Bioremediation potential of cadmium by recombinant Escherichia coli surface expressing metallothionein MTT5 from Tetrahymena thermophila

Cadmium (Cd) is a common heavy metal contaminant in industrial wastewater that causes many diseases in humans. Metallothionein (MT), a cysteine-rich metal-binding protein, is well known in chelate-heavy metals. In this study, we expressed MTT5 of Tetrahymena thermophila fused with Lpp-OmpA in the outer membrane of Escherichia coli to determine its ability to accumulate and adsorb Cd. Our results revealed that our recombinant E. coli had a 4.9-fold greater Cd adsorption compared to wild E. coli. Adsorption isothermic analysis demonstrated that the adsorption behavior for Cd in our recombinant bacteria was better fitted into the Freundlich isotherm model than Langmuir isotherm model. Fourier-transform infrared spectroscopy indicated that phosphate and organic phosphate groups were involved in the interaction between Cd and the bacterial surface. Using quantitative reverse transcription polymerase chain reaction, we further showed that the expression of metal-resistance genes (dnaK and clpB) was downregulated due to surface MTT5 protected our recombinant bacteria from Cd²⁺ adsorption. Furthermore, we showed that our recombinant bacteria could adsorb Cd from the contaminated wastewater containing other metals and were suggested to be applied in the field study.