Characterizing and Classifying Viruses, Viroids, and Prions: Microbiology Study Notes

Characterizing and Classifying Viruses, Viroids, and Prions

Characteristics of Viruses

Viruses are minuscule, acellular, infectious agents that possess either DNA or RNA as their genetic material. They are responsible for numerous diseases in humans, animals, plants, and bacteria. Viruses cannot carry out metabolic processes, grow, or respond to their environment independently. Instead, they rely on host cells to reproduce and increase their numbers. Viruses lack cytoplasmic membranes, cytosol, and organelles, and exist in two states: extracellular (virion) and intracellular.

• Virion: The extracellular state of a virus, consisting of a protein coat (capsid) surrounding nucleic acid. Some virions have a phospholipid envelope.

• Intracellular State: The capsid is removed, and the virus exists as nucleic acid within the host cell.

Genetic Material of Viruses

Viruses display greater diversity in their genomes compared to cells. Their genetic material can be DNA or RNA, but never both. Viral genomes may be double-stranded (dsDNA or dsRNA) or single-stranded (ssDNA or ssRNA), and can be linear, segmented, or circular. Viral genomes are much smaller than those of cells.

Hosts of Viruses

Viruses typically infect specific host cells due to the affinity between viral surface proteins and complementary proteins on the host cell surface. Some viruses are highly specific, infecting only certain cell types, while others are generalists, infecting multiple cell types or hosts. All organisms are susceptible to some form of virus.

Capsid Morphology and Viral Shapes

The capsid provides protection for viral nucleic acid and facilitates attachment to host cells. Capsids are composed of protein subunits called capsomeres, which may be made of one or several types of proteins. Viruses are classified by their shape, which can be helical, polyhedral, or complex.

• Helical: Rod-shaped, with nucleic acid wound inside a spiral capsid.

• Polyhedral: Many-sided, often icosahedral.

• Complex: More intricate structures, such as bacteriophages.

The Viral Envelope

Some viruses acquire an envelope from the host cell during replication or release. The envelope consists of a phospholipid bilayer and proteins, including virally coded glycoproteins (spikes) that play a role in host recognition. Enveloped viruses are generally more fragile than naked viruses.

Classification of Viruses

Viruses are classified based on their type of nucleic acid, presence of an envelope, shape, and size. Viral genera are organized into families, but relationships among viruses are not fully understood.

Viral Replication

Viral replication depends on host organelles and enzymes. The lytic replication cycle results in the death and lysis of the host cell and consists of five stages:

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Lysogenic Replication of Bacteriophages

Lysogenic replication is a modified cycle where infected host cells grow and reproduce normally for generations before lysing. Temperate phages can enter a dormant state called a prophage. Lysogenic conversion occurs when phages carry genes that alter the phenotype of a bacterium.

Replication of Animal Viruses

Animal viruses follow a similar replication pathway to bacteriophages, with differences due to the presence of an envelope, eukaryotic nature of animal cells, and lack of cell wall. Attachment involves chemical attraction between viral proteins and cell receptors, often mediated by glycoprotein spikes. Entry and uncoating can occur via direct penetration, membrane fusion, or endocytosis.

Synthesis of DNA and RNA Viruses in Animals

• dsDNA viruses: Replicate genome in nucleus, proteins made in cytoplasm.

• ssDNA viruses: ssDNA folds to form dsDNA, replicated by cellular DNA polymerase.

• RNA viruses: Replication occurs in cytoplasm. Positive-sense (+) RNA acts as mRNA; negative-sense (−) RNA must be transcribed to mRNA.

• Retroviruses: Use DNA intermediary transcribed by reverse transcriptase.

Assembly and Release of Animal Viruses

• Most DNA viruses assemble in the nucleus; most RNA viruses assemble in the cytoplasm.

• Enveloped viruses are released via budding, which can result in persistent infections.

• Naked viruses are released by exocytosis or lysis.

Latency of Animal Viruses

Some animal viruses remain dormant in host cells as latent viruses or proviruses. Latency may last for years with no viral activity, and incorporation of provirus into host DNA is permanent.

The Role of Viruses in Cancer

Cell division is tightly regulated by genetic controls. Neoplasia is uncontrolled cell division, resulting in tumors. Malignant tumors are cancers, which can metastasize. Proto-oncogenes promote cell growth; their uncontrolled activation can lead to cancer. Environmental factors, including viruses, can activate oncogenes. Viruses may cause 20–25% of human cancers by carrying oncogenes, promoting host oncogenes, or interfering with tumor repression.

• Examples: Burkitt’s lymphoma, Hodgkin’s disease, Kaposi’s sarcoma, cervical cancer.

Culturing Viruses in the Laboratory

Viruses cannot grow in standard media and must be cultured inside host cells. Three types of media are used: mature organisms, embryonated eggs, and cell cultures.

Culturing Viruses in Mature Organisms

• Phages are grown in bacteria in liquid cultures or on agar plates.

• Lysis produces plaques, allowing estimation of phage numbers by plaque assay.

Culturing Viruses in Embryonated Chicken Eggs

• Eggs are inexpensive, large, free of contaminants, and contain nourishing yolk.

• Embryonic tissues provide ideal sites for growing viruses; some vaccines are prepared in chicken cultures.

Culturing Viruses in Cell (Tissue) Culture

• Cells are isolated and grown on media or broth.

• Two types: diploid cell cultures and continuous cell cultures.

Other Parasitic Particles: Viroids and Prions

Characteristics of Viroids

Viroids are extremely small, circular pieces of ssRNA that are infectious and pathogenic in plants. They lack a capsid and do not code for proteins. Viroid RNA adheres to complementary plant RNA, leading to degradation and disease.

Characteristics of Prions

Prions are proteinaceous infectious agents. Normal cellular PrP has α-helices, while prion PrP has β-pleated sheets. Prion PrP induces refolding of cellular PrP into the disease-causing form. Prion diseases include spongiform encephalopathies, which cause large vacuoles in the brain and a spongy appearance. Examples include BSE, scrapie, kuru, CWD, and vCJD. Prions are transmitted by ingestion, transplantation, or contact with infected tissues. Standard sterilization does not deactivate prions; they are destroyed by incineration or autoclaving in concentrated sodium hydroxide.

Comparison of Viruses, Viroids, and Prions

• Viruses: Infectious agents with both protein (capsid) and nucleic acid.

• Viroids: Infectious RNA, no capsid.

• Prions: Infectious protein, no nucleic acid.

Summary Table: Properties of Viruses, Viroids, Prions, and Bacterial Cells

Agent	Genetic Material	Protein Coat	Envelope	Host Range
Virus	DNA or RNA	Yes (capsid)	Sometimes	Humans, animals, plants, bacteria
Viroid	ssRNA	No	No	Plants
Prion	No	No	No	Mammals
Bacterial Cell	DNA	No	No	Various

Additional info: This table is inferred from the comparison sections and summarizes the main differences among viruses, viroids, prions, and bacterial cells.