G. BACTERIOPHAGE LIFE CYCLES
1. The Lytic Life Cycle
The overall purpose of this Learning Object is to learn the lytic live cycle of lytic bacteriophages.
LEARNING OBJECTIVES FOR THIS SECTION
Viruses are infectious agents with both living and nonliving characteristics.
1. Living characteristics of viruses
a. They reproduce at a fantastic rate, but only in living host cells.
b. They can mutate.
2. Nonliving characteristics of viruses
a. They are acellular, that is, they contain no cytoplasm or cellular organelles.
b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.
c. The vast majority of viruses possess either DNA or RNA but not both.
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 are termed temperate phages. 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 courtesy of Dennis Kunkel's Microscopy.
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:
Attachment sites on the phage 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).
In the case of phages that adsorb to the bacterial cell wall, a phage enzyme "drills" a hole in the bacterial wall and the phage injects its genome into the bacterial cytoplasm (see Fig. 2). Some phages accomplish this by contracting a sheath which drives a hollow tube into the bacterium. This begins the eclipse period. The genome of phages 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.
Animation showing adsorption and penetration of coliphage T7
Movie S5: Originally published in Science Express on January 10 2013 by Hu, Margolin, Molineux, and Liu
From Seyet, LLC. This animation takes some time to load.
Enzymes coded by the phage genome shut down the bacterium's macromolecular (protein, RNA, DNA) synthesis. The phage replicates its genome and uses the bacterium's metabolic machinery to synthesize phage enzymes and phage structural components (see Fig. 3 and Fig. 4).
The phage parts assemble around the genomes (see Fig. 5).
Usually, a phage-coded lysozyme breaks down the bacterial peptidoglycan causing osmotic lysis and release of the intact bacteriophages (see Fig. 6).
From 50 to 200 phages may be produced per infected bacterium.
To view illustrations of the lytic bacteriophage coliphage T4 and a QuickTime movie of Escherichia coli being lysed by coliphage T4, see the Cells Alive web site.
To review the lytic life cycle of a lytic bacteriophage such as coliphage T4, (see Fig. 1 through 6).
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