Exploring the mechanism of competence development in Escherichia coli using quantum dots as fluorescent probes

The mechanism of divalent Ca2+ cation induction of Escherichia coli competence is still not fully understood, though it is a common method for introducing recombinant DNA into bacterial cells in gene engineering. Quantum dots (QDs), as a new fluorescent probe of being applied in biology research, have aroused great interest. In this paper, we explored the mechanism of E. coli competence development using QDs for the first time. Results showed that water-soluble QDs of diameter 3-4 nm could go into competent cells, but could not enter noncompetent cells. This result was further confirmed using atomic force microscopy and DNA transforming experiments, suggesting that nonphysiological, high concentrations of Ca2+ enhanced the penetrability of cell membranes so that QDs, which cannot enter cells normally due to their greater diameter (3-4 nm), can do so easily into competent cells. Therefore, we believe that, at least for E. coli, the mechanism of Ca2+-induced competence development is mediated physicochemically rather than physiologically.