We use DH5 alpha E. coli for transformation and all of our vectors are only amp resistant. We had prepared competent cell and while checking their competency efficiency, there was a lawn of bacterial overgrowth on amp+ plate. How are the untransformed cells growing on amp+ plate with such vigorous growth? We have used both amp and E. coli stock from 3 other labs (Where it is working fine), we still get the same result. This thing is repeatedly happening. Can someone help with this? Our entire work is stuck because of this.
Amp is finicky and I'm guessing your amp plates are bad. I'd make fresh plates using freshly prepared amp stock soln (100 mg/mL) from a relative recently purchased bottle of solid amp. Do not add the amp to the cooling agar too soon and make sure your amp plates are stored at 4 C in the dark.
In addition to the answer that Thomas J Raub provided, there is something very weird about the plate in the picture. Either the plate was very fresh and wet and the condensation on the plate surface was moving the cells around, or you are seeing some contamination by a very motile bacterium that is not actually E. coli. So maybe you have contamination in some common reagent such as what you are using to make the cells competent.
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