ABSTRACT
The plaque assay, an indispensable tool for the study of bacteriophage, was described in the earliest publications on the discovery of these viruses. The plaque assay was validated as showing that a single phage infecting a bacterium was sufficient to initiate formation of a plaque, because there was linear proportionality between the plaques observed and the dilution of the phage sample. However, the “efficiency of plating” of a bacteriophage preparation can vary extensively depending on conditions, especially the bacterial host strain. Varying susceptibility of different bacterial strains to different bacteriophage in fact was exploited for many years in the identification of bacteria, in a procedure called “phage typing.” Dilution fluid can be the growth medium for the host bacteria, but since this would be wasteful and expensive, in practice a simpler liquid diluent is used. Different phage exhibit different morphologies in plaque assays. Some plaques are small and clear, with sharply defined borders.
