Chapter 13: Viruses

I. Introduction

  • This chapter focuses on viruses, a topic of significant relevance due to current global health situations.

  • We will:

    • Define what a virus is.

    • Understand how viruses operate, beginning with bacteriophages, which are a model for learning about virus behavior.

    • Discuss various types of viruses, including those that infect animals, particularly humans.

    • Note: The term "virus" here refers to the most common type called "virion." For information on other types like viroids and prions, students should refer to their textbooks.

II. Defining Characteristics of a Virus

A. Acellularity

  • Viruses are acellular:

    • Definition: They are not composed of cells.

B. Composition

  1. Viruses consist of:

    • A nucleic acid core:

      • Types: Either DNA or RNA, never both.

      • Forms: Can be single-stranded (ss) or double-stranded (ds), e.g., ssRNA, dsRNA, ssDNA, dsDNA.

      • Notably, viruses challenge the norms of living organisms; the presence of dsRNA and ssDNA appears unconventional.

  2. Protein coat:

    • The protein coat surrounding the nucleic acid is referred to as a capsid (distinct from a capsule).

  3. Size:

    • Viruses are extremely small, measured in nanometers (nm).

      • Relationship: 1 nm = 1,000 µm (micrometers).

III. Morphology of Viruses

A. Shapes of Viruses

  1. Polyhedral:

    • Most common shape among viruses.

    • Definition: Many-sided; typically exhibits an icosahedral shape (20 sides and 12 corners).

  2. Helical:

    • Another morphological type shown in relevant figures.

  3. Complex:

    • Characterized by additional protein structures attached to the capsid, typically seen in bacteriophages but rare in animal viruses.

  4. Enveloped:

    • Defined: The capsid is surrounded by a lipid and protein-rich envelope, common in animal viruses.

B. Obligate Intracellular Parasites

  • Viruses are obligate intracellular parasites, meaning they cannot replicate outside a host cell.

Key Objective:

  • The primary purpose of a virus is to replicate (generate more of itself).

IV. Host Cell Dependency

A. Requirement for Host Cell Machinery

  1. Viral replication necessitates the cellular machinery found in host cells, as viruses lack the components required for nucleic acid replication and protein synthesis.

  2. The virus provides the nucleic acid template, while the host cell supplies the machinery needed for replication.

V. Viral Life Cycles

  • Understanding the cycles of viruses is crucial; they display patterns as defined by the chosen model virus, aiding in grasping how viruses operate:

    • There are 7 primary cycles to study.

    • Note: Medical microbiology courses cover approximately 40 to 50 viruses, but this chapter focuses on only 7 major examples.

VI. Virulent Bacteriophage: Lytic Cycle

A. T-4 Bacteriophage

  1. A complex virus characterized by double-stranded DNA (dsDNA).

  2. Targets and infects Escherichia coli (E. coli).

B. Steps of the Lytic Cycle

  1. Attachment:

    • The first step in any viral cycle.

    • The virus attaches to the receptor on a host cell; receptors can be proteins or sugars.

    • T-4 uses tail fibers for attachment.

  2. Entry:

    • The virus enters the host cell by injecting its DNA into it; the capsid remains outside the cell.

  3. Biosynthesis:

    • Host cell machinery is commandeered to synthesize viral nucleic acids and proteins.

    • Early viral genes are expressed to produce proteins such as:

      • Enzymes to degrade host DNA (preventing the host from making its own proteins).

      • DNA Polymerase for replicating viral DNA.

  4. Assembly and Packaging:

    • Viral parts are assembled, and the viral nucleic acid is packaged into the capsid.

  5. Release:

    • Newly formed viruses exit the host cell using lysozyme to cause cell lysis (the reason behind the term "lytic cycle").

VII. Temperate Bacteriophage

  • Definition: A phage that does not always kill the host cell.

A. Lambda (λ) Phage

  1. A complex virus with double-stranded DNA that infects E. coli.

  2. Possesses two cycles:

    • Lytic Cycle: Similar to the T-4 phage, leads to cell death.

    • Lysogenic Cycle: Virus remains dormant within the host cell without killing it.

B. Lambda Viral Cycle Steps

  1. Attachment:

  2. Entry:

  3. Choice Between Lytic & Lysogenic Cycle:

    • Genes involved in determining the fate of the viral cycle are present; if repressor genes are activated during transcription, the other cycle is inhibited.

  4. If Lytic Cycle is Chosen:

    • Follow the same path as in the T-4 lytic cycle.

  5. If Lysogenic Cycle is Chosen:

    • The phage DNA integrates into the host chromosome, becoming a prophage and remains inactive.

    • Copies of the prophage are inherited by the future generations of bacteria.

    • The prophage can stay dormant for an extended period.

  6. Induction:

    • Prophage can be excised and activated to enter the lytic cycle, this can occur either spontaneously or reactively by DNA-damaging agents such as chemicals or radiation.

VIII. Phage Conversion

  • Definition: Also known as prophage conversion; signifies that the expression of certain genes from the prophage confers new properties to the host bacterium.

A. Example - Corynebacterium diphtheriae

  1. Produces the diphtheria toxin, crucial in causing the disease diphtheria.

    • The bacterium's virulence is reliant upon the toxin, which originates from a prophage infection.

    • References to chapters specifying this disease should be undertaken for better understanding.

IX. Transduction

A. Definition

  • The method by which bacterial DNA is horizontally transferred between two bacterial cells via a bacteriophage.

  1. Generalized transduction:

    • Occurs during the lytic cycle; when phage mistakenly packages bacterial DNA instead of viral DNA.

  2. Specialized transduction:

    • Happens during the lysogenic cycle; during the induction phase, the excised prophage may include adjacent host genes, combining bacterial and viral DNA.