I streaked DH5 α culture in nalidixic and kanamycin plate to make sure free from contamination. I initiated three ml culture, then 30ml culture there also I added nalidixic acid. Then, I prepared cc followed in a standard protocol. After preparation I added LB broth with cc alone, 4 to 5 colonies were grown in nalidixic(30mg) kanamycin(50mg) plates. I tried many times, some time colonies were grown, some time not grown. I want to know possibility of contamination during transformation and whether any other antibiotics we can use to grow only DH5 α to prevent contamination.
I am not getting your question but I have a suggestion based on my understanding as follows.
E. coli DH5alpha cells will not contain any antibiotic resistance markers, E. coli can grow in the presence of antibiotics only when they are resistant (when you transfer antibiotic resistant plasmids). At this point, your competent cells should be empty so it is impossible to grow in presence of any antibiotics. But there is a possibility that, in case you culture the untransformed cells in the presence of antibiotics there is a possibility that some cells might develop mutuant and grow (resistance). Anyway these are not to be used later for selection. So you have to prepare new competent cells without using antibiotic.
Make sure that the prepared competent cells are E.coli DH5alpha.
DH5alpha will be resistant to nalidixic acid because of the gyrA mutation, so that is not a problem. However unless your strain is already carrying a plasmid then it should not be resistant to kanamycin.
I would ask why are you so worried about contamination, if you use good sterile technique then you should be concerned and not need to use any antibiotics to prevent contamination.
I have isolated plasmid DNA from those contaminated colonies and the cells are not get lysed during lysis stage (when i had solution II)
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