by Microbiology Doctor dr (doctor-dr)(doctor_dr)
Plant Diversity
BACTERIOPHAGES
Structure and Composition
Bacteriophages, like other viruses, have a nucleic acid core covered by a protein coat, the capsid, which is made up of subunits called capsomeres. Bacterial viruses come in a variety of shapes, but most contain a tail via which they inject viral nucleic acid into the host cell. Coleophages T2 or T4 virion infecting Escherichia coli have a hexagonal head, a stiff tail with a contractile sheath and tail fibres, and are the most complicated kind. Most phages have one of two structural forms: cubic or helical symmetry. Cubic phages have a typical solid look, whereas helical phages have a rod-like appearance. The form of polyhedral phages is icoshedral. Tobacco mosaic virus has the simplest icoshedral capsid (TMV). Double-stranded DNA is found in all tailed phages, but single-stranded DNA is found in some, and single-stranded RNA is found in others. The DNA of phage lambda is linear in the virion, but the cohesive ends unite to create a circular once it enters the host cell.
Ultrastructure of T-even Phage
T-even coliphages have tadpole-like shapes. It has two parts: a head and a tail. The head is shaped like a bipyramidal hexagonal prism, and the tail is connected to one end of the head in the form of a cylindrical cylinder.
A proteinaceous membrane surrounds a core of double-stranded viral DNA in the head. There are four parts that make up the tail. During infection, viral DNA travels via a central helical hollow tube or core. The centre is protected by a helical proteinaceous covering that may compress longitudinally. At the head end, the sheath was linked to a narrow disc or collar. A hexagonal basal plate with a complicated structure is linked to the sheath's distal end. Every corner of the plate bears a pin. The plate, together with its pins, is attached to six long thin tail fibres that serve as attachment organs to the host cell's wall.
There are two main types of bacterial viruses:
i. Lytic or Virulent Phages
After infection, these phages kill the bacterial cell that they infect. Inside the cell, they multiply and create a huge number of viruses. When the host cell explodes, additional phages are released, infecting neighbouring bacterial cells. They go through a lytic cycle.
ii. Temperate or Avirulent Phages
These are phages that do not damage or destroy the bacterial cell they are infected with. For many generations, the viral nucleic acid is transported and reproduced in the host bacterial cells without causing any harm. Temperate phages, on the other hand, may spontaneously become virulent and lyse the host cells in a future generation, exhibiting a lysogenic cycle.
Life Cycle
Two different types of life cycles are exhibited by bacteriophages:
Lytic Life Cycle
The lytic life cycle occurs when the phage multiplies inside the host cell, resulting in the lysis or destruction of the host bacteria cell. The offspring is discharged into the environment to attack fresh bacterial cells.
The following steps can be noticed during the life cycle of a lytic bacteriophage:
Adsorption is the initial stage in a phage's infection of a host bacterial cell. Ionic bonds or more or fewer particular receptor sites that engage with certain proteins in the capsids or virion allow the virion to adhere to the host cell. The penetration phase is the next stage. The virus's tail tip binds to receptor sites on the bacterial cell surface. To secure the tail pins and base plate to the cell surface, the tail fibres flex. The hollow spike is forced into the cell when the tail sheath contracts. The presence of lysozyme in the base plate aids the process. The nucleic acid of the virus is injected into the host cell. Outside the cell, the protein coat remains.
Lysogeny or Lysogenic Life Cycle of a Bacteriophage
Not all phage infections of bacterial cells result in lysis. In certain circumstances, a completely new connection between the virus and its bacterial host, known as lysogeny, or the ability to lyse, may arise.
Instead of taking over the function of the cell's genes, the viral DNA of the temperate phage is integrated into the host DNA and forms a prophage in the bacterial chromosome, serving as a gene. The bacteria metabolises and reproduces normally during lysogeny, and the viral DNA is passed down through the generations to each daughter cell. The viral DNA is occasionally removed from the host chromosome for unknown causes, and the lytic cycle begins. This is known as spontaneous induction. Irradiation with UV light or exposure to certain substances can occasionally cause a shift from lysogeny to lysis.
Coliphage lambda has been the finest model for lysogeny research. Because the genes responsible for phage multiplication and lysis are turned off, phage multiplication and lysis are suppressed inside the infected cell. The phage has a gene that codes for a repressor protein that protects the cell from being lysed by the prophage or another lytic virus. It was possible to extract and purify the repressor protein. It's a protein that's acidic. When the phage lambda is exposed to UV light, it causes the host cell to produce a protein. The repressor protein is cleaved by this protein, which prevents it from binding to the lambda prophage. This possibly induces lysis.
