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

Viruses, Viroids, and Prions: Tiny but Deadly

Introduction

This chapter explores three types of acellular infectious agents—viruses, viroids, and prions—which are extremely small but can cause significant diseases in humans, animals, and plants. Understanding their discovery, 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. He referred to these tiny life forms as "animalcules." His discoveries laid the foundation for the field of microbiology.

• Animalcules: Early term for microorganisms observed by Leeuwenhoek.

• 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 the Link Between Microorganisms and Disease

Koch's postulates are a set of criteria used to establish a causative relationship between a microbe and a disease. Developed by Robert Koch, these postulates are fundamental in medical microbiology.

• The microorganism must be observed in every case of the disease.

• It must be isolated and grown in pure culture.

• When inoculated into a healthy animal, the pure culture must cause the disease.

• The microorganism must be recovered from the experimentally infected animal.

Example: Koch used these postulates to identify the causative agent of tuberculosis (Mycobacterium tuberculosis).

Discovery of Viruses

Early Experiments and the Chamberland Filter

The discovery of viruses began with the observation that certain infectious agents could pass through filters designed to trap bacteria. The Chamberland filter, invented in 1884, was a porcelain water filter that could remove known microbes.

• Chamberland filter was used to filter out bacteria from solutions.

• Some diseases persisted even after filtration, suggesting the presence of smaller infectious agents.

Tobacco Mosaic Virus (TMV)

In the late 19th century, tobacco growers reported a disease affecting their crops. Adolf Mayer demonstrated that sap from infected plants could transmit the disease, but he could not isolate bacteria or fungi as the cause.

• Filtered sap from infected plants still caused disease, even after passing through a Chamberland filter.

• No microbes were visible under the microscope, indicating a new type of infectious agent.

• This led to the identification of the Tobacco Mosaic Virus (TMV), the first virus discovered.

Example: TMV infects tobacco plants, causing mottled leaves and reduced crop yield.

Further Discoveries

Martinus Beijerinck and Dmitri Ivanovsky repeated these experiments and coined the term "virus," meaning poison. Later, viruses were found to cause diseases in animals, such as foot-and-mouth disease.

• Viruses are much smaller than bacteria and cannot be seen with light microscopes.

• They require living hosts for replication.

Key Definitions

• Virus: An acellular infectious agent composed of genetic material (DNA or RNA) surrounded by a protein coat; requires a host cell for replication.

• Viroid: A small, circular RNA molecule that infects plants; lacks a protein coat.

• Prion: An infectious protein that can cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins.

Summary Table: Comparison of Viruses, Viroids, and Prions

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

Additional info:

• Viruses, viroids, and prions are all considered acellular infectious agents, meaning they lack cellular structure and cannot carry out independent metabolism.

• Viruses are important in global health due to their role in pandemics (e.g., influenza, COVID-19).

• Prions are unique in that they are composed solely of protein and can induce abnormal folding in host proteins, leading to neurodegenerative diseases.