Heavy growth in LB agar plates with two antibiotics - (Dec/06/2013 )

Hi Experts

Here's a problem of growing transformed Agrobacterium GV3101 on LB agar plates which is resistant to streptomycin. My plasmid is resistant to Kanamycin.

I plated transformed culture to the plates containing both these antibiotics (100μg/ mL) and found heavy growth very next morning. 

Then as a control I plated the same culture on LB agar plates with Streptomycin and LB agar plates with Kanamycin. I found nice and well spaced colonies after two days in both plates. 

Second control was the incubation of sealed plate without any culture. No colony observed even after four days.

As a third control I plated Un-transformed GV3101 competent cells on the LB agar plates with streptomycin. I found just very normal growth pattern after two days of incubation at 28°C.

I need to know weather there is some sort of drug interaction has occurred as the plates seem to promote the growth of some other bacteria rather than agrobacterium. kindly help.

Did you also do the other obvious control - plate the untransformed on Kan plates?  If not - it would pay to check that your kan hasn't gone off.