Escherichia coli absorbing external-DNA under condition of lower Ca2+ concentration

Escherichia coli was inoculated in Luria Bertani (LB) medium containing lower CaCl2 concentration at 37 °C to approximate OD600 0.5. Plasmid pBR322 was added before or after incubation. The transformants in the culture were screened from LB plates containing ampicillin (1×10-4 g /mL) after deposited at 4 °C for a certain time. The result indicated that E. coli could take in external-DNA under the conditions of resemble to the nature and the DNA absorbed can replicate and express regularly. Moreover, the transformation frequency was related to Ca2+ concentration and a proper time was required for accomplishing natural genetic transformation. The result is significant in understanding natural genetic transformation of E. coli and assessing the risk of the application of genetically engineed microorganism. (GEMs).