Other Pathogenic Agents

Viruses: Properties and Composition

Viruses are not typically classified as living organisms due to their inability to replicate without a host cell.
They are acellular and composed of a nucleic acid (DNA or RNA) core surrounded by a protein coat (capsid).
Some viruses have an additional lipoprotein envelope derived from host cell.
Viral nucleic acid can be single-stranded or double-stranded, linear or circular. This mirrors the variation of microbial genomes.

Viral Infections and Reproduction

Infections begin when a virus attaches to a specific host cell receptor.
After attachment, the virus penetrates the host cell and uncoats, releasing its nucleic acid.
Viral reproduction is heavily dependent on host machinery. The viral nucleic acid in the infected cell instructs the cell to make viral proteins and to copy the viral nucleic acid.
After assembly of new virions, the viruses leave the host cell either by budding or lysis.
- Budding off a host cell envelops the virus in a portion of the host cell’s plasma membrane.
- Lysis leads to death of the host cell.

Prions: Properties and Disease

Prions are infectious proteins that cause a group of diseases known as transmissible spongiform encephalopathies (TSEs).
Prions lack nucleic acid.
Misfolded prions can cause normal proteins to also misfold, leading to a chain reaction that can result in disease.
Prion diseases are characterized by neuronal death leading to ‘spongy’ brain tissue.
Examples of TSEs include Creutzfeldt-Jakob disease and Bovine spongiform encephalopathy (“Mad Cow” disease).

Viroids: Properties and Diseases

Viroids are smallest known pathogens that consist of tiny strands of circular RNA devoid of a protein coat.
They only infect plants.
Unlike viruses, they don’t code for proteins. They interfere with plant’s cellular machinery, causing disease symptoms.

By understanding the wide spectrum of pathogenic agents, we can delve into the complexities of microbial diseases and their impacts. This knowledge is crucial in the prevention and treatment of many types of diseases.