You now take a nitrocellulose membrane and press it on the plate [by doing so you transfer part of the bacteria cells from the colonies on the plate to the nitrocellulose membrane. The nitrocellulose membrane serves as a replica (=copy) of the colonies on the plates.] You immerse the nitrocelluse paper in lysis buffer [in order to lyse the bacteria and expose the proteins in the cytoplasm of the bacteria] Following fixation [you don't want the proteins of each colony to diffuse and mix with proteins from other colonies or be washed away from the nitrocellulose paper] you expose the nitrocellulose paper to the specific antibody derived against Cancer Antigen 1 (CA1). [If the colony contains a plasmid with the cDNA encoding for CA1 and CA1  is efficiently translated in the bacteria, the antibody will form a complex with this protein.] You then perform several washes [to get rid of unbound antibodies]  and use a detection method that allows visuallization of antigen-antibody complexes. Click on the icon to get more information about the detection method. then
Click on the icon to read about screening of expression libraries
(Look at the bottom of the page for a nice  animation
illustrating the screening of expression libraries)

then
examine your results.