I had transformed E. coli (DH5-α) with a plasmid vector and plated them on LB-agar plates (100 ug/mL ampicillin). The next day I found in all the plates there are smears but no individual colonies. My E. coli cells are not contaminated as I had assured it beforehand. I had transformed a 50 ul of E. coli cells with 1 ul of pcDNA3.1 vector. Please help.
Check your LB-agar plates, I had these problems when the ampicillin expired. You might want to make sure the SOC is not contaminated too. Please also check the concentration of your plasmid vector. Good Luck!
Did you plate 2 or 3 µl on LB-agar to check comtamination of your media ? To obtain individual colonies, I usually plate different volums of E.coli cells. Like Xiaoli Zhang said, you may have a problem with ampicillin. Try an other ampicillin stock or a higher concentration if you don't have an other stock solution.
Thank you so much for comment. Before that, I have not used different volume of E.coli. I will try it this time. Also I have prepared again fresh stock of Ampicillin.
Thanks again.
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