A bacteriophage is a kind of virus that can infect and replicate itself inside bacterial cells. The virus has a protein-encapsulated DNA or RNA genome and can have simple or complex anatomies. There are many types of bacteriophages including M13, T phage, lambda phage, MS2, G4, and Phix174.
One of the characteristics of bacteriophages is their temperateness. Temperateness refers to the ability of some bacteriophages, particularly lambda phage, to choose between two cycles: lysogenic or lytic. “Temperance” generally refers to the moderation of actions, and in the case of phages, moderation is seen through the ability to not express anti-bacterial virulence.
Viruses cannot multiply through the division of cells because they are acellular (they do not have cells). Instead, they seek a host cell in which they replicate and assemble themselves using the metabolism and machinery of the host cell. Different species of viral populations undergo different viral life cycles, but for temperate phages, as previously mentioned, they must pick between two.
The lytic-lysogeny decision
Decision making isn’t just done by people; it is also done by temperate phages as they need to choose between two different life cycles, productive (lytic) or reductive (lysogenic). There is a predominance of lytic among temperate phages, as induction can cause lysogenic to convert to lytic.
However, in most cases, temperate phages reel toward the lysogenic cycle especially when phage absorption in the infected bacteria is apparent. It is inferred that other local bacteria are undergoing the same phage infection, making the bacteria decrease in density. Because of this “crisis,” the go-to cycle is lysogenic.
On the other hand, when there is an abundance of uninfected bacteria, undergoing the lytic cycle is preferable because to increase the number of healthy bacteria, phages that have productive infections are needed.
In the lysogenic cycle, the genomes of temperate phages are not expressed. However, they are integrated into the genome of the bacteria and produce prophages, which are created without disrupting the bacterial cell. Moreover, because of this integration, passive replication of the bacteriophage occurs when daughter bacterial cells are produced. These prophage-containing bacteria cells are called lysogens – phages that can exist as dormant DNA within its host cell. These lysogens have the ability to stay in the lysogenic cycle for a very long time, but through induction, they can be directed to the lytic cycle at any point in time. When induction occurs, prophage DNA is cut off from the bacterial genome and coat proteins are produced via transcription and translation of the prophage DNA for the regulation of lytic growth.
The lytic cycle is similar to the lysogenic cycle in that the host DNA machinery is used to replicate the phage, but the phage is considered a separate molecule from the host DNA. When a temperate phage undergoes the lytic cycle, it becomes a virulent phage. The infected cell and its membrane disintegrate as the viral DNA, which is considered a separate molecule from the bacterial cell, replicates separately from the DNA of the host bacteria, eventually overwhelming it.
The lytic cycle is divided into different stages. The first stage is the penetration in which the virus enters the host cell and injects its nucleic acids into it, releasing genetic material (either DNA or RNA) and infecting the cell. Viral components are then produced using the machinery of the host cell, culminating in the biosynthesis of mRNA and protein production. The host cell begins to copy the viral nucleic acids, which combine with viral proteins to produce phage particles within the cell. When there are already too many viral particles within the host, its membrane splits and the released viruses begin infecting other cells.
Temperate phages have various biological and molecular applications. They can be used to genetically manipulate eukaryotic cells, especially species that have large genomes like plants and mammals. Gene therapy, manipulation of cell lines, and construction of transgenic organisms can also employ phage enzymes. The temperate phage Mu-1 has a DNA-modifying function, which is of great importance especially in virology. Various food and biotechnology products and chemicals also employ the bacterial fermentation of phages.
In most laboratories, temperate phages are considered more of lytic phages because most lytic-lysogenic decisions result in the former. However, whether phages are lytic or lysogenic, it is apparent that even they are capable of making a decision, particularly for replication.
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