Question

The quantitative counts of virus particles may be achieved using a .......

• A. Ion exchange assay
• B. MPN assay
• C. Plaque assay
• D. Selective medium assay

Hint:

The plaque assay is the gold standard for quantifying infectious viral titers, as it specifically measures viable, replication-competent virus particles, whereas molecular methods like qPCR count both infectious and non-infectious viral genomes.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Viruses are obligate intracellular parasites that require host cells to replicate.
Because they do not form visible colonies on agar plates like bacteria, specialized virological assays are required to quantify their concentration in a sample.

Step 2: Detailed Explation:
The quantitative count of infectious virus particles is commonly achieved using a Plaque Assay.
In this method, a confluent monolayer of susceptible host cells is inoculated with serial dilutions of the viral sample.
An semi-solid overlay (such as agar or carboxymethyl cellulose) is applied to restrict the movement of released progeny viruses.
As the virus infects a cell, replicates, and lyses it, the infection spreads only to neighboring cells.
This localized cycle of infection and cell death creates a clear zone of lysed cells called a plaque.
By counting the number of plaques, the titer of the virus can be calculated and expressed as Plaque-Forming Units (PFUs) per milliliter.
To compare other options:
• MPN (Most Probable Number) and Selective medium assays: These are microbiological methods used to enumerate bacteria, not viruses.

• Ion exchange assay: This is a chromatography-based separation method for proteins or ions, not a biological viral enumeration assay.

Step 3: Fil Answer:
The correct option is (C).