Chapter 6: Viruses and Acellular Pathogens – Structure, Classification, and History

Viruses and Acellular Pathogens

Introduction to Acellular Pathogens

Acellular pathogens are infectious agents that do not consist of, divide into, or contain cells. They include viruses, virusoids, viroids, and prions, each with distinct structural and functional properties. These agents are significant in microbiology due to their unique replication strategies and impact on health.

• Viruses: Composed of at least protein and nucleic acid (DNA or RNA).

• Virusoids: Made of only RNA, often dependent on other viruses for replication.

• Viroids: Made of only RNA, typically infect plants.

• Prions: Consist solely of protein, causing neurodegenerative diseases.

Additional info: Viruses are obligate intracellular parasites, meaning they require a host cell to replicate and cannot carry out metabolic processes independently.

Viral Structure and Classification

Fundamental Differences Between Viruses and Other Acellular Infectious Agents

Viruses differ from other acellular pathogens in their composition and replication mechanisms. Unlike prions and viroids, viruses possess both nucleic acid and a protein coat, and some have lipid envelopes.

• Obligate Intracellular Parasites: Viruses must infect host cells to reproduce, hijacking cellular machinery for replication.

• Size: Viruses are ultramicroscopic, typically ranging from 20 nm to 450 nm in diameter, much smaller than most bacteria.

• Protein Synthesis: Viruses cannot produce their own proteins; they rely on host ribosomes.

Viral Structure

Viruses have a simple but efficient structure designed for protection and delivery of their genetic material.

• Capsid: The proteinaceous shell that protects the viral nucleic acid.

• Capsomer: Identical protein subunits that make up the capsid.

• Nucleocapsid: The combination of the capsid and the viral nucleic acid.

• Envelope: A lipid membrane surrounding some viruses, derived from the host cell membrane. Viruses lacking this are termed naked viruses.

• Spike Proteins: Glycoproteins on the envelope or capsid that mediate attachment to host cells.

Functions of Capsid/Envelope

• Protection: Shields the nucleic acid from environmental damage.

• Attachment: Facilitates binding to host cell receptors.

• Penetration: Assists in entry of viral genome into host cells.

Viral Morphology

Viruses are classified based on the shape and structure of their capsid and envelope.

• Helical Capsid: Capsomers arranged in a spiral, forming a rod-shaped structure.

• Icosahedral Capsid: Capsomers arranged in a 20-sided polygon, providing a compact and stable structure. (can lack an outer envelop)

• Complex Viruses: Exhibit intricate structures, such as bacteriophages with polyhedral heads and helical tails.

Example: Herpes simplex virus (enveloped, icosahedral), Influenza virus (enveloped, helical), T4 bacteriophage (complex).

Viral Genomes

Viruses possess diverse types of genomes, which influence their replication strategies.

• DNA Viruses: Can be double-stranded (dsDNA) or single-stranded (ssDNA).

• RNA Viruses: Can be double-stranded (dsRNA) or single-stranded (ssRNA), with positive-sense (+) or negative-sense (−) polarity.

• Segmented Genomes: Some viruses have genomes split into multiple segments.

Classification of Viruses

Viruses are classified by structure, chemical composition, and genetic makeup. The International Committee on the Taxonomy of Viruses (ICTV) oversees viral classification, using family and genus names with standardized suffixes (e.g., -viridae for families, -virus for genera).

• Mutation Rate: High mutation rates make classification challenging.

• Examples of Families: Herpesviridae, Retroviridae, Coronaviridae.

History of Virology

Discovery and Impact

The study of viruses began in the late 19th century with the work of Ivanoski and Beijerinck on Tobacco mosaic disease. They discovered that the infectious agent was not a bacterium but something smaller, leading to the term "virus" (Latin for "poison").

• Pandemics: Viral epidemics such as the 1918-1919 influenza pandemic, polio outbreaks, Ebola, and the recent SARS-CoV-2 pandemic have highlighted the importance of virology.

• Endogenous Retroviruses: Human genomes contain endogenous retroviruses (HERVs), remnants of ancient viral infections.

Additional info: Between 1-8% of human DNA is estimated to be HERVs.

Comparison Table: Types of Acellular Infectious Agents

Agent	Composition	Example	Host
Virus	Protein + Nucleic Acid (DNA or RNA)	Influenza virus	Animals, plants, bacteria
Virusoid	RNA only	Hepatitis D virusoid	Animals (requires helper virus)
Viroid	RNA only	Potato spindle tuber viroid	Plants
Prion	Protein only	PrPSc (scrapie prion)	Animals, humans

Key Terms and Definitions

• Capsid: Protein shell enclosing viral nucleic acid.

• Capsomer: Subunit of the capsid.

• Nucleocapsid: Capsid plus nucleic acid.

• Envelope: Lipid membrane surrounding some viruses.

• Spike Protein: Glycoprotein involved in host cell attachment.

• Obligate Intracellular Parasite: Requires host cell for replication.

Formulas and Equations

• Viral Size Range: $20\,\text{nm} \leq \text{diameter} \leq 450\,\text{nm}$

• Genome Types: $\text{Genome} = \begin{cases} \text{dsDNA} \\ \text{ssDNA} \\ \text{dsRNA} \\ \text{ssRNA} \end{cases}$

Summary

Viruses and other acellular pathogens are unique infectious agents with distinct structures and replication strategies. Their study is essential for understanding infectious diseases, epidemiology, and the development of antiviral therapies.