Don't understand replica plating...

[WiseIdiot]

my textbook says that the mutagenized E.Coli colonies (I assumed that they were mutants that were deficient in lactose metabolism) were grown in a medium containing just glucose. Then, some of the cells from each colony were transferred to medium with just lactose, i.e., the replica plate. Subsequently, the cells that differed from the replica plate that were on the master (original) plate were unable to metabolize lactose.
This is redundant since the mutagenized cells were initially selected for their defects in metabolizing lactose. Why would this last step even occur?

[MrMistery]

i think you are actually working with cells that are unable to metabolize lactose but have been transformed with a plasmid that bears the lacZ gene. And the answer you are looking for is the protein known as CAP.
Good luck