How are plaques used to measure the number of viruses in a culture?

In microbiology, plaques are used to quantify the number of viruses in a culture. This method involves observing clear areas, known as plaques, that form on an opaque field of bacteria. These clearings indicate that the bacterial cells have been inhibited or dissolved by the presence of viruses.1

To measure virus numbers, a culture containing viruses is applied to a layer of bacteria grown on agar. The viruses infect and lyse the bacteria, creating visible plaques. By counting the number of plaques and knowing the dilution factor of the original virus sample, scientists can determine the concentration of infectious viruses, often expressed as plaque-forming units (PFU) per unit volume. This technique, adapted by Renato Dulbecco, is crucial for studying and quantifying viruses.2