Question

In the plaque assay, what is the precise origin of a single plaque?

Short answer

Answer: The precise origin of a single plaque in a plaque assay can be traced back to a single infectious unit (such as a bacteriophage or a viral particle) that initially infected a single host cell. As the virus replicates and spreads through nearby cells, it creates a plaque. Therefore, a single plaque represents the progeny of a single infectious unit.

Step-by-step solution

Understanding Plaque Assay

Plaque assay is a method used to estimate the concentration of an infective agent like viruses in a sample. In this process, the sample containing the virus is added to a monolayer of host cells (usually bacteria or animal cells), and then a semi-solid overlay medium is placed over the cells to restrict the virus's movement. As the viruses infect and replicate within the cells, they cause localized areas of cell destruction or lysis, called plaques.

Formation of plaques

The formation of a single plaque in the plaque assay originates from a single infectious unit, which can be a bacteriophage or a viral particle. The infectious unit binds to the host cell, entering and infecting it. As the virus replicates and assembles inside the cell, it eventually causes lysis or apoptosis of the cell, releasing many progeny virus particles.

Progeny virus particles and plaque expansion

The released progeny virus particles move on to infect nearby cells, leading to a cascade of infections. This process leads to the formation of a visible, clear zone or plaque on the cell monolayer. Each plaque corresponds to a single infectious unit from the original virus suspension, as the overlay medium restricts the movement of the virus to nearby cells only.

The precise origin of a single plaque

The precise origin of a single plaque in a plaque assay can be traced back to a single infectious unit that initially infected a single host cell. As the virus replicates and spreads through nearby cells, it creates a plaque. Therefore, a single plaque represents the progeny of a single infectious unit.