Viruses

Overview and Distinction Between Viruses and Bacteria

Definition of Obligatory Intracellular Parasites: Viruses are obligate intracellular parasites, meaning they absolutely require a living host cell to replicate and multiply. Outside of a host cell, they lack independent metabolic activity and cannot synthesize their own biological components.
Metabolic and Structural Capabilities:
- Viruses contain either DNA or RNA, but never both simultaneously.
- Viruses contain a protein coat (capsid) surrounding their nucleic acid genome.
- Viruses multiply inside living host cells by commandeering the cell's metabolic and synthesizing machinery.
- Viruses possess no ribosomes for protein synthesis.
- Viruses possess no ATP-generating metabolism.

Systemic Comparison Matrix:
- Intracellular Parasitism:
  - Typical Bacteria: No
  - Rickettsias / Chlamydias: Yes
  - Viruses: Yes
- Plasma Membrane:
  - Typical Bacteria: Yes
  - Rickettsias / Chlamydias: Yes
  - Viruses: No
- Binary Fission Replication:
  - Typical Bacteria: Yes
  - Rickettsias / Chlamydias: Yes
  - Viruses: No
- Pass Through Bacteriological Filters:
  - Typical Bacteria: No
  - Rickettsias / Chlamydias: No or Yes
  - Viruses: Yes
- Possess Both DNA and RNA:
  - Typical Bacteria: Yes
  - Rickettsias / Chlamydias: Yes
  - Viruses: No
- ATP-Generating Metabolism:
  - Typical Bacteria: Yes
  - Rickettsias / Chlamydias: Yes or No
  - Viruses: No
- Ribosomes Present:
  - Typical Bacteria: Yes
  - Rickettsias / Chlamydias: Yes
  - Viruses: No
- Sensitivity to Antibiotics:
  - Typical Bacteria: Yes
  - Rickettsias / Chlamydias: Yes
  - Viruses: No
- Sensitivity to Interferon:
  - Typical Bacteria: No
  - Rickettsias / Chlamydias: No
  - Viruses: Yes

Host Range and Receptor Interactions

Concept of Host Range: Host range defines the specific spectrum of host cells a virus is capable of infecting.
Specificity Determinants: Most individual viruses are restricted to infecting specific types of cells in a single host species. Host range is determined by:
- The presence of specific host receptor attachment sites on the surface of host cells.
- Availability of required cellular host factors necessary for viral replication.
Bacteriophages (Bacterial Viruses):
- Target receptor sites may be part of the bacterial cell wall, fimbriae, or flagella.
Animal Viruses:
- Target receptor sites are typically specific proteins or glycoproteins located on the host cell plasma membrane.

Bacteriophage Lambda interacting with LamB receptor

Molecular Case Study - Bacteriophage $\lambda$ Interaction with Host Receptor:
- Research Citation: Nature Communications (Published 17 May 2024, Volume 15, Article number: 4185) by Xiaofei Ge & Jiawei Wang: "Structural mechanism of bacteriophage lambda tail's interaction with the bacterial receptor".
- Host Receptor Identity: LamB is a transmembrane protein pore located in the outer membrane (OM) of Escherichia coli.
- Physiological Function of LamB: Serves natively as a porin to transport maltose (sugar) into the cell as a biological nutrient source.
- Viral Attachment & Entry: Phage $\lambda$ attaches specifically via its tail tip proteins to the LamB receptor on the outer bacterial cell surface and injects its DNA across the outer membrane into the periplasmic space and cytoplasm.
- Tail Tip Components: Cryo-electron microscopy (cryo-EM) reveals structural proteins comprising the tail tip interaction complex: gpM, gpL, gpI, gpJ-a, gpJ-b, and gpJ-c.
- Structural Dimensions: The total tail tip structural height is $17.6\,nm$ , with interaction cross-sections measuring $13.9\,nm$ , $15.7\,nm$ , and $10.5\,nm$ , and a central channel width measuring $3\,nm$ .

Physical Size Spectrum of Biological Entities

Size Spectrum from Prions to Human Blood Cells

Quantitative Size Hierarchy:
- Prion: $20\,nm$
- Bacteriophages f2, MS2: $24\,nm$
- Poliovirus: $30\,nm$
- Enterovirus rhinovirus: $30\,nm$
- Coronavirus: $90\,nm$
- Rabies virus: $170 \times 70\,nm$
- Bacteriophage T4: $225\,nm$
- Tobacco mosaic virus: $250 \times 18\,nm$
- Viroid: $300 \times 10\,nm$
- Vaccinia virus: $300 \times 200 \times 100\,nm$
- Bacteriophage M13: $800 \times 10\,nm$
- Ebola virus: $970\,nm$
- Chlamydia bacterium elementary body: $300\,nm$
- Escherichia coli bacterium: $3000 \times 1000\,nm$
- Human red blood cell: $10000\,nm$ in total diameter (with a plasma membrane thickness of $10\,nm$ ).

Chemical Structure of Viruses

Virion: A fully developed, structurally complete, infectious viral particle.
Nucleic Acid Genome:
- Composed of either DNA or RNA, but never both in the same virus.
- Genome structure can be single-stranded ( $ss$ ) or double-stranded ( $ds$ ).
- Physical arrangement can be linear, circular, or segmented.
- Total genome length spans from a few thousand nucleotides up to $250000\text{ nucleotides}$ .

Structure of a nonenveloped polyhedral capsid

Capsid and Capsomeres:
- The capsid is a protective protein shell enclosing the viral genome.
- Capsids are constructed from repetitive protein subunits known as capsomeres.

Envelope:
- An outer lipid, protein, and carbohydrate layer present surrounding the capsid in some viruses.
- Partially acquired from the host cell's plasma membrane during the process of viral budding.
Spikes:
- Projections extending outward from the viral surface in certain enveloped or nonenveloped viruses.
- Composed of glycoprotein (carbohydrate-protein complexes).
- Function primarily as attachment factors to adhere to host cell surface receptors.

Viral Morphological Classifications

Helical Viruses:
- Characterized by hollow, cylindrical capsids structured in a helical spiral surrounding the nucleic acid.
- Examples: Rabies virus ( $170 \times 70\,nm$ ) and Ebola virus ( $970\,nm$ ).

*   *Rabies Virus Components:* Contains an internal helical RNA genome complexed with Large protein (L), Nucleoprotein (N), and Phosphoprotein (P), bounded by Matrix protein (M) and an outer lipid envelope studded with Glycoprotein (G) spikes.

Polyhedral Viruses:
- Multi-sided structure; most predominantly formatted as an icosahedron (having $20$ triangular facets and $12$ corners).
- Examples: Adenoviruses and Poliovirus.
Enveloped Viruses:
- Viruses equipped with an outer membrane envelope, taking on a roughly spherical overall shape.

*   *Coronavirus Architecture:* Enveloped sphere incorporating Spike Glycoprotein (S), Membrane Protein (M-Protein), Hemagglutinin-esterase dimer (HE), Envelope Protein (E-Protein), and internal viral RNA bound to Nucleocapsid Protein (N).

Complex Viruses:
- Possess intricate, elaborate structural features attached to capsids.

Complex Viruses including T-even bacteriophage and Orthopoxvirus

*   *T-even Bacteriophage Components:* Features a polyhedral capsid head ( $65\,nm$  wide) encapsulating viral DNA, attached to a helical sheath, baseplate, pins, and tail fibers.
*   *Orthopoxvirus:* Complex brick-shaped viral morphology containing internal viral cores.

Isolation, Cultivation, and Identification Techniques

General Cultivation Requirement: Viruses must be provided with living host cells to reproduce due to their total lack of autonomous synthetic machinery.
Culturing Bacteriophages:
- Bacteriophages are grown in bacterial cultures suspended in or on solid agar media.

*   *Plaque Method:* A mix of bacteriophages and host bacteria is plated on agar. As viruses lyse host bacterial cells, clear zones called plaques form on an opaque lawn of confluent bacterial growth. Each plaque corresponds to a single viral infection event.

Culturing Animal Viruses:
1. Living Animals: Infection of laboratory host models such as mice, rabbits, or guinea pigs.
2. Embryonated Eggs: Inoculation of viral samples into specialized compartments of fertilised avian eggs.

    *   *Inoculation Routes:* Chorioallantoic membrane inoculation, Amniotic cavity inoculation, Allantoic cavity inoculation, or Umbilical vesicle (yolk sac) inoculation.
3.  *Cell Cultures:* Growing host cells in artificial liquid growth media inside specialized laboratory tubes or tissue culture flasks.
    *   *Primary Cell Lines:* Derived directly from host tissue samples; survive for only a limited number of cell generations before dying, requiring continuous fresh tissue sources.
    *   *Continuous Cell Lines:* Established from transformed or cancerous cells (immortalized cell lines) that can be propagated indefinitely.

Methods of Viral Identification:
1. Cytopathic Effects (CPE): Morphological changes and structural damage induced in host cell cultures during viral infection.
2. Serological Testing: Detection of specific virus-antibody binding reactions (e.g., Enzyme-Linked Immunosorbent Assay / ELISA).
3. Nucleic Acid Assays: Amplification and sequence identification of viral DNA or RNA genomes using Polymerase Chain Reaction (PCR).

Viral Kinetics and Multiplication Pathways

Viral One-Step Growth Curve:
- Eclipse Period: The phase immediately following infection during which viral genomes uncoat and direct intracellular biosynthesis. Free, fully assembled, infectious virions cannot be detected outside the host cells.
- Acute Infection Phase / Release: Following viral maturation, intracellular virions are released in massive quantities from host cells, resulting in a steep surge in detectable extracellular virion counts.
Bacteriophage Replication Cycles:
- Lytic Cycle (Virulent Phages): Leads directly to host cell lysis and death.
  1. Attachment: Phage tail fibers bind to specific bacterial cell surface receptors.
  2. Entry: Tail sheath contracts, forcing viral DNA through the cell wall/membrane into host cytoplasm.
  3. Biosynthesis: Phage DNA directs synthesis of viral proteins and replication of viral nucleic acid while degrading host genomic DNA.
  4. Maturation: Viral components self-assemble into complete progeny virions.
  5. Release: Phage lysozyme breaks down the bacterial cell wall, lysing host cells and releasing virions.

*   **Lysogenic Cycle (Temperate Phages, e.g., Phage  $\lambda$ ):**
    1.  *Integration:* Following entry, viral DNA integrates into the host bacterial chromosome, becoming a latent prophage.
    2.  *Lysogenic Propagation:* As the host bacterium replicates its genome during standard cell division, the prophage genome is replicated along with host chromosomes through many cellular generations.
    3.  *Induction:* Environmental stress triggers excision of prophage DNA from the host chromosome, initiating the lytic cycle.

Comparative stages between Bacteriophages and Animal Viruses

Comparison of Bacteriophage and Animal Virus Multiplication:
- Attachment Stage:
  - Bacteriophages: Tail fibers attach to bacterial cell wall proteins or appendages.
  - Animal Viruses: Attachment sites bind plasma membrane surface proteins and glycoproteins.
- Entry Stage:
  - Bacteriophages: Viral DNA is injected across cell envelopes; capsid remains outside.
  - Animal Viruses: Entire naked or enveloped viral capsid enters cell via receptor-mediated endocytosis or membrane fusion.
- Uncoating Stage:
  - Bacteriophages: Not required (capsid remains extracellular).
  - Animal Viruses: Enzymatic removal of outer protein capsid inside host endosomes or cytoplasm.
- Biosynthesis Location:
  - Bacteriophages: Occurs in host cytoplasm.
  - Animal Viruses: DNA viruses replicate in host cell nucleus; RNA viruses replicate in host cell cytoplasm.
- Chronic / Latent Infection Potential:
  - Bacteriophages: Manifests as lysogeny.
  - Animal Viruses: Manifests as cellular latency, slow persistent viral infections, or oncogenic host cell transformation (cancer).
- Release Mechanism:
  - Bacteriophages: Host cell envelope is lysed by viral enzymes.
  - Animal Viruses: Enveloped viruses bud out gradually (acquiring membrane host envelope); non-enveloped viruses rupture/lyse host plasma membranes.