C.5

Viruses Microbiology Lecture Study Notes

Overview of Lecture

  • Main Topics Covered:
    1. Defining "virus"
    2. Important questions concerning viruses
    3. Viral classification & composition
    4. Viral morphology
    5. Bacteriophages
    6. Prions, viroids, & oblisks
    7. The current status of finding a cure for HIV infection

Defining “Virus”

  • Definition:
    • Viruses are entities that infect every type of cell, including:
    • Bacteria
    • Algae
    • Fungi
    • Protozoa
    • Plants
    • Animals
  • Characteristics of Viruses:
    • Not Living:
    • Not composed of cells.
    • Cannot reproduce on their own.
    • Size: Smaller than cells, with very few exceptions.

Size Comparison of Viruses with Cells & Prions

  • Visual Reference: Not provided.
  • Typical observing tool: Electron Microscope, capable of 10,000X total magnification or more is necessary to view viruses.

First Discovery of Viruses

  • Historical Context:
    • First discovery involved the Tobacco Mosaic Virus (TMV) affecting tobacco plants.
    • Remarkably small, viruses go undetected when bacteria are filtered out due to their size.

Important Questions Concerning Viruses

  • Key Questions Addressed:
    • What role did viruses play in the evolution of life?
    • How can such small and seemingly insignificant particles lead to diseases?
    • What is the association between viruses and cancer?
  • Implication: Even though viruses do not perform life processes typical of cells, they can direct those processes and exhibit active influence.

The Vital Role of Viruses in Evolution

  • Influence on Genetic Makeup:
    • Viruses infect cells and affect their genetic composition, shaping evolution.
    • Contribution to Human Genome:
    • Approximately 8% of the human genome originates from viral sequences.
    • 10 to 20% of bacterial DNA has been found to contain viral sequences.
  • Definition: Viruses described as obligate intracellular parasites, unable to multiply independently from host cells.

The Varied Properties of Viruses

  • Types of Viral Nucleic Acid:
    • Either DNA or RNA, but not both. Types of nucleic acids include:
    • Double-stranded DNA (dsDNAdsDNA)
    • Single-stranded DNA (ssDNAssDNA)
    • Double-stranded RNA (dsRNAdsRNA)
    • Single-stranded RNA (ssRNAssRNA)
  • Virus Specificity:
    • Molecules on virus surfaces grant them specific attachment to host cells.
    • Viruses replicate by commandeering the host cell's genetic material. Lack cellular machinery such as enzymes and ribosomes to perform metabolic processes.

Classification & Naming of Viruses

  • New Classification Systems Focus on:
    • Host types and caused diseases
    • Structural characteristics (icosahedral, cylindrical, spiral, complex)
    • Chemical composition
    • Genetic makeup similarities
  • Baltimore Classification System:
    • Based on the virus's nucleic acid and its replication mechanism, developed by Dr. David Baltimore (Nobel Laureate, 1978).

General Viral Composition

  • Structure comprises:
    • Capsid: External protein shell.
    • Core: Contains nucleic acid segments (either DNA or RNA, but not both).
    • Generally possess a small number of genes.
  • Enveloped Viruses:
    • Some viruses have an external lipid membrane, similar to a plasma membrane.
    • Envelopes feature proteins for cell attachment and are common in viruses infecting animal cells.
    • Specific enzymes may be included for replication that the host cell typically does not possess (e.g., HIV utilizing reverse transcriptase).

Virion Definition

  • Virion: A fully formed virus capable of infecting a host cell.

Types of Virus Morphology

  • Common Structures:
    • Glycoproteins, Capsid, Lipid membrane, RNA or DNA encapsulates, and more.
  • Examples of Virus Types:
    • Bacteriophage, Influenza, Adenovirus, Tobacco Mosaic Virus, Ebola Virus.

Replication Cycles in Animal Viruses

  • Phases of Replication:
    1. Adsorption: Getting into host cell.
    2. Penetration & Uncoating: Occurs within cell cytoplasm.
    3. Synthesis: Replicating viral genetic material.
    4. Assembly: Forming new virions.
    5. Release: New virions exit host cell to infect others.
  • Host Range: Defines the specific cells a virus can infect.

Cytopathic Effects (CPEs)

  • Definition: Damage inflicted by a virus on host cells.

Persistent / Chronic Viral Infections

  • Characteristics:
    • Some cells retain the virus without immediate destruction, which can last from weeks to a lifetime.
    • Example: HIV is a chronic infection.

Viruses & Cancer

  • Statistics: Approximately 20% of cancers are attributed to viral infections (IMPORTANT).
  • Transformation: Cancer-causing viruses lead to significant changes in host cells, resulting in cancer.
  • Viruses Linked to Cancer:
    • HPV - cervical cancer
    • HIV - Kaposi's sarcoma
    • HCV - liver carcinoma
    • EBV - Burkitt's lymphoma

Viruses That Infect Bacteria (Bacteriophages)

  • Most bacteriophages consist of double-stranded DNA (dsDNAdsDNA).
  • Widely distributed, especially in sewage and aquatic environments; may help combat antibiotic-resistant bacteria.

Bacteriophage Life Cycles

  • Lytic Cycle: Bacteriophages causing immediate lysis of host cells.
  • Lysogenic Cycle: Bacteriophages infecting but remaining dormant within host cells.

Cultivating Animal Viruses in the Laboratory

  • Medium Requirements: Viruses require living cells.
    • In Vivo: Using living animals and embryonic tissues.
    • In Vitro: Cell and tissue culture methods.
    • Purpose includes understanding replication or producing virus for vaccine development.

Detection of Viral Growth in Tissue Culture

  • Observation: Growth is noted by the degeneration and lysis of infected cells.
  • Plaques: Areas in a culture indicating destruction from virus-infected cells.

Prions

  • Composition: Made entirely of protein, lacking nucleic acids.
  • Mode of Infection: Features long protein fibrils deposited in the brain, linked to diseases like Creutzfeldt-Jakob Disease and Mad Cow Disease.
    • Transmission Routes: Genetic mutation, contaminated medical instruments, organ transplants, and infected meat consumption.

Viroids

  • Nature: Virus-like agents impacting plants, composed of naked RNA strands without a capsid.
  • Economic Impact: Significant pathogens to crops such as tomatoes, potatoes, cucumbers, and citrus trees.

Treatment of Viral Infections in Humans & Animals

  • Ineffectiveness of Antibiotics: Antibiotics cannot combat viral infections, as they target cellular machinery.
  • Preventive Measures: Developing vaccines is more feasible than treatments for viral diseases.
  • Treatments for Management: Such as HAART therapy for HIV, which inhibits viral enzymes.

Side Effects of Anti-HIV Therapy

  • Common Side Effects:
    • Lipodystrophy: Fat redistribution linked to enzyme inhibitors (e.g., “Buffalo Hump”).
    • Skin Rashes: Potential effects during treatment.

Resistance to HIV Infection

  • CCR5 Coreceptor: 1 to 3% of the population lacks this coreceptor, providing resistance to HIV.
  • Berlin Patient Case (2009): A patient who received a bone marrow transplant from a donor with a resistant mutation, leading to undetectable virus levels post-treatment.

CRISPR to Cure HIV (2022)

  • CRISPR-Cas9: A bacterial gene-editing tool utilized to eliminate HIV from human cells, with ongoing clinical trials.
  • EBT-101: A specific application intending to remove HIV proviral DNA using CRISPR.

Helpful Resources

  • Video and quiz resources available for further study on viral concepts.