IV. VIRUSES

G. BACTERIOPHAGE LIFE CYCLES

1. The Lytic Life Cycle

Fundamental Statements for this Learning Object:

1. Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages,
2. Adsorption is the attachment sites on the phage adsorb to receptor sites on the host bacterium.
3. Specific strains of bacteriophages can only adsorb to specific strain of host bacteria (viral specificity).
4. In the case of bacteriophages that adsorb to the bacterial cell wall, a bacteriophage enzyme "drills" a hole in the bacterial wall and the bacteriophage injects its genome into the bacterial cytoplasm.
5. The bacteriophage replicates its genome and uses the bacterium's metabolic machinery to synthesize bacteriophage enzymes and bacteriophage structural components.
6. During maturation, the bacteriophage parts assemble around the phage genomes.
7. A phage-coded lysozyme breaks down the bacterial peptidoglycan causing osmotic lysis and release of the intact bacteriophages.

 

LEARNING OBJECTIVES FOR THIS SECTION

As mentioned in an earlier 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 lytic life cycle of bacteriophages.

The Lytic Life Cycle of Lytic Bacteriophages

As mentioned in an earlier section, bacteriophages are viruses that only infect bacteria ( see Fig. 1C and Fig. 2E). Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages.

- Scanning electron micrograph of the lytic bacteriophage coliphage T4. 

After infecting bacteria with lytic bacteriophages in the lab, plaques can be seen on the petri plates. Plaques are small clear areas on the agar surface where the host bacteria have been lysed by lytic bacteriophages. The lytic life cycle is somewhat similar to the productive life cycle of animal viruses (def) and consists of the following steps:

 

1. Adsorption

Attachment sites on the bacteriophage adsorb to receptor sites on the host bacterium (see Fig. 1). Most bacteriophages adsorb to the bacterial cell wall, although some are able to adsorb to flagella or pili. Specific strains of bacteriophages can only adsorb to specific strain of host bacteria. This is known as viral specificity (def).

html5 animation showing adsorption of a lytic 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

2. Penetration

In the case of bacteriophages that adsorb to the bacterial cell wall, a bacteriophage enzyme "drills" a hole in the bacterial wall and the bacteriophage injects its genome into the bacterial cytoplasm (see Fig. 2). Some bacteriophages accomplish this by contracting a sheath which drives a hollow tube into the bacterium. This begins the eclipse period. The genomes of bacteriophages which adsorb to flagella or pili enter through these hollow organelles. In either case, only the phage genome enters the bacterium so there is no uncoating stage.

html5 animation showing penetration of a lytic 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

 

YouTube Animation showing adsorption and penetration of coliphage T4.

 

3. Replication

Enzymes coded by the bacteriophage genome shut down the bacterium's macromolecular (protein, RNA, DNA) synthesis. The bacteriophage replicates its genome and uses the bacterium's metabolic machinery to synthesize bacteriophage enzymes and bacteriophage structural components (see Fig. 3 and Fig. 4).

html5 animation showing replication of a lytic 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

 

4. Maturation

The phage parts assemble around the genomes (see Fig. 5).

html5 animation showing maturation of a lytic 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

5. Release

Usually, a bacteriophage-coded lysozyme breaks down the bacterial peptidoglycan causing osmotic lysis and release of the intact bacteriophages (see Fig. 6).

html5 animation showing release of a lytic 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

 

YouTube movie showing the lysis of Escherichia coli by coliphage T4. Courtesy of Jim Sullivan, Cells Alive

 

6. Reinfection

From 50 to 200 bacteriophages may be produced per infected bacterium.

GIF Animation summarizing the lytic life cycle of a lytic 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

Concept Map for the Lytic Life Cycle of Bacteriophages

 

 

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