Plaque assays for bacteriophages (Figure 10) are performed by mixing the phage into a layer of bacteria, which are spread out as an overlay on the surface of an agar plate. As plate is incubated, the bacteria grow and they become visible as a turbid layer on the plate. The plaques form clear areas in this lawn.
The plaque assay is useful as it allows quantitation of phage as well as identification of pure phage strains (since each plaque arises from a single phage). This is very important in the molecular biological applications of phage vectors.
Bacteriophages transduce bacterial DNA from one cell to another. They are natural vectors. Phage vectors have a natural advantage over plasmids as they infect cells much more efficiently than plasmids transform cells, so the quantity of clones yielded with phage vectors is typically greater.
Using phages, clones are not colonies of cells, rather they are the plaques formed. Each plaque is derived from a single phage that infects the cell, killing it and the cells in vicinity.
The degradation process continues until a visible patch, or plaque, of dead cells appears. Since these phages in the plaque derive from one original phage, they are all genetic reprints of one another, or clone.