I have been doing cloning and transformation almost since past 10 years, but have never experienced the following. In the lab which I have newly joined, my student is trying make the competent cells. While checking the efficiency by transforming pUC18, she is getting some background contamination. This sometimes looks like a mat growth made up of millions of tiny colonies onto which we get larger E. coli colonies. Few other times we get two type of colonies-larger and smaller; I presume that the larger ones are transformed E. coli. When only water is transformed containing no plasmid, no growth is seen suggesting that the contamination is not being introduced during the transformation experiments and that ampicillin which is being used is fine. Even after switching to new plasmid which was newly procured and untouched, same results were observed. Since negative control (water transformation) is fine, I am really wondering what might be going on wrong!!
Has anyone experienced the same thing any time?
NB: I have also tried making the competent cells again (from another strain brought from a different lab) which has given the same results.
Edited by rmbio, 16 May 2015 - 12:10 AM.