my competent cell grow on any agar plate - (Jan/04/2013 )

I have been given a batch of XL1 blue cells. I made them competent using calcium chloride procedure. To test them, i plated them out on agar plate containing: Kanamycin, Tetracyclin, Ampicilin and control. After overnight incubation, there is bacteria colonies in all plates. These colonies are so dense we cant not see them separately.
Since I have kept my original stock of cells before making them competent, I did the same test on the original stack and i saw colonies only on the tetracyclin plates (XL1 Blue is resistant to Tet). This means the procedure of producing the competent cell has somehow made my Ecoli strand a multiresistant strain.
My question is if this is possible and if not why this has happened to my batch?

Are you sure that your agar plates are alright.? Do you have positive and negative controls.? And check your protocol doing the competent cells.

Another possibility is that you have contaminated the cells during your competent cell prep with something other than E. coli. Tap water or ice often contains resistant organisms.

Pangea, yes, all plates even those with no antibiotics and those with tet+kan showed growth,
Phage434, thanks for letting me know the source of this problem. probably ice or water contained these resistant organism

Were they fresh plates?- could easily be that your antibiotics are old or no longer working, this could especially be the case if the agar was too hot when the antibiotics were added.

hey guys,
i carried out the experimental procedure again to make new competent cell from new stock of xl1 blue. I tested them with right antibiotics. They only grew on Tet and control (o antibiotic), It is great news. Its because I autoclaved my equipments before use and I made sure the whole procedure is absolute sterile eliminating the contamination problems.

Under which condition are you growing the stock solution cells before transforming with plasmid to have enought cells? Or are you using a stock aliqout?

we have aliquots of XL1 blue, i spread them on agar plate containing tet, i took out a colony, did a 15ml starter culture, then did the 500ml culture, i centrifuged out the pellet and using calcium chloride, i made the cells competent. Then I tested them on agar plates containing antibiotics, they worked perfectly.
Therefore i did not do any transformation. I just did normal testing on the cells