Genetic engineering of bacteria and their potential for Hg2+ bioremediation

Ion exchange or biosorptive processes for metal removal generally lack specificity in metal binding and are sensitive to ambient conditions, e.g. pH, ionic strength and the presence of metal chelators. In this study, cells of a genetically engineered Escherichia coli strain, JM109, which expresses metallothionein and a Hg2+ transport system after induction were evaluated for their selectivity for Hg2+ accumulation in the presence of sodium, magnesium, or cadmium ions and their sensitivity to pH or the presence of metal chelators during Hg2+ bioaccumulation. The genetically engineered E. coli cells in suspension accumulated Hg2+ effectively at low concentrations (0-20 microM) over a broad range of pH (3 to 11). The presence of 400 mM sodium chloride, 200 mM magnesium chloride, or 100 microM cadmium ions did not have a significant effect on the bioaccumulation of 5 microM Hg2+, indicating that this process is not sensitive to high ionic strength and is highly selective against sodium, magnesium, or cadmium ions. Metal chelators usually interfere with ion exchange or biosorptive processes. However, two common metal chelators, EDTA and citrate, had no significant effect on Hg2+ bioaccumulation by the genetically engineered strain. These results suggest that this E. coli strain could be used for selective removal of Hg2+ from waste water or from contaminated solutions which are resistant to common treatments. A second potential application would be to remove Hg2+ from Hg(2+)-contaminated soil, sediment, or particulates by washing them with a Hg2+ chelator and regenerating the chelator by passing the solution through a reactor containing the strain.