Bacteria mixed with phages to produce a bacterial lawn and thereby support phage plaquing.
Indicator bacteria usually but not always consists of permissive host bacteria that support efficient plating. See, however, efficiency of plating.
Note that the bacterium hosting the initial phage infection during plaque formation need not be the same as the bacterial host serving as the indicator bacteria. Keeping these two hosts distinct allows for the design of experiments or assays that can yield more information than just simple plaquing. See for example efficiency of center of infection.
This is the definition from Adams (1959), p. 440, "Bacteria sensitive to a specified kind of phage, usually also resistant to other specified kinds."
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