Enterobacter cloacae CYS-25 strain was isolated from a chromate plant. This bacterium was capable of resisting high hexavalent chromium concentration and reducing Cr(VI) under aerobic condition. CrO42- stimulated the increase of bacterial size and production of compact convex paths containing chromium on the bacterial surface. The increase of bacterial size was caused by integrative growth but not extracellular polymeric substance hyperplasia. IR and SDS-PAGE analyses showed the extracellular polymeric substance (EPS) components were mainly proteins and had no obvious changes whether the strains were induced by Cr(VI) or not. The EPS was amorphous and contained trivalent chromium. Under CrO42- growth condition, the extracellular substance of Enterobacter cloacae CYS-25 strains and Cr(VI) had redox reaction. The products were Cr3+-protein complexes which formed a piece of compact convex paths on the surface of bacteria and prevented Cr(VI) from entering into cells.
Ochrobactrum anthropi CTS-325 isolated from a chromium-contaminated site had better resistance to Cr(Ⅵ) in LB medium under aerobic condition.Meanwhile,it was found that the reduction of Cr(Ⅵ) is not complete during the experimental process.Therefore,a series of small molecule energy sources including nitrogen and carbon sources were added into the LB medium in the bacterial stationary phase to promote the chromium reducibility.The result showed that the bacterial growth was positively correlated with the chromium reduction.SDS-PAGE analysis indicated that the protein groups were changed when the bacteria were stimulated by the chromium.Additionally,it was revealed that O.anthropi CTS-325 could utilize the cheaper alternative of sugar(sucrose residue leaching solution) well for further growth and restart the chromium reduction,which offered a new method for practical appli-cations.
A Gram-negative, chromium(Ⅵ) tolerant and reductive strain CTS-325, isolated from a Chinese chromate plant, was identified as Ochrobactrum anthropi based on its biochemical properties and 16S rDNA sequence analysis. It was able to tolerate up to 10 mmol/L Cr(Ⅵ) and completely reduce 1 mmol/L Cr(Ⅵ) to Cr(Ⅲ) within 48 h. When the strain CTS-325 was induced with Cr(Ⅵ), a protein increased significantly in the whole cell proteins. Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis revealed that this protein was a superoxide dismutase (SOD) homology. The measured superoxide dismutase activity was 2694 U/mg after three steps of purification. The SOD catalyzes the dismutation of the superoxide anion (O2.-) into hydrogen peroxide and molecular oxygen. This protein is considered to be one of the most important anti-oxidative enzymes for O. anthropi as it allows the bacterium to survive high oxygen stress environments, such as the environment produced during the reduction process of Cr(Ⅵ).
Bioremediation has been a considerable method for treating Cr(Ⅵ ) contamination. Bacterial surface changes of Ochrobactrum anthropi during Cr biosorption was investigated in this study. We found that Cr adsorption capacity increased with the increase of initial Cr(Ⅵ ) concentration. Atomic force microscope (AFM) morphologic analysis combined with surface roughness analysis indicated that the bacterial surfaces became rougher during Cr uptake process. X-ray photoelectron spectroscopy (XPS) showed that Cr(Ⅲ) was adsorbed on the bacterial surfaces. Fourier transform infrared (FT-IR) analysis showed that surface functional groups including C-O and C-N might be involved in the Cr biosorption process.
