Viruses, Viroids, and Prions

Introduction

This chapter explores the unique characteristics of viruses, viroids, and prions, their replication cycles, methods of cultivation and identification, and their roles in disease and cancer. Understanding these infectious agents is crucial for microbiology students, as they differ fundamentally from cellular life forms such as bacteria.

Viruses vs. Bacteria

Key Differences

• Viruses are obligate intracellular parasites—they can only reproduce inside a living host cell.

• Viruses contain either DNA or RNA (never both), surrounded by a protein coat (capsid).

• Viruses lack ribosomes and ATP-generating mechanisms.

• Viruses use the host cell’s machinery for replication.

• Bacteria are complete cells with both DNA and RNA, ribosomes, and independent metabolism. They reproduce by binary fission and generate ATP.

Host Range: The spectrum of species, strains, or cell types that a virus can infect.

Bacteriophages: Viruses that specifically infect bacteria.

Structure of Viruses

Enveloped vs. Nonenveloped Viruses

• All viruses contain nucleic acid (DNA or RNA) and a capsid (protein coat made of capsomeres).

• Nonenveloped viruses: Consist only of nucleic acid and capsid.

• Enveloped viruses: Have an additional envelope composed of lipid, protein, and carbohydrate, often derived from the host cell membrane. The envelope may contain spikes (glycoproteins) that aid in attachment to host cells.

Virion: A complete, fully developed, infectious viral particle.

Classification of Viruses

• Viruses are classified based on their nucleic acid type (DNA or RNA) and differences in their protein coats and structures.

Cultivation of Viruses

Bacteriophage Cultivation

• Grown in living bacterial cells.

• Plaque method: Bacteriophage sample is mixed with host bacteria and melted agar, poured onto a solid medium. Each virus infects a bacterium, multiplies, and lyses the cell, creating a plaque (clear area) on the bacterial lawn.

• Plaque-forming units (PFU): Each plaque corresponds to a single virus.

Animal Virus Cultivation

• Living animals: Animals are inoculated and observed for disease signs.

• Embryonated eggs: Virus is injected into eggs; changes or embryo death are monitored. Used for vaccine production.

• Cell cultures:

  • Diploid cell lines: Derived from human cells, can be maintained for ~100 generations (e.g., for rabies vaccine).

  • Continuous cell lines: Derived from cancerous cells, can be maintained indefinitely (e.g., HeLa cells).

Identification of Viruses

• Cytopathic effects: Observable changes in host cells due to viral infection.

• Serological tests: Detection using antibodies (e.g., ELISA, rapid antigen tests).

• Nucleic acid tests: PCR and related molecular techniques.

Replication Cycles of Bacteriophages

Lytic Cycle (T-even Bacteriophages)

The lytic cycle results in the destruction of the host cell and release of new phages.

1. Attachment: Phage attaches to bacterial cell via tail fibers.

2. Penetration: Phage lysozyme opens the cell wall; tail sheath contracts, injecting viral DNA.

3. Biosynthesis: Host cell machinery is hijacked to produce viral DNA and proteins; host protein synthesis stops.

4. Maturation: New virions are assembled.

5. Release: Phage lysozyme lyses the cell wall, releasing new virions.

Lysogenic Cycle (Lambdavirus)

The lysogenic cycle allows the phage genome to persist in the host without killing it immediately.

1. Phage attaches and injects DNA into host.

2. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

3. Bacterium reproduces normally, copying prophage DNA with each division.

4. Prophage may excise and enter the lytic cycle.

Outcomes of Lysogeny:

• Immunity to reinfection by the same phage.

• Phage conversion: new properties conferred to the host bacterium.

• Specialized transduction: specific bacterial genes transferred to another bacterium via phage.

Multiplication of Animal Viruses

General Steps

1. Attachment: Virus binds to host cell membrane.

2. Entry: By receptor-mediated endocytosis or membrane fusion.

3. Uncoating: Viral nucleic acid is separated from the capsid by host or viral enzymes.

4. Biosynthesis: Production of viral nucleic acids and proteins.

5. Maturation: Assembly of viral components.

6. Release: By budding (enveloped viruses) or rupture (nonenveloped viruses).

DNA vs. RNA Virus Replication

• DNA viruses: Replicate DNA in the nucleus using host enzymes; capsid proteins are synthesized in the cytoplasm and assembled in the nucleus.

• RNA viruses: Replicate in the cytoplasm using RNA-dependent RNA polymerase.

  • Positive-sense RNA: Viral RNA acts as mRNA.

  • Negative-sense RNA: Viral RNA must be transcribed to positive-sense RNA before translation.

• Retroviruses: Use reverse transcriptase to synthesize DNA from RNA, which integrates into the host genome as a provirus.

Viruses and Cancer

Oncogenes and Transformation

• Proto-oncogenes: Normal genes involved in cell growth; mutations can convert them to oncogenes that trigger uncontrolled cell division (cancer).

• Transformed cell: A cell that has acquired properties leading to cancer, often expressing tumor-specific transplantation antigens (TSTA).

Viral Oncogenesis

• Oncogenic DNA viruses: May integrate into host DNA, disrupting normal regulation and leading to tumors.

• Oncogenic RNA viruses (Retroviruses): Use reverse transcriptase to integrate viral DNA into host genome, potentially activating oncogenes.

Latent and Persistent Viral Infections

Latent Infections

• Virus remains dormant in host cells for long periods without causing symptoms.

• Can reactivate when host immunity changes (e.g., Varicellovirus causes chickenpox, can reactivate as shingles).

Persistent (Chronic) Infections

• Virus is continuously released; symptoms gradually increase in severity over time.

• Often fatal (e.g., subacute sclerosing panencephalitis from measles virus).

Viruses, Viroids, and Prions: Definitions and Comparisons

Prions: Infectious Proteins

• Prions are infectious, misfolded proteins that induce normal proteins to misfold, leading to accumulation and disease (e.g., spongiform encephalopathies such as mad cow disease, Creutzfeldt-Jakob disease).

• Prions contain no detectable nucleic acids.

Key Terms and Concepts

• Prophage: Phage DNA integrated into bacterial chromosome.

• Provirus: Viral DNA integrated into a eukaryotic host genome (e.g., retroviruses).

• Reverse transcriptase: Enzyme used by retroviruses to synthesize DNA from RNA.

• TSTA (Tumor-specific transplantation antigen): Antigen found on the surface of transformed cells.

Summary Table: Viral Replication Cycles

Examples

• Latent infection: Varicellovirus (chickenpox/shingles)

• Persistent infection: Measles virus causing subacute sclerosing panencephalitis

• Prion disease: Mad cow disease, Creutzfeldt-Jakob disease

Additional info: Retroviruses such as HIV are clinically significant because their integration into the host genome makes them difficult to eradicate and can lead to long-term latent infections.