Bacteria grew on plates but not in liquid culture - (Jul/26/2013 )

I was doing simple ligation to get my PCR-amplified insert into pGEX vector to express recombinant protein. After ligation, I screened colonies by colony PCR and inoculated positives clones into amp-containing LB for overnight. However, none of them grew. I tried it twice, used different batches of cut vectors, freshly made new plates and still, I got colonies on plates but no growth once inoculated into liquid culture. In the beginning, I suspected it might be the ligation efficiency (e.g. all clones I picked were false positive), or ampicillin was too old, etc. However, it was not the case because I did the following experiments.

I took the empty vector, amp-resistant pGEX, and transformed it into XL1-blue competent cells. At the same time, I used non-transformed XL1-blue competent cells as control. I plated 120 uL non-transformed (NT) and transformed (T) cells into LB agar plates (+/-amp) or inoculated 2 uL directly into LB medium (+/-amp or carb). Here are the results:

NT cells:
1. LB plate -amp: grew and covered the whole plate
2. old plate (~1 week) +amp: none grew
3. new plate +amp: none grew
4. LB medium -amp: grew, turbid
5. LB medium +amp from our lab: none grew
6. LB medium +amp from lab A: none grew
7. LB medium +carb from lab B: none grew


T cells:
1. LB plate -amp: grew and covered the whole plate
2. old plate (~1 week) +amp: many single colonies
3. new plate +amp: many single colonies
4. LB medium -amp: grew, turbid
*5. LB medium +amp from our lab: none grew
*6. LB medium +amp from lab A: none grew
*7. LB medium +carb from lab B: none grew

As you can see, * were the problems, I got colonies on the plates, but the same vector-transformed cells couldn't grow in liquid culture. Does anyone have suggestion what the problem might be?

Two potential problems could be - 1, false +ve from the colony PCR (often due to picking up DNA from the transformation mix that has been spread on the plate) and 2, a toxic insert that will support some growth for a limited period of time.