Novel selection for tetracycline- or chloramphenicol-sensitive Escherichia coli

A method for selecting tetracycline- or chloramphenicol-sensitive Escherichia coli cells from a population of predominantly resistant cells is described. This method depends on the inability of drug-sensitive cells to induce lambda receptors in the presence of chloramphenicol or tetracycline, protein synthesis-inhibiting drugs. The addition of bacteriophage lambda vir to a mixture of drug-sensitive and drug-resistant cells, induced for lambda receptors in the presence of tetracycline or chloramphenicol, preferentially kills the drug-resistant cells (which are capable of inducing lambda receptors). The result is a culture enriched for the sensitive cells. Several common strains used for transformation were compared for their ability to be selected. E. coli 294 was found to be superior.