An insight into the mechanism of biosorption of copper by Bacillus polymyxa

Abstract

Our main objective was to isolate bacteria from soil contaminated with effluents from electroplating wastewater and assess their potential to remove copper from the aqueous phase. The strain identified as Bacillus polymyxa accumulated copper inside the cell, where it bound to the cell wall. The intracellular metal accumulation led to the inhibition of dehydrogenases, which is essential for the energy deriving reactions. Addition of thiol group containing compounds, like reduced glutathione, dithiothreitol and mercapto-ethanol, revived the enzyme activity, implicating the sulfhydryl groups as the target of metal action. However, when the cells were exposed to higher concentration of Cu(II), irreversible enzyme denaturation occurred. Desorption with DTT and citrate showed that 82% of the metal was accumulated on cell surface and the contribution from metabolism-dependent intracellular accumulation was only 10-12%. The surface characterization of B. polymyxa indicated the presence of carboxyl, imidazolium, amino and phenolic groups, which might be responsible for metal uptake. Electron paramagnetic resonance (EPR) spectroscopy studies indicated that the metal coordinating environment could be either CuN2O2 or CuNO3 chromophores, which may be the result of binding of Cu(II) to oxygen atoms of carboxylic groups of cell wall peptidoglycan and nitrogen atoms of amino-sugars or structural proteins. This environment got slightly altered after several treatments of B. polymyxa, leading to the formation of CuO4, suggesting the coordination of copper to four oxygen atoms derived from carboxyl groups of peptidoglycan.