Plaques assay

Plaques are visible structures formed in a cell culture contained within a nutrient medium. (e.g the plaquing medium). These clearings has no cells in it because the virus infect the cell and eventually lysing it.The cells that were initially seeded in the well and allow the virus to infect it then overlay with plaquing medium for the cells to attach to the agarose and grow. Just like how each colony is derived from a single E.coli cell in an agar plate, each clearing is derived from one virus attaching to one cell, and when the virus infects the cell and multiple it continues to lyse the other cells around and forms a clearing around that area. As that clearing, also known as plaque, arise from one virus, we are able to determine the number of viruses (which is also known as titer which is also = no. of plaque forming units/ ml) by counting the number of plaques.

In plaque assay, virus undergoes serial dilution.

http://www.slic2.wsu.edu:82/hurlbert/micro101/images/virus28.gif

I have generally found that plaques assay is not a very good way to find virus concentration. There are many ways for virus to hide, and so this assay is usually an underestimate of the actual number of virus present. For instance, a pfu (plaque forming unit) may not find a cell by the time the attachment period is ended or it may stick to the plate where no cell is present and by the time the monolayer reaches it, it's too far behind to form a visible plaque.