Can Ca2+-dependent competence be repeatedly induced in the same Escherichia coli cells?

With the help of devised multicycle consecutive transformation (MCT) it is shown that Ca2+-dependent competence can be repeatedly induced in the same population of Escherichia coli cells. The same fraction of cells is induced to competence and transformed during MCT. In contrast to the results on classical transformation with mixed DNA preparations, no double transformants are observed in MCT. The competent cells and transformants are found to be more fragile than nontransformed cells. The latter are represented presumably by the cells that have not absorbed exogenous plasmid DNA. The results suggest that there is strong interference between plasmid DNAs during MCT, and that the presence of exogenous DNA makes the cells more sensitive to the apparently harmful procedure of repeated competence induction.