I plated out DH10B + BIBAC2 vector on Kan (40 mg/L) + Sucrose (5%) plates and got some colonies. What is going on?
A better way to test for sucrose sensitivity is to patch individual colonies from a LB + Kan (40 mg/ml) plant onto a LB + Kanamycin (40 mg/L) master, then replica plate (print) to a Kan + sucrose plate – this gives an accurate result. If, for example you streak out DH10B (BIBAC2) onto Kan + Sucrose you will always get some colonies – there is a very strong selection pressure to survive. Most likely the cells that survive will have deletions or rearrangements in the sacB gene. A related problem is: when constructing libraries there will always be some background, that is, vectors without inserts that appear on Kan + sucrose plate. This is true for lacZ, sacB and essentially any marker system.