Viruses, Viroids, and Prions: Tiny but Deadly (Chapter 13 Study Notes)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Viruses, Viroids, and Prions: Tiny but Deadly

Introduction

This chapter explores three major types of acellular infectious agents: viruses, viroids, and prions. These entities are much smaller than bacteria and are responsible for a wide range of diseases in humans, animals, and plants. Understanding their structure, replication, and impact is essential for microbiology students.

History of Microbiology

Antonie van Leeuwenhoek: The Father of Microbiology

Antonie van Leeuwenhoek (1632–1723) was the first person to observe bacteria and other microorganisms using a microscope he designed himself. He referred to these tiny organisms as "animalcules." His discoveries laid the foundation for the field of microbiology.

Animalcules: Early term for microscopic organisms.
Observed bacteria, sperm cells, and banding patterns in muscle fibers.
Contributed to the understanding that life exists beyond what is visible to the naked eye.

Koch's Postulates: Establishing Microbial Disease Causation

Robert Koch developed a set of criteria to prove the link between a microorganism and a specific disease. These postulates remain fundamental in medical microbiology.

The microorganism must be found in all cases of the disease.
It must be isolated and grown in pure culture.
The cultured microorganism should cause disease when introduced into a healthy host.
The microorganism must be re-isolated from the experimentally infected host.

Discovery of Viruses

Chamberland Filter and the Concept of Viruses

The Chamberland filter, invented in 1884, was designed to remove bacteria from water. It played a crucial role in the discovery of viruses, as it could filter out known microbes but not viruses, which are much smaller.

Viruses are smaller than bacteria and can pass through filters that retain bacteria.
Early experiments showed that filtered sap from diseased plants could still transmit disease, indicating the presence of an infectious agent smaller than bacteria.

Tobacco Mosaic Virus: The First Virus Discovered

In the late 19th century, researchers studying diseased tobacco plants found that the infectious agent could not be seen under a microscope or cultured like bacteria or fungi. The agent was later named virus, meaning "poison." This led to the identification of the Tobacco Mosaic Virus (TMV) as the first virus.

Filtered sap from infected plants caused disease in healthy plants.
Viruses could not be isolated or visualized by conventional means.
Martinus Beijerinck and Dmitri Ivanovsky were key figures in these discoveries.

Structure and Properties of Viruses

General Characteristics

Viruses are acellular entities that require a host cell for replication. They are composed of genetic material (DNA or RNA) surrounded by a protein coat called a capsid.

Extremely small (typically 20–300 nm).
Cannot carry out metabolic processes independently.
Do not grow or respond to the environment outside a host.
Obligate intracellular parasites.

Viral Genome

The viral genome can be composed of either DNA or RNA, but never both. It may be single-stranded (ss) or double-stranded (ds), and can be segmented or non-segmented.

Types of viral genomes: dsDNA, ssDNA, dsRNA, ssRNA.
Genomes are much smaller than those of cellular organisms.
Some viruses have partial genomes.

Capsid and Envelope

The capsid is the protein shell that encases the viral genome. Some viruses also possess an envelope, a lipid membrane derived from the host cell during viral release.

Capsids are made of protein subunits called capsomeres.
Three basic shapes: helical, icosahedral, and complex.
Enveloped viruses acquire their envelope from the host cell membrane and may have viral glycoproteins for host recognition.

Viral Replication Cycles

Lytic Cycle

The lytic cycle is a viral replication process that results in the destruction of the host cell and release of new viral particles.

Stages: Attachment, Entry, Synthesis, Assembly, Release.
Host cell is lysed, releasing progeny viruses.

Lysogenic Cycle

Some viruses, such as bacteriophage lambda, can integrate their genome into the host's DNA, remaining dormant for a period before entering the lytic cycle.

Viral DNA is incorporated into the host genome (prophage).
Host cell survives and replicates the viral DNA along with its own.
Environmental triggers can induce the lytic cycle.

Replication in Animal Viruses

Animal viruses follow similar basic steps but may differ in entry and release mechanisms due to the presence of an envelope and the nature of animal cells.

Entry by membrane fusion or endocytosis.
Replication may occur in the nucleus or cytoplasm.
Release by cell lysis or budding (enveloped viruses).

Special Types of Infectious Agents

Viroids

Viroids are small, circular RNA molecules that infect plants. They lack a protein coat and are much smaller than viruses.

Consist solely of RNA.
Cause significant agricultural damage.
Do not encode proteins.

Prions

Prions are infectious proteins that cause neurodegenerative diseases in mammals. They induce abnormal folding of normal cellular proteins, leading to brain damage.

Composed only of protein; no nucleic acid.
Cause diseases such as Creutzfeldt-Jakob disease and mad cow disease.
Resistant to standard sterilization methods; destroyed by incineration or autoclaving.

Viruses and Human Disease

Oncogenes and Cancer

Some viruses can contribute to cancer development by carrying oncogenes or by integrating into host DNA and disrupting normal cell regulation.

Oncogenes promote uncontrolled cell growth.
Examples: Human papillomavirus (HPV) and hepatitis B virus (HBV).
Viruses are responsible for approximately 15% of human cancers.

Laboratory Cultivation of Viruses

Methods of Cultivation

Viruses cannot be grown on standard media; they require living cells for replication. Common methods include:

Inoculation into live animals or plants.
Use of embryonated eggs (e.g., chorioallantoic membrane).
Cell (tissue) culture using isolated cells grown in vitro.

HeLa cells are a famous example of immortal cell lines used for viral research.

Summary Table: Comparison of Viruses, Viroids, and Prions

Agent	Genetic Material	Protein Coat	Host Range	Diseases Caused
Virus	DNA or RNA	Present (capsid)	Animals, plants, bacteria	Influenza, HIV, TMV, etc.
Viroid	RNA (circular)	Absent	Plants	Potato spindle tuber disease, etc.
Prion	None	Present (abnormal protein)	Animals (especially mammals)	Creutzfeldt-Jakob disease, mad cow disease

Key Definitions

Capsid: Protein shell surrounding the viral genome.
Envelope: Lipid membrane derived from host cell, present in some viruses.
Oncogene: Gene that has the potential to cause cancer.
Prion: Infectious protein causing neurodegenerative disease.
Viroid: Infectious RNA molecule affecting plants.

Important Equations and Concepts

Burst size: Number of viruses released per lysed cell.
Burst time: Time required for a virus to complete its replication cycle and lyse the host cell.

Conclusion

Viruses, viroids, and prions are unique infectious agents with profound impacts on health, agriculture, and research. Their study is essential for understanding disease mechanisms and developing effective treatments and preventive measures.