In real life, the 50% end point does not usually fall exactly on a dilution as shown in the example. The virus stock in this example contains 10 5 ID 50 per ml. This number can be calculated from the data and expressed as 50% infectious dose (ID 50) per milliliter. This is the end point: the dilution of virus at which 50% of the cell cultures are infected. Half of the cell cultures showed cytopathic effects at the 10 -5 dilution. At low dilutions, every cell culture is infected. At high dilutions, none of the cell cultures are infected because no particles are present. After an incubation period, plates that displayed cytopathic effects were scored with a +. In this example of an end-point dilution assay, 10 monolayer cell cultures were infected with each virus dilution. ![]() The number of cell cultures that are infected is then determined for each virus dilution, usually by looking for cytopathic effect. Serial dilutions of a virus stock are prepared and inoculated onto replicate cell cultures, often in multi-well formats (e.g. The end-point dilution assay was used to measure virus titer before the development of the plaque assay, and is still used for viruses that do not form plaques. ![]() ![]() Fortunately there are several alternative methods available, including the end-point dilution assay. The plaque assay is a terrific method for determining virus titers, but it doesn’t work for all viruses.
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