5.3 BACTERIOPHAGE | Acellular Life | Biology 11

A bacteriophage, or simply phage, is a virus that specifically infects and replicates within bacteria.

Structure of Bacteriophage

A bacteriophage typically has a tadpole-like shape, consisting of three main parts: a head, a neck or collar, and a tail.

Figure 5.1: Structure of a bacteriophage

Head: An icosahedral (20-sided) protein shell that contains the viral genetic material, which is typically single-stranded DNA.
Neck or Collar: A narrow region that connects the head to the tail.
Tail: A complex structure composed of:
- Hollow Core Tube: A channel through which viral nucleic acid is injected into the host bacterium.
- Contractile Sheath: A protein sheath surrounding the core tube that contracts during infection to drive the core tube through the bacterial cell wall.
- Base Plate: A plate at the end of the tail with tail pins or spikes on its lower surface.
- Tail Fibers: Six fibers attached to the base plate that recognize and bind to specific receptors on the bacterial cell surface.
Lysozyme: An enzyme located at the base of the tail core. It is released upon contraction of the sheath and digests a portion of the bacterial cell wall, allowing for penetration.

Life Cycle of Bacteriophage

Bacteriophages exhibit two primary types of life cycles: the Lytic Cycle and the Lysogenic Cycle. Both cycles begin with the same initial infection process.

Infection Process

Adsorption: The phage attaches to the surface of a specific host bacterium. This binding is mediated by the tail fibers, which recognize specific receptor sites on the bacterial cell wall.
Penetration: The phage's tail sheath contracts, driving the hollow core tube through the bacterial envelope. The enzyme lysozyme digests a small hole in the cell wall to facilitate this process.
Genome Injection: The phage injects its DNA into the cytoplasm of the host bacterium. The rest of the phage structure remains outside the cell.

Following genome injection, the phage can enter either the lytic or lysogenic cycle.

Figure 5.3: Lytic and lysogenic life cycle of bacteriophage

Lytic Cycle

This cycle results in the rapid destruction (lysis) of the host cell. Phages that only undergo this cycle are called virulent phages.

Master-Slave Relationship: The phage DNA takes immediate control of the host cell's metabolic machinery.
Host DNA Disintegration: The phage produces a DNAase enzyme that breaks down the host's genomic DNA. The phage's own DNA is chemically modified to be protected from this enzyme.
Synthesis of Phage Components: The host cell's machinery is used to synthesize viral mRNA, proteins (for head, tail, and other components), and enzymes.
Assembly and Maturation: The newly synthesized components are assembled into mature phage particles. DNA is packaged into the heads, and tails are attached.
Lysis and Release: The cell produces phage lysis proteins (like lysozyme), which break down the bacterial cell wall. The cell bursts, releasing approximately 200 new phage particles in 20–25 minutes to infect other bacteria.

Lysogenic Cycle

This cycle allows the phage genome to coexist with the host genome without immediately killing the cell. Phages capable of this cycle are called temperate phages.

Host-Guest Relationship: The phage DNA does not take over the cell immediately. Instead, it integrates into the host bacterium's chromosome.
Prophage: The integrated phage DNA within the host chromosome is called a prophage.
Lysogeny: The state where a bacterium harbors a prophage is called lysogeny, and the bacterium is a lysogenic bacterium. The prophage is replicated along with the host DNA every time the cell divides, passing the viral DNA to all daughter cells.
Induction: The lysogenic state can be terminated, causing the prophage to exit the host chromosome and enter the lytic cycle. This is triggered by adverse environmental conditions such as UV radiation, desiccation, or exposure to certain chemicals.
Lysogenic Conversion: The prophage contains genes that can be expressed by the host bacterium, giving it new properties. For example, the toxins produced by Clostridium botulinum are encoded by prophage genes.

Feature	Lytic Cycle	Lysogenic Cycle
Phage Type	Virulent Phage	Temperate Phage
Host Cell Fate	Destroyed (lysed)	Survives and reproduces
Phage DNA	Remains separate, directs synthesis of new phages	Integrates into host chromosome (Prophage)
Relationship	Master-Slave	Host-Guest
Replication	Immediate production of new phage particles	Phage DNA is replicated with host DNA
Outcome	Release of new phages	Phage DNA passed to daughter cells
Activation	Occurs immediately after infection	Can be induced to enter the lytic cycle

Usage of Bacteriophages in Genetic Engineering

Bacteriophages are valuable tools in biotechnology and genetic engineering.

Vectors: Phage DNA is used as a vector to carry and transfer genes from one organism to another, particularly in creating genomic libraries (collections of all the genes of an organism).
Phage Therapy: Genetically engineered phages can be used to specifically target and kill pathogenic bacteria. This is a potential alternative to antibiotics, with the advantage of a narrow spectrum (killing only target bacteria while leaving beneficial bacteria unharmed).
Drug-Delivery Vehicles: Phages are being explored as targeted vehicles to deliver drugs to specific cells or tissues in the body, a field known as antibacterial nanomedicine.

References

Derived from FBISE textbook

A bacteriophage, or simply phage, is a virus that specifically infects and replicates within bacteria.

Structure of Bacteriophage

A bacteriophage typically has a tadpole-like shape, consisting of three main parts: a head, a neck or collar, and a tail.

Figure 5.1: Structure of a bacteriophage

Head: An icosahedral (20-sided) protein shell that contains the viral genetic material, which is typically single-stranded DNA.
Neck or Collar: A narrow region that connects the head to the tail.
Tail: A complex structure composed of:
- Hollow Core Tube: A channel through which viral nucleic acid is injected into the host bacterium.
- Contractile Sheath: A protein sheath surrounding the core tube that contracts during infection to drive the core tube through the bacterial cell wall.
- Base Plate: A plate at the end of the tail with tail pins or spikes on its lower surface.
- Tail Fibers: Six fibers attached to the base plate that recognize and bind to specific receptors on the bacterial cell surface.
Lysozyme: An enzyme located at the base of the tail core. It is released upon contraction of the sheath and digests a portion of the bacterial cell wall, allowing for penetration.

Life Cycle of Bacteriophage

Bacteriophages exhibit two primary types of life cycles: the Lytic Cycle and the Lysogenic Cycle. Both cycles begin with the same initial infection process.

Infection Process

Adsorption: The phage attaches to the surface of a specific host bacterium. This binding is mediated by the tail fibers, which recognize specific receptor sites on the bacterial cell wall.
Penetration: The phage's tail sheath contracts, driving the hollow core tube through the bacterial envelope. The enzyme lysozyme digests a small hole in the cell wall to facilitate this process.
Genome Injection: The phage injects its DNA into the cytoplasm of the host bacterium. The rest of the phage structure remains outside the cell.

Following genome injection, the phage can enter either the lytic or lysogenic cycle.

Figure 5.3: Lytic and lysogenic life cycle of bacteriophage

Lytic Cycle

This cycle results in the rapid destruction (lysis) of the host cell. Phages that only undergo this cycle are called virulent phages.

Master-Slave Relationship: The phage DNA takes immediate control of the host cell's metabolic machinery.
Host DNA Disintegration: The phage produces a DNAase enzyme that breaks down the host's genomic DNA. The phage's own DNA is chemically modified to be protected from this enzyme.
Synthesis of Phage Components: The host cell's machinery is used to synthesize viral mRNA, proteins (for head, tail, and other components), and enzymes.
Assembly and Maturation: The newly synthesized components are assembled into mature phage particles. DNA is packaged into the heads, and tails are attached.
Lysis and Release: The cell produces phage lysis proteins (like lysozyme), which break down the bacterial cell wall. The cell bursts, releasing approximately 200 new phage particles in 20–25 minutes to infect other bacteria.

Lysogenic Cycle

This cycle allows the phage genome to coexist with the host genome without immediately killing the cell. Phages capable of this cycle are called temperate phages.

Host-Guest Relationship: The phage DNA does not take over the cell immediately. Instead, it integrates into the host bacterium's chromosome.
Prophage: The integrated phage DNA within the host chromosome is called a prophage.
Lysogeny: The state where a bacterium harbors a prophage is called lysogeny, and the bacterium is a lysogenic bacterium. The prophage is replicated along with the host DNA every time the cell divides, passing the viral DNA to all daughter cells.
Induction: The lysogenic state can be terminated, causing the prophage to exit the host chromosome and enter the lytic cycle. This is triggered by adverse environmental conditions such as UV radiation, desiccation, or exposure to certain chemicals.
Lysogenic Conversion: The prophage contains genes that can be expressed by the host bacterium, giving it new properties. For example, the toxins produced by Clostridium botulinum are encoded by prophage genes.

Feature	Lytic Cycle	Lysogenic Cycle
Phage Type	Virulent Phage	Temperate Phage
Host Cell Fate	Destroyed (lysed)	Survives and reproduces
Phage DNA	Remains separate, directs synthesis of new phages	Integrates into host chromosome (Prophage)
Relationship	Master-Slave	Host-Guest
Replication	Immediate production of new phage particles	Phage DNA is replicated with host DNA
Outcome	Release of new phages	Phage DNA passed to daughter cells
Activation	Occurs immediately after infection	Can be induced to enter the lytic cycle

Usage of Bacteriophages in Genetic Engineering

Bacteriophages are valuable tools in biotechnology and genetic engineering.

Vectors: Phage DNA is used as a vector to carry and transfer genes from one organism to another, particularly in creating genomic libraries (collections of all the genes of an organism).
Phage Therapy: Genetically engineered phages can be used to specifically target and kill pathogenic bacteria. This is a potential alternative to antibiotics, with the advantage of a narrow spectrum (killing only target bacteria while leaving beneficial bacteria unharmed).
Drug-Delivery Vehicles: Phages are being explored as targeted vehicles to deliver drugs to specific cells or tissues in the body, a field known as antibacterial nanomedicine.

References

Derived from FBISE textbook