Viruses, Viroids, and Prions: Structure, Characteristics, and Comparison with Bacteria

Viruses, Viroids, and Prions

Introduction

This section introduces the fundamental properties of viruses, viroids, and prions, focusing on their structure, replication, and how they differ from bacteria. Understanding these differences is crucial for microbiology students, as viruses and prions represent unique infectious agents with distinct biological characteristics.

General Characteristics of Viruses

Obligatory Intracellular Parasites

• Viruses are obligate intracellular parasites, meaning they require living host cells to multiply.

• They are acellular (not composed of cells).

• Viruses contain either DNA or RNA as their genetic material, but never both.

• They lack ribosomes and do not carry out protein synthesis independently.

• No ATP-generating mechanism; they rely entirely on the host cell for energy and biosynthetic processes.

• Each virus is surrounded by a protein coat called a capsid.

Comparison: Viruses vs. Bacteria

The following table summarizes the key differences between viruses and bacteria:

Characteristic	Virus	Bacteria
Cellular Structure	No (acellular)	Yes (cellular)
Obligate Intracellular Parasite	Yes	No (most are free-living)
Plasma Membrane	No	Yes
Binary Fission	No	Yes
Pass Through Bacteriological Filters	Yes	No
Both DNA and RNA	No (one or the other)	Yes
ATP-Generating Mechanism	No	Yes
Ribosomes	No	Yes
Antibiotic Sensitivity	No	Yes

Host Range of Viruses

Definition and Determinants

• The host range is the spectrum of host cells a virus can infect.

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

• Host range is determined by specific host attachment sites and cellular factors.

• Bacteriophages are viruses that infect bacteria.

Examples of host range specificity:

• Plant viruses infect plant cells (cell wall contains cellulose).

• Animal viruses infect animal cells (cell membrane contains cholesterol, proteins, etc.).

• Human viruses infect human cells.

Viral Structure

Virion and Its Components

• A virion is a complete, fully developed viral particle capable of infection.

• Contains nucleic acid (DNA or RNA), which can be single- or double-stranded, linear or circular.

• The capsid is a protein coat made of subunits called capsomeres.

• Some viruses have an envelope—a lipid, protein, and carbohydrate layer surrounding the capsid.

• Spikes are projections from the outer surface, often involved in host cell attachment.

Viral Morphology

Types of Viral Shapes

• Helical viruses: Hollow, cylindrical capsid (e.g., tobacco mosaic virus).

• Polyhedral viruses: Many-sided, usually icosahedral (20 faces) (e.g., adenovirus).

• Enveloped viruses: Viruses with a lipid envelope (e.g., influenza virus).

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

Strength of viral structure: Non-enveloped (naked) viruses are generally more resistant to environmental factors (e.g., heat, detergents) than enveloped viruses, whose lipid envelopes are sensitive to desiccation and disinfectants.

Viroids and Prions

Viroids

• Viroids are infectious agents composed solely of short strands of circular, single-stranded RNA without a protein coat.

• They infect plants and can cause significant agricultural diseases.

• Viroids do not encode proteins and replicate within host cells using host enzymes.

Prions

• Prions are infectious proteins that cause neurodegenerative diseases in animals and humans (e.g., scrapie in sheep, Creutzfeldt-Jakob disease in humans).

• They lack nucleic acids (no DNA or RNA).

• Prions propagate by inducing normal cellular proteins to adopt the abnormal prion form, leading to accumulation in neural tissue.

• Transmission can occur via ingestion, transplantation, or contaminated surgical instruments.

Summary Table: Viruses, Viroids, and Prions

Agent	Genetic Material	Protein Coat	Envelope	Cellular Structure	Example Disease
Virus	DNA or RNA	Yes	Sometimes	No	Influenza, HIV
Viroid	RNA	No	No	No	Potato spindle tuber disease
Prion	None	No	No	No	Creutzfeldt-Jakob disease

Key Equations and Concepts

• Viral yield: The number of progeny virions produced per infected cell can be up to 1,000 or more.

• Viral replication: Viruses multiply by taking over the host cell's machinery to synthesize viral components, which are then assembled into new virions.

Examples and Applications

• Bacteriophage therapy is being explored as an alternative to antibiotics for treating bacterial infections.

• Prion diseases highlight the importance of protein folding in health and disease.

• Viroids are significant in agriculture due to their impact on crop yields.