Selection of recombinants due to inactivation of antibiotics is a laborious process as it requires:
i. a vector with two antibiotic resistance markers,
ii. preparation of two kinds of media plates, with one antibiotic each.
Transformed cells are first plated on the antibiotic plate which has not been insertionally inactivated (say, ampicillin) and incubated overnight for growth of transformants. For selection of recombinants, these transformants are replica-plated on second antibiotic (say, tetracycline) plate (which got inactivated due to insertion of gene). Nonrecombinants grow on both the plates (one carrying ampicillin and the other carrying tetracycline) while recombinants will grow only on ampicillin plate.
This entire exercise is laborious and takes more time (two overnight incubation) as well. However, if we choose insertional inactivation of a marker that produces colour in the presence of a chromogenic compound, we can distinguish between the recombinants and non-recombinants on a single medium plate (containing one antibiotic and the chromogenic compound) after overnight growth.
