Engineering TCE-degrading rhizobacteria for heavy metal accumulation and enhanced TCE degradation

Many superfund sites are currently co-contaminated with organic pollutants such as trichloroethene (TCE) and heavy metals. A promising strategy to address these mixed-waste situations is the use of TCE-degrading rhizobacteria that will survive and thrive in soil heavily polluted with heavy metals. In this work, a gene coding for the metal-binding peptide, EC20, was introduced into rhizobacteria engineered for TCE degradation, resulting in strains with both metal accumulation and TCE degradation capabilities. EC20 was displayed onto the cell surface of Pseudomonas strain Pb2-1 and Rhizobium strain 10320D using an ice-nucleation protein (INP) anchor. Expression of EC20 was confirmed by Western blot analysis and cells with EC20 expression showed sixfold higher cadmium accumulation than non-engineered strains in the presence of 16 μM CdCl ₂. As expected, the TCE degradation rate was reduced in the presence of cadmium for cells without EC20 expression. However, expression of EC20 (higher cadmium accumulation) completely restored the level of TCE degradation. These results demonstrated that EC20 expression enhanced not only cadmium accumulation but also reduced the toxic effect of cadmium on TCE degradation. We expect that similar improvements will be observed when these engineered rhizobacteria are inoculated onto plant roots.