What would prevent the growth of these bacteria in the LB/amp/ara broth?

Although your instructor worked quickly to transfer a sample of bacteria expressing rfp, there is a good chance that some non-rfp-expressing bacteria were transferred as well. What would prevent the growth of these bacteria in the LB/amp/ara broth? The bacteria was exposed to a recombinant DNA called pARA-R. What would prevent the growth of the non-rfp-expressing bacteria in the LB/amp/ara broth but not the rfp-expressing bacteria?