Question

Explain the process by which a single microscopic bacteriophage can produce a plaque in a lawn of bacteria that is visible to the naked eye: infection, replication etc.

Answer 1

Bacteriophage as its name suggested, eats bacteria. It is Viruses that live as intracellular parasites. They have capability of multiplication and replication inside the bacterial host. They use the host machinery for the replication process. They complete their life cycle inside the host cell in two ways, either thay can be lysogenic or lytic. In case of lysogeny the virus completes its life cycle inside the host and after one or two round of replication leaves the host cell intact, the bacteria continues normal replication in this case. While in case of lytic cycle the virus uses whole biosynthetic machinery of the bacteria to replicate and exits by lysing the bacterial cell, leading to the death of the bacteria.

The virus enters the bacterial cell by attaching on the cell wall of the bacteria, followed by the release of its DNA into the host cytoplasm. Its DNA acts as template for production of phage proteins leads to the formation of capsid. These proteins also subjugates the the host cell, eventually causing lysis and death of the host. This killing of bacteria can be seen as clear plaque in the lawn of bacteria on media plate.

Since viruses cannot be seen via light microscope, this plaque assay is the best method to count the phage particles. In this technique the infected bacteria have been poured into the media with bacterial lawn grown on the agar plate. Since the cells are infected and killed by the released viruses, a clear spot on the agar in the bacterial lawn develops, called a plaque. The plaques are countable as number of virus particles or virions in the original specimen.

This can be quantitated as viruses/ml of plaque-forming units/ml(PFUs).