I want to select for two plasmids in E. Coli. One contains a Chloramphenicol marker and one contains a spectinomycin marker. I selected transformants on LB + Cm (30μg/ml) + Sp (50μg/ml) plate. However, the colonies seem to grow very slow (After overnight, I can barely see visible colonies, I do get small colonies after 24 hours). I am wondering whether the combination of these two drugs are too harsh for bacteria? Thank you in advance for your help!!
My first question is why you are using 2 protein inhibitors like chloramphenicol and spectinomycin. The organism (E.coli) is under severe stress and only those cells survive which will have the resistance mechanism to both the antibiotics. Hence you see a very slow growth. I am quite sure that had it been P. aeruginosa,it would be a different story.
I would plate your transformants on each of the antibiotics alone to see if the problem is specific to one of the antibiotics or to the combination of the two.
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