Lab 9 Background

Plaque titer assay

Plaque titer assay is one of the most accurate methods to quantify viruses, or more accurately, viral particles, through the counting of plaques. In this method, phages are introduced into soft agar along with bacterial host cells. This soft agar mixture is laid over a hard agar base. After 6-24 hours of incubation, phages lyse the bacterial cells in their vicinity, resulting in zones of clearing on the plate that are known as plaques. Each plaque represents a single viral particle in the original viral stock. By counting the number of plaques on a plate, the concentration of viral particles in the original viral stock can be calculated.

Plaque assay is most accurate when the number of plaques on a plate is somewhere between 30 and 300. To achieve the desired plaque number with a sample with an unknown viral concentration, serial dilution of the virus is often performed. Plating multiple dilutions ensure that at least one plate will contain the desired plaque number. Each viral particle that forms a plaque is called a plaque-forming unit (PFU).

We will complete the first part of the plaque titer assay in this lab, and we will calculate the PFU in the next lab period.

Reference