Library Screening

There are several methods of screening for transformed colonies that contain recombinant vectors. For example, where the cloning site in the vector lies within an antibiotic resistance gene, successful integration of the insert will lead to inactivation of the resistance gene and recombinant colonies can be identified by a technique known as replica plating. In this method, the pattern of colonies in the original Petri dish is printed on to a nutrient agar plate containing the selective antibiotic. The position of the recombinant colonies, i.e. those that fail to grow on the selective antibiotic, is noted so that they can be picked from the master plate. In other vectors, a method called blue/white selection can be performed. In blue/white selection, successful integration of a foreign DNA molecule in the vector destroys an enzyme gene (the LacZ gene of b-galactosidase) that otherwise forms a blue product when the transformed colonies are exposed to the substrate X-gal. Thus recombinant colonies are white and non-recombinants are blue.

Replica plating to detect recombinant plasmids