Plaque assays are essential laboratory techniques used to quantify bacteriophages, which are viruses that infect bacteria. Since bacteriophages are obligate intracellular parasites, they require a host cell for replication. The plaque assay allows researchers to determine the concentration of phage particles in a sample by observing the formation of plaques, which are clear zones in a layer of cultured bacterial cells that indicate cell death due to phage infection.
Each plaque corresponds to a plaque forming unit (PFU), representing a single phage particle. By counting the number of plaques formed on a culture plate, scientists can estimate the number of phage particles in the original sample. To achieve accurate results, dilutions of the phage sample are prepared to control the number of PFUs on the plate, which ultimately helps in calculating the titer. The titer is defined as the concentration of phage particles in the undiluted sample.
The process begins with a lawn of bacteria grown on a petri dish. When bacteriophages are introduced, they infect the bacteria, leading to cell lysis through the lytic cycle. As the infected bacteria die, they create clear areas or plaques on the dish. Each plaque signifies a region where bacteria have been lysed by phages, and thus, the number of plaques directly correlates to the number of PFUs and, consequently, the concentration of phages in the original sample.
In summary, plaque assays are a vital method for quantifying bacteriophages, allowing researchers to determine the titer of phage particles based on the formation of plaques in bacterial cultures. This technique not only aids in understanding phage biology but also has applications in phage therapy and microbiological research.
