Cadmium resistance mechanism in Escherichia coli P4 and its potential use to bioremediate environmental cadmium

A cadmium-resistant bacterium was isolated from industrial wastewater and identified as Escherichia coli (dubbed as P4) on the basis of morphological, biochemical tests and 16S rRNA ribotyping. It showed optimum growth at 30 °C and pH 7. E. coli P4 found to resist Cd+2 (10.6 mM) as well as Zn+2 (4.4 mM), Pb+2 (17 mM), Cu+2 (3.5 mM), Cr+6 (4.4 mM), As+2 (10.6 mM), and Hg+2 (0.53 mM). It could remove 18.8, 37, and 56 % Cd+2 from aqueous medium after 48, 96, and 144 h, respectively. Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and Energy-dispersive X-ray (EDX) analysis also confirmed the biosorption of Cd+2 by E. coli P4. However, temperature and pH were found to be the most critical factors in biosorption of Cd+2 by E. coli P4. Cd+2 stress altered E. coli P4 cell physiology analyzed by measuring glutathione (GSH) and non-protein thiol (cysteine) levels which were increased up to 130 and 48 %, respectively. Quantitative real-time polymerase chain reaction (qRT-PCR) showed alteration in the expression levels of ftsZ, mutS, clpB, ef-tu, and dnaK genes in the presence of Cd+2. Total protein profiles of E. coli P4 in the absence and presence of Cd+2 were compared by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), which showed remarkable difference in the banding pattern. czcB gene, a component of czcCBA operon, was amplified from genomic DNA which suggested the chromosomal-borne Cd+2 resistance in E. coli P4. Furthermore, it harbors smtAB gene which plays a significant role in Cd+2 resistance.