Viruses: Structure, Classification, and Roles in Microbiology

Chapter 18: Viruses

Introduction

Viruses are unique infectious agents that play significant roles in microbiology, biotechnology, and medicine. This chapter covers the definition, structure, classification, replication cycles, and ecological and medical significance of viruses, as well as related entities such as viroids and prions.

Definition and Discovery of Viruses

What is a Virus?

A virus is an infectious agent composed of genetic material (either DNA or RNA) encased in a protein coat called a capsid. Viruses are non-cellular and require a host cell for replication.

• Nucleic acid: Can be DNA or RNA, single-stranded (ss) or double-stranded (ds).

• Capsid: Protein shell that protects the genome and determines the virus's shape.

• Envelope (optional): Lipid and glycoprotein layer acquired from the host cell membrane.

• Spikes: Glycoprotein projections involved in host cell attachment and entry.

Example: Bacteriophage (infects bacteria), Influenza virus (infects animals)

Historical Discovery

• Dmitri Ivanovsky (Russian botanist) and Martinus Beijerinck (Dutch microbiologist and botanist) discovered the Tobacco mosaic virus in plants in 1892 and 1898, respectively.

Comparison: Viruses vs. Cells

Structural and Functional Differences

Viruses differ fundamentally from prokaryotic and eukaryotic cells:

• Cellular structure: Viruses are non-cellular; cells are either prokaryotic or eukaryotic.

• Organelles: Viruses lack organelles and cytoplasm.

• Metabolism: Viruses do not metabolize nutrients or generate energy.

• Reproduction: Viruses cannot reproduce independently; they require a host cell.

• Genetic material: Viruses possess either DNA or RNA, but not both.

Are viruses alive? Viruses share some characteristics of living things (e.g., genetic material, ability to mutate), but do not meet all criteria for life.

Major Components of Viruses

Structural Features

• Nucleic acid core: The viral genome (DNA or RNA).

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope: Lipoprotein layer outside the capsid (present in some viruses).

• Spikes: Glycoprotein projections for host cell recognition and attachment.

Characteristics of Viruses

• Obligate intracellular parasites: Require host cells for replication.

• Small size: Typically 20–300 nanometers (nm).

• Host specificity: Infect only certain cell types ("lock and key" fit between viral antigens and host receptors).

• Host range: Includes plant viruses, animal viruses, and bacterial viruses (bacteriophages).

Measurement of Viruses

Units of Measurement

Viruses are measured in nanometers (nm).

• 1 nm = $10^{-9}$ meters

• Bacteria: 1–10 micrometers ($ext{μm}$)

• Viruses: 20–300 nm

Shapes of Viruses

General Shapes and Examples

Viruses exhibit three main shapes:

• Helical: Coiled or spiral capsid (e.g., Rabies virus, Influenza virus, Ebola virus).

• Polyhedral: Icosahedral structure with 20 triangular sides and 12 corners (e.g., Herpes virus, Polio virus).

• Complex: Irregular or complicated shapes (e.g., Bacteriophage, Smallpox virus).

Viral Host Specificity

Mechanism of Infection

Viruses are host-specific due to the interaction between viral surface markers (antigens) and host cell receptors. Only cells with matching receptors can be infected.

• Example: Rabies virus infects only nervous system cells.

Viral Replication Cycles

Lytic Cycle vs. Lysogeny

• Lytic cycle: Virus infects host, replicates, and causes cell lysis, releasing new virions.

• Lysogeny: Viral genome integrates into host DNA and replicates passively until triggered to enter the lytic cycle.

• Temperate phage: Can undergo lysogeny.

• Virulent phage: Only undergoes the lytic cycle.

Methods for Culturing Viruses

Laboratory Techniques

Viruses are cultured using living cells, such as:

• Bacterial cultures: For bacteriophages.

• Animal cell cultures: For animal viruses.

• Plant cell cultures: For plant viruses.

Diseases Caused by Viruses

Examples of Viral Diseases

• HIV: Causes AIDS.

• Herpes viruses: Cause oral/genital herpes, chickenpox, shingles.

• Ebola virus: Causes hemorrhagic fever.

Viroids and Prions

Viroids

• Viroid: Infectious RNA molecule without a protein coat, primarily infects plants.

Prions

• Prion: Infectious protein causing neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

Beneficial Roles of Viruses

Ecological and Biotechnological Importance

Viruses contribute to natural ecosystems, biotechnology, and medicine:

• Biocontrol: Bacteriophages used to target harmful bacteria in food production (e.g., LISTEX P100 targets Listeria).

• Gene therapy: Viral vectors deliver therapeutic genes.

• Vaccines: Engineered viruses stimulate immune responses.

Table: Comparison of Virus, Prokaryotic Cell, and Eukaryotic Cell

Feature	Virus	Prokaryotic Cell	Eukaryotic Cell
Cellular Structure	Non-cellular	Cellular	Cellular
Genetic Material	DNA or RNA	DNA	DNA
Organelles	None	None	Present
Reproduction	Requires host	Binary fission	Mitosis/meiosis
Metabolism	None	Present	Present

Summary

Viruses are obligate intracellular parasites with diverse structures and replication strategies. Their study is essential for understanding infectious diseases, biotechnology applications, and ecological processes.