Viruses, Viroids, and Prions: Structure, Classification, and Pathogenicity

Viruses, Viroids, and Prions

Viral Discovery and History

The discovery of viruses marked a significant milestone in microbiology, as these infectious agents could not be seen with light microscopes and were initially described as 'contagium vivum fluidum' or contagious living fluid. The term virus (Latin for poison) was adopted in the 1930s to describe these filterable agents.

• 1886 – Adolf Mayer: Investigated tobacco mosaic disease and found it was infectious, but could not identify the causative microbe.

• 1892 – Dmitri Ivanovsky: Used a Chamberland filter to show that the infectious agent could pass through bacteria-proof filters, suggesting it was smaller than bacteria or a toxin.

• 1930s – Electron Microscope: Enabled direct observation of viruses, allowing scientists to study their structure and replication.

Distinctive Features of Viruses

Viruses are unique infectious agents with several distinctive features that set them apart from other microorganisms.

• Obligatory intracellular parasites: Require living host cells to multiply.

• Genetic material: Contain either DNA or RNA, never both.

• Protein coat: Surrounded by a protein capsid.

• No ribosomes or ATP-generating mechanisms: Depend entirely on host cell machinery for replication.

Virus Size and Visualization

Viruses are much smaller than bacteria and can only be visualized using electron microscopy. Their size typically ranges from 20 to 300 nanometers.

Virus Structure and Categories

A virion is a complete, fully developed, infectious viral particle. Viruses are classified based on their nucleic acid type and the structure of their protein coats.

• Nucleic Acid: DNA or RNA, single- or double-stranded, linear, circular, or segmented.

• Capsid: Protein coat made of capsomeres.

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

• Spikes: Protein structures that facilitate attachment and entry into host cells.

Types of Viruses: Naked vs. Enveloped

Viruses are categorized based on the presence or absence of an envelope:

• Naked (Non-enveloped) Viruses: Consist only of nucleic acid and capsid.

• Enveloped Viruses: Have a nucleic acid-capsid core surrounded by a lipid envelope, often with spikes.

Virus Morphology

Viruses exhibit several morphological types:

• Helical: Rod-shaped, with nucleic acid inside a helical capsid (e.g., rabies, Ebola).

• Polyhedral: Many-sided, typically icosahedral (e.g., poliovirus).

• Enveloped: Spherical, with helical or polyhedral nucleocapsids (e.g., influenza, Marburgvirus).

• Complex: Complicated structures, such as bacteriophages with heads, tails, and fibers.

Virus Classification

Viruses are classified by their nucleic acid type, capsid symmetry, presence or absence of an envelope, and host range. Medically important viruses are grouped by these features.

Genome Type	Capsid Shape	Envelope	Example
dsDNA	Icosahedral	Naked/Enveloped	Herpesvirus
ssRNA (+)	Helical	Enveloped	Influenza virus
ssRNA (-)	Helical	Enveloped	Rabies virus
dsRNA	Icosahedral	Naked	Rotavirus
Complex	Complex	Naked	Bacteriophage

Host Range and Transmission

The host range is the spectrum of host cells a virus can infect, determined by specific attachment sites and cellular factors. Transmission can occur via direct contact, fomites, or vectors (e.g., mosquitoes, ticks).

• Bacteriophages: Infect bacteria, attach to cell wall, fimbriae, or flagella.

• Animal Viruses: Attach to plasma membrane receptors.

• Vectors: Arthropods can transmit viruses biologically (internal) or mechanically (external).

Zoonoses and Reverse Zoonoses

Some viruses can be transmitted from animals to humans (zoonoses), such as avian influenza. Reverse zoonoses occur when viruses move from humans to animals.

Antigenic Drift and Shift

Genetic changes in viruses, especially RNA viruses, can lead to new strains:

• Antigenic Drift: Accumulation of small mutations over time, allowing evasion of immune responses.

• Antigenic Shift: Reassortment of RNA segments from different viruses, creating new subtypes with pandemic potential.

Viral Multiplication in Animal Cells

Viral replication in animal cells follows a series of steps:

1. Attachment: Virus binds to host cell membrane.

2. Entry: By receptor-mediated endocytosis or fusion.

3. Uncoating: Viral genome is released by host or viral enzymes.

4. Biosynthesis: Production of viral nucleic acids and proteins.

5. Maturation: Assembly of viral components into new virions.

6. Release: By budding (enveloped viruses) or cell rupture (naked viruses).

Viruses and Cancer

Some viruses can cause cancer by integrating their genetic material into the host genome, leading to transformation of normal cells into cancerous cells. These are called oncogenic viruses.

• Proto-oncogenes: Normal genes that regulate cell growth; mutations can convert them to oncogenes.

• Transformation: Process by which cells acquire cancerous properties.

Latent and Persistent Viral Infections

Viruses can establish long-term infections:

• Latent Infections: Virus remains dormant in host cells and can reactivate (e.g., herpesviruses).

• Persistent Infections: Virus is continuously present and often fatal (e.g., HIV).

Prions

Prions are infectious proteins that cause neurodegenerative diseases. They are inherited or transmissible by ingestion, transplantation, or contaminated surgical instruments.

• Diseases: Spongiform encephalopathies (e.g., mad cow disease, Creutzfeldt-Jakob disease, Kuru).

• Mechanism: Prions induce abnormal folding of normal proteins in the brain, leading to cell death.