IV. VIRUSES

G. BACTERIOPHAGE LIFE CYCLES

2. The Lysogenic Life Cycle

Fundamental Statements for this Learning Object:

1. Bacteriophages capable of a lysogenic life cycle are termed temperate phages.
2. When a temperate bacteriophage infects a bacterium, it either replicates by means of the lytic life cycle and cause lysis of the host bacterium, or, incorporates its DNA into the bacterium's DNA and become a non-infectious prophage whereby the bacteriophage DNA replicates as a part of the bacterium's DNA so that every daughter bacterium now contains the prophage.
3. In rare cases spontaneous induction occurs. The bacteriophage genes become activated and new bacteriophages are produced by the lytic life cycle.

 

LEARNING OBJECTIVES FOR THIS SECTION

Bacteriophages are viruses that only infect bacteria. There are two primary types of bacteriophages: lytic bacteriophages and temperate bacteriophages.

1. Bacteriophages that replicate through the lytic life cycle (def) are called lytic bacteriophages (def), and are so named because they lyse the host bacterium as a normal part of their life cycle.

2. Bacteriophages capable of a lysogenic life cycle (def) are termed temperate phages (def). When a temperate phage infects a bacterium, it can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or, it can incorporate its DNA into the bacterium's DNA and become a noninfectious prophage.

We will now look at the lysogenic life cycle of bacteriophages.

The Lysogenic Life Cycle of Temperate Bacteriophages

Bacteriophages capable of a lysogenic life cycle are termed temperate bacteriophages. When a temperate bacteriophage infects a bacterium, it can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or, it can incorporate its DNA into the bacterium's DNA and become a noninfectious prophage (def) (see Fig. 1).

In the latter case, the cycle begins by the bacteriophage adsorbing to the host bacterium or lysogen (def) and injecting its genome as in the lytic life cycle (see Fig. 2 and Fig. 3). However, the bacteriophage does not shut down the host cell. Instead, the bacteriophage DNA inserts or integrates into the host bacterium's DNA (see Fig. 4). At this stage the virus is called a prophage. Expression of the bacteriophage genes controlling bacteriophage replication is blocked by a repressor protein, and the phage DNA replicates as a part of the bacterium's DNA so that every daughter bacterium now contains the prophage (see Fig. 5).

html5 animation showing adsorption of a temperate bacteriophage. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

html5 animation showing penetration of a temperate bacteriophage. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

html5 animation showing prophage formation. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

The number of viruses infecting the bacterium as well as the physiological state of the bacterium appear to determine whether the temperate bacteriophage enters the lytic cycle or becomes a prophage.

In about one out of every million to one out of every billion bacteria containing a prophage, spontaneous induction occurs. The bacteriophage genes are activated and new bacteriophages are produced by the lytic life cycle (see Fig. 5A, Fig. 6, Fig. 7, Fig. 8, and Fig. 9).

html5 animation showing spontaneous induction. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

 

html5 animation showing replication of a temperate bacteriophage. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

 

html5 animation showing maturation of a temperate bacteriophage. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

 

html5 animation showing release of a temperate bacteriophage. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

 

TPS Question

GIF Animation summarizing the lysogenic life cycle of a temperate bacteriophage. by Gary E. Kaiser, Ph.D. Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus This work is licensed under a Creative Commons Attribution 4.0 International License. Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html. Last updated: August, 2019 Please send comments and inquiries to Dr. Gary Kaiser

 

Concept Map for the Lysogenic Life Cycle of Bacteriophages

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