Acellular Infectious Agents: Viruses, Viroids & Prions: Videos & Practice Problems

Acellular infectious agents, which include viruses, viroids, and prions, are entities that do not consist of or contain cells. The term "acellular" indicates that these agents lack cellular structures, and as a result, they are not classified as living organisms. The fundamental unit of life is the cell, and anything that does not possess this structure cannot be considered alive. Therefore, viruses, viroids, and prions are categorized as non-living infectious agents that can contaminate and affect living cells.

In the context of the microbial world, acellular infectious agents occupy a distinct position separate from cellular organisms. Cellular organisms include prokaryotes, such as bacteria and archaea, as well as microscopic eukaryotes like fungi, protists, algae, and protozoa. These cellular organisms represent the living aspect of the microbial world, while the acellular infectious agents are positioned on the periphery, highlighting their non-living status.

As we delve deeper into the study of these acellular infectious agents, we will explore each group in detail, examining their structures, functions, and impacts on living organisms. Understanding these agents is crucial for comprehending their roles in health and disease, as well as their interactions with cellular life.

Viruses are fascinating entities that can be defined as obligate intracellular parasites, meaning they can only replicate within a host cell. They consist of either DNA or RNA, but not both, which is encased in a protein coat. In some cases, this genetic material may also be surrounded by a lipid envelope. The term "obligate intracellular parasite" highlights that viruses depend entirely on their host for reproduction and survival, as they cannot replicate independently.

Viruses have the ability to infect all forms of life, including bacteria, archaea, and eukarya. While many viruses lead to the destruction of the cells they infect, some can remain dormant within the host without causing immediate harm. This duality of behavior is an important aspect of viral biology that will be explored in greater detail later in the course.

For example, bacteriophages are viruses that specifically target bacteria. A well-known example is the Escherichia coli T4 bacteriophage, which has a distinct structure featuring a protein coat and genetic material. On the other hand, human viruses, such as SARS-CoV-2, which is responsible for COVID-19, exhibit different structural characteristics but still share the fundamental components of a protein coat and genetic material.

In summary, viruses are unique biological entities that play significant roles in various ecosystems and can have profound effects on their hosts. Understanding their structure and function is crucial for comprehending their impact on health and disease, which will be further discussed in subsequent lessons.

Viroids are unique infectious agents that, like viruses, are obligate intracellular parasites, meaning they require a living cell to replicate. However, unlike viruses, which consist of DNA or RNA along with proteins and sometimes lipids, viroids are composed solely of a single short strand of RNA that forms a closed ring structure. This distinctive feature allows them to base pair with themselves, creating a stable configuration.

Currently, viroids are known to infect only plants, leading to various plant diseases. Their ability to cause disease can result in symptoms such as wilting and ultimately death of the infected plants. One notable example of a viroid is the potato spindle tuber viroid (PSTV), which specifically infects potatoes, transforming healthy potatoes into infected ones.

Despite their significance in plant pathology, the origins of viroids and the mechanisms by which they induce disease remain largely unknown, prompting ongoing research in the field. Understanding viroids is crucial for developing strategies to manage plant diseases and protect agricultural crops.

Prions, short for proteinaceous infectious agents, are unique infectious agents composed solely of proteins. Unlike viruses and viroids, which are also obligate intracellular parasites, prions do not contain any nucleic acids; they are entirely made up of misfolded proteins. This misfolding is critical because prions can induce normal proteins in the body to misfold as well, leading to a cascade of dysfunction.

The misfolded proteins lose their normal function, which can result in severe neurodegenerative diseases. Notable examples of prion-related diseases include scrapie in sheep and mad cow disease in cattle. The structural differences between normal proteins and prions are significant; the misfolded prion proteins adopt an abnormal shape that disrupts normal cellular processes.

In a healthy brain, proteins maintain their proper structure, allowing them to function correctly. However, when prions are present, they can convert these normal proteins into misfolded versions, ultimately leading to a diseased state. This transformation is visually represented by comparing the structure of a normal protein with that of a prion, highlighting the critical role that protein conformation plays in cellular health.

Understanding prions is essential as they represent a unique class of infectious agents that challenge traditional concepts of disease causation, emphasizing the importance of protein structure in biological function.