The extrusion of cAMP can be assessed by using an indicator E. coli mutant strain that develops red colonies on maltose MacConkey agar, when supplemented with minimal level of cAMP.  A culture drop of the strain to be analyzed for cAMP extrusion is plated on a lawn of the indicator strain.  After an overnight incubation, a red halo develops around the culture drop of a crp strain.   The width of the halo is proportional to the amount of cyclic AMP released.

This method of visualization published in 1990 [Microbiology] was used to screen E. coli mutant strains impaired in cAMP production.