Bacteriophages: Viruses that Attack Bacteria
If you’ve been through a weekend with flu and cold you know all about them. At the other end of the spectrum HIV continues to have a devastating influence on the planet. But viruses do not only infect humans and animals; they also target bacteria. Viruses that infect bacteria are called bacteriophages. The name is derived from the Greek word “phage” which means to eat or to devour.
Initial observations of bacteriophages were done on bacterial plates, where an unknown entity devoured the bacterial population, hence the name. Although the bacteriophages were initially seen as a way to combat bacteria, they actually served a very different purpose for science and mankind. In fact some of the most illuminating findings and knowledge that we currently possess in the field of molecular biology comes from the study of these bacteriophages. Some of the best geneticists and Nobel laureates have in some way or other worked on phages, such as Max Delbruck, Salvador Luria, James Watson & Francis Crick.
Structure of the T4 Phage
1st Stage: This is identification of suitable receptors for attachment on the cell. Bacteriophages do not infect all bacteria; they can only infect bacteria that possess a specific receptor site which is recognised by the phage. It binds to the receptor with the help of the base plate with their attachment hooks, and the tail fibers for support. Once it fixes itself on the cell then the peptidoglycan based cell wall is acted upon by lysozymes - enzymes that break down peptidoglycan. Once that is accomplished the bacteriophage genetic material is injected into the bacterial cell.
2nd Stage: Once the viral genetic material has entered the cell, the next step is for it to be propagated. The virus carries its own copy of T4 DNA polymerase which is used to replicate the viral genetic material with the help of the host’s molecular machinery.
3rd stage: Once the viral genetic material has been synthesized and the appropriate head and tail structures have also been manufactured, they are released into the surrounding area when the bacterial cell bursts open. This type of bacteriophage life cycle is known as the lytic life cycle.
Life Cycles of Bacteriophages
After a phage infects a bacterial cell it usually multiplies and exits the hosts by killing it. However, there is another life cycle apart from the lytic one, and it is called as the lysogenic life cycle.
When a bacteriophage enters a lysogenic life cycle the viral genetic material is not immediately processed and released. Instead it goes into a dormant state and allows the bacterial cell to survive. But under suitable/specific conditions the viral genetic material is expressed leading to multiplication and release by breaking open the cell. Once the bacterial host is in the lysogenic phase it cannot be infected by another phage. This is brought about by making the receptor needed for phage binding unavailable. The choice of a bacteriophage to go lytic or lysogenic is determined by a repressor protein. This repressor has the ability to put the viral genetic material into a dormant phase by binding to promoter regions and thereby preventing genes required for the lytic cycle from being expressed.
The bacterial cell itself has developed several methods of tackling the bacteriophage threat, like making the receptors that are required for phage binding unavailable and carrying special repressor proteins that do not allow the viral genetic material to replicate.
Influence of Bacteriophages on Science
The extent to which the study of bacteriophages has served humanity can be summed up in a single significant finding; the elucidation of the structure of the genetic material - DNA. It is worth noting that both Crick and Watson were part of the Phage group at the Cold Spring Harbour Labs. It was founded by JH Miller and members included most of the prominent scientists of the time and later Nobel Prize winners in biology. Most of our initial understanding about gene regulation, transcription and DNA recombination came from the ideas and concepts derived out of the study of bacteriophages.
Source of the T4 bacteriophage image