Effect of different antibiotics on efficiency of transformation of bacteria by electroporation

We noted large differences in the transformation efficiency of Escherichia coli when different methods were used for transformation. To analyze these observations, the ampicillin and tetracycline resistance plasmid, pBR322, was transformed into HB101 and DH5 alpha E. coli using calcium chloride or electroporation, and the clones containing the plasmids were selected under various conditions. Electroporation yielded around 5 x 10(7) transformants per microgram of plasmid DNA when ampicillin was used for selection, but only 5 x 10(5) transformants per microgram when tetracycline was used. A calcium-chloride method of transformation showed no differences between the two antibiotics. The loss of efficiency of electroporation in the presence of tetracycline was also seen with three tetracycline-related antibiotics and could be blocked by chelating agents. It was also overcome by a 3-h preincubation of the bacteria before exposure to antibiotic. D-Cycloserine showed a selective effect on electroporated bacteria, but Polymyxin-B, chloramphenicol or phosphonomycin did not. The mechanism of inhibition by tetracycline-related antibiotics is not known, but the results show that the recovery of electroporated bacteria may be significantly decreased if these antibiotics are used.