical cancer; Hepatitis B virus (HBV) causes liver cancer.

Prions and Viroids

Prions and viroids are subviral entities with unique properties.

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease, mad cow disease).

• Viroids: Infectious RNA molecules (short, circular, single-stranded RNA) that cause plant diseases (e.g., potato spindle tuber disease).

Antiviral Drugs

Antiviral drugs target various stages of the viral life cycle to inhibit replication.

• Entry and fusion inhibitors: Block virus entry into host cells.

• Nucleic acid synthesis inhibitors: Prevent viral genome replication.

• Assembly and release inhibitors: Interfere with formation and release of new virions.

• Neuraminidase inhibitors: Prevent release of influenza viruses.

Antivirals for Treating HIV/AIDS

HIV is treated with a combination of antiretroviral drugs targeting different viral enzymes and processes.

• Nucleoside analogs: e.g., zidovudine

• Nucleotide analogs: e.g., tenofovir

• Non-nucleoside inhibitors: e.g., nevirapine

• Protease inhibitors: e.g., atazanavir

• Integrase inhibitors: e.g., raltegravir

• Entry inhibitors: e.g., maraviroc

• Fusion inhibitors: e.g., enfuvirtide

Interferons

Interferons are proteins produced by virus-infected cells that inhibit the spread of viral infections.

• Induce antiviral states in neighboring cells.

• Used therapeutically for some viral infections and cancers.

Additional info:

• Some details on viral taxonomy, prion diseases, and antiviral drug mechanisms were expanded for clarity and completeness.