I'm using Kanamycin for selection after electroporation of my plasmid containing a kanamycin-resistance gene at a concentration of 50mg/L prepared in 1000x stock that is usually kept in frozen aliquots.
When preparing LB plates for selection, I usually prepare a 300ul dilution of the 1000x stock (30ul kan + 250ul h20) and spread that onto my plates evenly. This usually works fine.
However, on some occasions, if I let the plates sit for a week or so, I'll go ahead and reapply more antibiotic just in case.
Is there an amount of kanamycin that is toxic to the E. coli, even if they've been successfully transformed? Are there any side effects I should be aware?