5.3.2 Life Cycle of Bacteriophage

Introduction

Bacteriophages show two types of life cycles: the lytic cycle and the lysogenic cycle. The life cycle of a bacteriophage comprises two main steps: the infection process and replication within the host cell. The initial steps in the infection process such as adsorption, penetration, and genome injection are quite similar in both cycles, but the mode of replication is much different in the lytic cycle and the lysogenic cycle.

Definition

Life Cycle of Bacteriophage: The life cycle of a bacteriophage is the sequence of events from infection of a bacterial cell to the release of new phage particles.

In simple words: The life cycle of a bacteriophage is how it infects a bacterium and makes more copies of itself.

Core Concept

• Bacteriophages show two types of life cycles:
• The life cycle of a bacteriophage comprises two main steps:
• The initial steps in the infection process are quite similar in both cycles:
• Adsorption
• Penetration
• Genome injection
• The mode of replication is much different in the lytic cycle and the lysogenic cycle.

Detailed Explanation

The Infection Process (Common to Both Cycles)

The common steps of the infection process of bacteriophages to their host are as follows:

1. Adsorption

• The first step in the infection process is the adsorption of the phage to the bacterial cell.
• This step is mediated by the tail fibres and tail pins/spikes.
• Bacteriophages attach to specific receptors on the bacterial cell.

Key Point: Adsorption is specific; the phage can only attach to bacteria with the correct receptors.

2. Penetration

• The binding of the phage to the bacterium results in the contraction of the sheath.
• This releases lysozyme that digests the portion of the bacterial envelope.
• As a result, the hollow core tube is pushed through the bacterial envelope.
• This insertion of the core tube is called penetration.

Key Point: Penetration allows the phage to inject its genome into the bacterium.

3. Genome Injection

• The penetration of the core results in the injection of viral DNA into the bacterial cytoplasm.
• The remainder of the phage remains on the outside of the bacterium.

Key Point: Only the nucleic acid enters the bacterium; the rest of the phage remains outside.

Lytic Cycle

Definition: The bacteriophage that performs the lytic cycle is called a lytic or virulent phage because it immediately causes lysis (breakdown) of its host cell after its own multiplication.

Key Points:

• It develops a Master-Slave relationship with the host cell.
• The host genomic DNA is immediately disintegrated by the virally encoded DNA digesting enzyme (DNAase).
• Since viral DNA is already undergone certain chemical modification, therefore, such enzymes do not affect it.
• The disintegration of host DNA enables the viral DNA to take over the control of the whole metabolic machinery of its host.
• In the lytic cycle, the subsequent steps are:

Steps of the Lytic Cycle:

Key Points:

• Master-Slave relationship: The phage controls the host cell's machinery.
• Host DNA degradation: Host DNA is destroyed by viral DNAase.
• Viral DNA protection: Viral DNA is chemically modified and is not affected by DNAase.
• Phage components: Structural proteins (head, tail) and lysis proteins are synthesized separately.
• Maturation: Assembly of phage components into mature infective phage particles.
• Release: Within 20 to 25 minutes, approximately 200 phage particles are produced.
• Lysis: Bacteria begin to lyse due to accumulation of phage lysis protein (lysozyme).

Lysogenic Cycle

Definition: The bacteriophages that perform the lysogenic cycle are called lysogenic or temperate phages. These phages can either multiply via the lytic cycle or enter a dormant state in the cell.

Key Points:

• Such phages develop a Host-Guest relationship.
• In this case, the phage DNA actually integrates into the host chromosome.
• It is replicated along with the host chromosome and passed on to the daughter cells.
• This integrated state of phage DNA is termed a prophage.
• This process is known as lysogeny.
• The bacteria harbouring prophage are called lysogenic bacteria.

Steps of the Lysogenic Cycle:

Key Points:

• Host-Guest relationship: The phage DNA integrates into the host chromosome.
• Prophage: The integrated state of phage DNA.
• Lysogeny: The process of integration.
• Lysogenic bacteria: Bacteria harbouring prophage.
• Induction: Termination of the lysogenic state.

Induction

Definition: The lysogenic state of a bacterium can get terminated anytime when it is exposed to adverse conditions. This process is called induction.

Conditions that favour the termination of the lysogenic state:

• Desiccation
• Exposure to UV or ionizing radiation
• Exposure to mutagenic chemicals

Key Points:

• The separated phage DNA then initiates the lytic cycle.
• This results in cell lysis and release of phages.
• Such phages are then capable of infecting new susceptible cells and render them lysogenic.

Comparison of Lytic and Lysogenic Cycles

Summary of the Life Cycle

Intermediate Level

Lytic Cycle in Detail

Lysogenic Cycle in Detail

Comparison of Lytic and Lysogenic Cycles

Important Points

• 1. Bacteriophages show two types of life cycles i.e., Lytic cycle and Lysogenic cycle.
• 2. The life cycle of bacteriophage comprises two main steps i.e., infection process and replication within the host cells.
• 3. The initial steps in the infection process such as adsorption, penetration and genome injection are quite similar in both cycles but mode of replication is much different in lytic cycle or lysogenic cycle.
• 4. Infection Process:
• - Adsorption: The first step in the infection process is the adsorption of the phage to the bacterial cell. This step is mediated by the tail fibres and tail pins/spikes. Bacteriophages attach to specific receptors on the bacterial cell.
• - Penetration: The binding of the phage to the bacterium results in the contraction of the sheath and release of lysozyme that digest the portion of bacterial envelope; as a result, the hollow core tube is pushed through the bacterial envelope. This insertion of core tube is called penetration.
• - Genome injection: The penetration of core results into the injection of viral DNA in the bacterial cytoplasm whereas, the remainder of the phage remains on the outside of the bacterium.
• 5. Replication of Bacteriophage in Lytic Cycle:
• - The bacteriophage that performs lytic cycle is called lytic or virulent phage because it immediately causes lysis (breakdown) of its host cell after its own multiplication.
• - It develops Master-Slave relationship with the host cell because host genomic DNA is immediately disintegrated by the virally encoded DNA digesting enzyme (DNAase).
• - Since viral DNA is already undergone certain chemical modification therefore, such enzymes do not affect it.
• - The disintegration of host DNA enables the viral DNA to take over the control of the whole metabolic machinery of its host.
• - In lytic cycle the subsequent steps are synthesis of phage components, assembly, maturation, lysis and release.
• - Soon after the disintegration of host DNA phage specified mRNAs and proteins are beginning to produce.
• - Structural proteins (head, tail) that comprise the phage as well as the proteins needed for lysis of the bacterial cell are separately synthesized.
• - Nucleic acid is then packaged inside the head and then tail is added to the head.
• - The assembly of phage components into mature infective phage particle is known as maturation.
• - Within 20 to 25 minutes, approximately 200 phage particles are produced.
• - In lysis and release phase the bacteria begin to lyse due to the accumulation of the phage lysis protein i.e., lysozyme and intracellular phage particles are released into the medium.
• 6. Replication of Bacteriophage in Lysogenic Cycle:
• - The bacteriophages that perform lysogenic cycle are called lysogenic or temperate phages.
• - These phages can either multiply via the lytic cycle or enter a dormant state in the cell.
• - Such phages develop Host-Guest relationship because in this case the phage DNA actually integrates into the host chromosome and is replicated along with the host chromosome and passed on to the daughter cells.
• - This integrated state of phage DNA is termed prophage.
• - This process is known as lysogeny and the bacteria harbouring prophage are called lysogenic bacteria.
• - The lysogenic state of a bacterium can get terminated anytime when it is exposed to adverse conditions. This process is called induction.
• - Conditions that favour the termination of the lysogenic state include: desiccation, exposure to UV or ionizing radiation, exposure to mutagenic chemicals, etc.
• - The separated phage DNA then initiates lytic cycle resulting in cell lysis and releases of phages.
• - Such phages are then capable of infecting new susceptible cells and render them lysogenic.
• 7. Usage of Bacteriophages in Genetic Engineering:
• - Beside bacterial plasmids the phage DNA has also been used as vector in genetic engineering techniques such as development of genomic library.
• - Phage therapy is the application of genetically engineered phages that can kill pathogenic bacteria.
• - Phage therapy has advantages over conventional antibiotic therapy.
• - As phages are fairly narrow in their spectrum of activity, meaning that with phage treatment it is possible to kill bacterial pathogens while avoiding harming of normal bacterial flora, i.e., our good bacteria.
• - Bacteriophages have been used for many years as tools for the treatment of bacterial infections but recently a new application in the area of antibacterial Nano medicines has been discovered in which bacteriophages can be formulated as targeted drug-delivery vehicles.

Remember

• - Bacteriophage = virus that attacks bacteria
• - Two life cycles: Lytic cycle and Lysogenic cycle
• - Infection process: Adsorption, Penetration, Genome injection
• - Lytic cycle: Virulent phage; Master-Slave relationship; host DNA degraded; host cell lysed; immediate release
• - Lysogenic cycle: Temperate phage; Host-Guest relationship; viral DNA integrates; prophage formed; host cell survives; delayed release
• - Prophage = integrated phage DNA
• - Induction = switching from lysogenic to lytic cycle
• - Examples: T4 phage (lytic), Lambda phage (lysogenic)

Common Mistakes

• 1. Confusing lytic and lysogenic cycles – Lytic = host lysed; Lysogenic = host survives.
• 2. Thinking the phage enters the bacterium – Only the nucleic acid enters; the rest of the phage remains outside.
• 3. Thinking the lysogenic cycle always leads to lysis – The lysogenic cycle can continue indefinitely; lysis occurs only after induction.
• 4. Forgetting the role of prophage – Prophage is the integrated phage DNA.
• 5. Confusing virulent and temperate phages – Virulent = lytic; Temperate = lysogenic.
• 6. Thinking the host DNA is always degraded – Host DNA is degraded only in the lytic cycle.
• 7. Forgetting the role of induction – Induction is the switch from lysogenic to lytic cycle.
• 8. Thinking the lytic cycle takes a long time – The lytic cycle takes only 20–25 minutes.
• 9. Confusing the terms "lysogeny" and "lysis" – Lysogeny = integration; Lysis = breakdown.
• 10. Forgetting the examples – T4 phage (lytic), Lambda phage (lysogenic).
• 11. Thinking bacteriophages are always harmful – Bacteriophages are used in phage therapy.
• 12. Confusing the infection process and replication – Infection process is common to both cycles; replication differs.

Quick Revision

• Bacteriophages show two types of life cycles i.e., Lytic cycle and Lysogenic cycle.
• The life cycle of bacteriophage comprises two main steps i.e., infection process and replication within the host cells.
• The initial steps in the infection process such as adsorption, penetration and genome injection are quite similar in both cycles but mode of replication is much different in lytic cycle or lysogenic cycle.
• Infection Process:
• Adsorption: The first step in the infection process is the adsorption of the phage to the bacterial cell. This step is mediated by the tail fibres and tail pins/spikes. Bacteriophages attach to specific receptors on the bacterial cell.
• Penetration: The binding of the phage to the bacterium results in the contraction of the sheath and release of lysozyme that digest the portion of bacterial envelope; as a result, the hollow core tube is pushed through the bacterial envelope. This insertion of core tube is called penetration.
• Genome injection: The penetration of core results into the injection of viral DNA in the bacterial cytoplasm whereas, the remainder of the phage remains on the outside of the bacterium.
• Replication of Bacteriophage in Lytic Cycle:
• The bacteriophage that performs lytic cycle is called lytic or virulent phage because it immediately causes lysis (breakdown) of its host cell after its own multiplication.
• It develops Master-Slave relationship with the host cell because host genomic DNA is immediately disintegrated by the virally encoded DNA digesting enzyme (DNAase).
• Since viral DNA is already undergone certain chemical modification therefore, such enzymes do not affect it.
• The disintegration of host DNA enables the viral DNA to take over the control of the whole metabolic machinery of its host.
• In lytic cycle the subsequent steps are synthesis of phage components, assembly, maturation, lysis and release.
• Soon after the disintegration of host DNA phage specified mRNAs and proteins are beginning to produce.
• Structural proteins (head, tail) that comprise the phage as well as the proteins needed for lysis of the bacterial cell are separately synthesized.
• Nucleic acid is then packaged inside the head and then tail is added to the head.
• The assembly of phage components into mature infective phage particle is known as maturation.
• Within 20 to 25 minutes, approximately 200 phage particles are produced.
• In lysis and release phase the bacteria begin to lyse due to the accumulation of the phage lysis protein i.e., lysozyme and intracellular phage particles are released into the medium.
• Replication of Bacteriophage in Lysogenic Cycle:
• The bacteriophages that perform lysogenic cycle are called lysogenic or temperate phages.
• These phages can either multiply via the lytic cycle or enter a dormant state in the cell.
• Such phages develop Host-Guest relationship because in this case the phage DNA actually integrates into the host chromosome and is replicated along with the host chromosome and passed on to the daughter cells.
• This integrated state of phage DNA is termed prophage.
• This process is known as lysogeny and the bacteria harbouring prophage are called lysogenic bacteria.
• The lysogenic state of a bacterium can get terminated anytime when it is exposed to adverse conditions. This process is called induction.
• Conditions that favour the termination of the lysogenic state include: desiccation, exposure to UV or ionizing radiation, exposure to mutagenic chemicals, etc.
• The separated phage DNA then initiates lytic cycle resulting in cell lysis and releases of phages.
• Such phages are then capable of infecting new susceptible cells and render them lysogenic.
• Usage of Bacteriophages in Genetic Engineering:
• Beside bacterial plasmids the phage DNA has also been used as vector in genetic engineering techniques such as development of genomic library.
• Phage therapy is the application of genetically engineered phages that can kill pathogenic bacteria.
• Phage therapy has advantages over conventional antibiotic therapy.
• As phages are fairly narrow in their spectrum of activity, meaning that with phage treatment it is possible to kill bacterial pathogens while avoiding harming of normal bacterial flora, i.e., our good bacteria.
• Bacteriophages have been used for many years as tools for the treatment of bacterial infections but recently a new application in the area of antibacterial Nano medicines has been discovered in which bacteriophages can be formulated as targeted drug-delivery vehicles.

60-Second Revision

• Bacteriophage = virus that attacks bacteria
• Two life cycles: Lytic cycle and Lysogenic cycle
• Infection process: Adsorption, Penetration, Genome injection
• Lytic cycle: Virulent phage; Master-Slave relationship; host DNA degraded; host cell lysed; immediate release
• Lysogenic cycle: Temperate phage; Host-Guest relationship; viral DNA integrates; prophage formed; host cell survives; delayed release
• Prophage = integrated phage DNA
• Induction = switching from lysogenic to lytic cycle
• Examples: T4 phage (lytic), Lambda phage (lysogenic)
• Warning: Lytic cycle lyses the host; lysogenic cycle does not. Only the nucleic acid enters the bacterium. Prophage is the integrated phage DNA. Induction switches from lysogenic to lytic cycle. The lytic cycle takes 20–25 minutes. T4 phage is lytic; Lambda phage is lysogenic. Bacteriophages are used in phage therapy and genetic engineering.