Bacterial reduction of hexavalent chromium

Summary Cr(VI)-reducing bacteria are widespread and Cr(VI) reduction occurs under both aerobic and anaerobic conditions. Under aerobic conditions, both NADH and endogenous cell reserves may serve as the electron donor for Cr(VI) reduction. Under anaerobic conditions, electron transport systems containing cytochromes appear to be involved in Cr(VI) reduction. High cell densities are necessary to obtain a significant rate of Cr(VI) reduction. Cr(VI) reduction by bacteria may be inhibited by Cr(VI), oxygen, heavy metals, and phenolic compounds. The optimum pH and temperature observed for Cr(VI) reduction generally coincide with the optimal growth conditions of cells. The optimum redox potential for Cr(VI) reduction has not yet been established.     

Hexavalent chromium reduction by bacteria from tannery effluent

Chromium is generated from several industrial processes. It occurs in different oxidation states, but Cr(III) and Cr(VI) are the most common ones. Cr(VI) is a toxic, soluble environmental contaminant. Some bacteria are able to reduce hexavalent chromium to the insoluble and less toxic Cr(III), and thus chromate bioremediation is of considerable interest. An indigenous chromium-reducing bacterial strain, Rb-2, isolated from a tannery water sample, was identified as Ochrobactrum intermedium, on the basis of 16S rRNA gene sequencing. The influence of factors like temperature of incubation, initial concentration of Cr, mobility of bacteria, and different carbon sources were studied to test the ability of the bacterium to reduce Cr(VI) under variable environmental conditions. The ability of the bacterial strain to reduce hexavalent chromium in artificial and industrial sewage water was evaluated. It was observed that the mechanism of resistance to metal was not due to the change in the permeability barrier of the cell membrane, and the enzyme activity was found to be inductive. Intracellular reduction of Cr(VI) was proven by reductase assay using cell-free extract. Scanning electron microscopy revealed chromium precipitates on bacterial cell surfaces, and transmission electron microscopy showed the outer as well as inner distribution of Cr(VI). This bacterial strain can be useful for Cr(VI) detoxification under a wide range of environmental conditions.     

Modeling hexavalent chromium reduction in Escherichia coli 33456

A model based on te analysis of the mechanism of enzymatic reactions was developed to characterize the rate and extent of microbial reduction of hexavalent chromium in Escherichia coli 33456. A finite reduction capacity (R(c)) was proposed and incorporated into the enzymatic model to regulate the toxicity effect on cells due to the oxidizing power of Cr(VI). The parameter values were determined by nonlinear least-square analysis using experimental data of anaerobic cultures. The obtained parameters were then used to predict Cr(VI) reduction in aerobic cultures along with a modification term of uncompetitive inhibition from molecular oxygen. The applicability of the developed model was demonstrated through excellent prediction of the results of batch studies conducted over range of initial Cr(VI) concentrations, initial cell densities, and DO levels. A sensitivity analysis revealed that the parameters obtained using the experimental data were unique, and neither change in K(c), the half-velocity constant, at high initial Cr(VI) concentrations nor change in R(c), the reduction capacity, at low initial Cr(VI) concentrations was sensitive to model prediction. (c) 1994 John Wiley & Sons, Inc.     

细菌六价铬还原及吸附机制研究进展

六价铬[Hexavalent chromium,Cr(Ⅵ)]是一种致癌物,其毒性远大于三价铬,因此会对人体健康和生态环境造成危害。Cr(Ⅵ)污染场地中的细菌主要通过生物还原和生物吸附降低Cr(Ⅵ)的毒性和迁移能力。Cr(Ⅵ)还原细菌的抗性机制与还原过程已被多次讨论,但现有综述还缺乏细菌类别、铬酸盐还原酶活性与吸附机制的总结。因此,本文通过系统发育树展示常见Cr(Ⅵ)还原细菌的类别,归纳细菌的Cr(Ⅵ)还原机制,总结现阶段铬酸盐还原酶的酶活性参数与反应条件,并讨论环境影响因子对细菌Cr(Ⅵ)还原的影响。其次,本文综述了细菌对Cr(Ⅵ)的吸附现象与机理。最后,本文对未来细菌修复Cr(Ⅵ)污染的机理研究进行了展望,以期加深对细菌Cr(Ⅵ)还原和吸附过程的了解。     

真菌还原Cr（VI）的研究

从不同来源的样品中分离筛选出几株抗Ｃｒ（ＶＩ）的真菌,他们能在含３００￣５００ｍｇ／ＬＫ２Ｃｒ２Ｏ７的蔗糖合成培养基中生长,其中ＢＳ－１菌株抗Ｋ２Ｃｒ２Ｏ７达９００ｍｇ／Ｌ．ＢＳ－１等４株真菌在含２００ｍｇ／Ｌ Ｋ２Ｃｒ２Ｏ７的培养基中生长４￣６ｄ后,培养液中的Ｃｒ（ＶＩ）已全部消失。这些真菌经鉴定为青霉菌ＢＳ－１和ＢＳ－３,黑曲霉ＢＲ－４和黄曲霉ＢＸ－１。经紫外可见光扫描及化学分析证实,高毒的Ｃ     

Measurement of bacterial sulfate reduction in sediments: Evaluation of a single-step chromium reduction method

A procedure which includes the Total Reduced Inorganic Sulfur (TRIS) in a single distillation step is described for the radiotracer measurement of sulfate reduction in sediments. The TRIS includes both Acid Volatile Sulfide (AVS: H₂S + FeS) and the remaining Chromium Reducible Sulfur (CRS: S⁰, FeS₂). The single-step distillation was simpler and faster than the consecutive distillations of AVS and CRS. It also resulted in higher (4–50%) sulfate reduction rates than those obtained from the sum of³⁵S in AVS and CRS. The difference was largest when the sediment had been dried after AVS but before CRS distillation. Relative to the³⁵S-AVS distillation alone, the³⁵S-TRIS single-step distallation yielded 8–87% higher reduction rates. The separation and recovery of FeS, S⁰ and FeS₂ was studied under three distillation conditions: 1) cold acid, 2) cold acid with Cr²⁺, and 3) hot acid with Cr²⁺. The FeS was recovered by cold acid alone while pyrite was recovered by cold acid with Cr²⁺. A smaller S⁰ fraction, presumably of the finer crystal sizes, was recovered also in the cold acid with Cr²⁺ while most of the S⁰ required hot acid with Cr²⁺ for reduction to H₂S.     

Kinetics and modeling of hexavalent chromium reduction in Enterobacter cloacae

Kinetics of bacterial reduction of toxic hexavalent chromium (chromate: CrO(4) (2-)) was investigated using batch and fedbatch cultures of Enterobacter cloacae strain HO1. In fedbatch cultures, the CrO(4) (2-) feed was controlled on the basis of the rate of pH change. This control strategy has proven to be useful for avoiding toxic CrO(4) (2-) overload. A simple mathematical model was developed to describe the bacterial process of CrO(4) (2-) reduction. In this model, two types of bacterial cells were considered: induced, CrO(4) (2-)-resistant cells and uninduced, sensitive ones. Only resistant cells were assumed to be able to reduce CrO(4) (2-). These fundamental ideas were supported by the model predictions which well approximated all experimental data. In a simulation study, the model was also used to optimize fed-batch cultures, instead of lengthy and expensive laboratory experiments. (c) 1993 John Wiley & Sons, Inc.     

Metabolic reduction of chromium,as related to its carcinogenic properties

At variance with Cr(III), Cr(VI) compounds easily cross cell membranes and exert genotoxic effects. No metabolic oxidation of Cr(III) could be detected, whereas Cr(VI) reduction was observed in the presence of body fluids and subcellular fractions of various tissues from several animal species. The differential efficiency of this process may account for the selection of target tissues in Cr(VI) carcinogenesis. For instance, reduction by saliva and gastric juice may explain a lack of carcinogenicity by the oral route; reduction inside erythrocytes may explain a lack of carcinogenicity at a distance from administration sites; reduction by the epithelial-lining fluid of terminal airways and by alveolar macrophages may be consistent with the occurrence of thresholds in lung carcinogenesis. Liver preparations displayed the top efficiency in reducing Cr(VI), whereas skeletal muscle, i.e., a typical target in experimental Cr(VI) carcinogenesis, had no detectable activity. Bronchial tree and peripheral lung parenchyma preparations from almost 100 individuals reduced Cr(VI) to a variable extent. The efficiency of lung parenchyma and of isolated alveolar macrophages was enhanced in cigarette smokers. In rats, Cr(VI) reduction by lung preparations was significantly stimulated by the repeated i.t. instillation of Cr(VI) itself. Among the electron donors (chiefly GSH) and enzymatic mechanisms responsible for the intracellular Cr(VI) reduction, such as cytochrome P-450 reductase, glutathione redactase, and aldehyde oxidase, an important role can be ascribed to cytosolic DT diaphorase activity, usually catalyzing a 2-electron reduction.     

Hexavalent chromium reduction by an actinomycete, Arthrobacter crystallopoietes ES 32

Environmental contamination by hexavalent chromium, Cr(VI), presents a serious public health problem. This study assessed the reduction of Cr(VI) by intact cells and a cell-free extract (CFE) of an actinomycete, Arthrobacter crystallopoietes (strain ES 32), isolated from soil contaminated with dichromate. Both intact cells and CFE of A. crystallopoietes, displayed substantial reduction of Cr(VI). Intact cells reduced about 90% of the Cr(VI) added within 12 h and Cr(VI) was almost completely reduced after 24 h. The K _M and V _max of Cr(VI) bioreduction by intact cells were 2.61 μM and 0.0142 μmol/min/mg protein, respectively. Cell-free chromate reductase of the A. crystallopoietes (ES 32) reduced hexavalent chromium at a K _M of 1.78 μM and a V _max of 0.096 μmol/min/mg protein. The rate constant (k) of chromate reduction was inversely related to Cr(VI) concentration and the half-life (t _1/2) of Cr(VI) reduction increased with increasing concentration. A. crystallopoietes produced a periplasmic chromate reductase that was stimulated by NADH. Results indicate that A. crystallopoietes ES 32 can be used to detoxify Cr(VI) in polluted sites, particularly in stressed environments.     

Effects of combining biological treatment and activated carbon on hexavalent chromium reduction

The objectives of the present work were: (a) to analyze the Cr(VI) removal by combining activated sludge (AS) with powdered activated carbon (PAC), (b) to analyze the effect of PAC and Cr(VI) on the growth kinetics of activated sludge, and (c) to determine if the combined method (AS-PAC) for Cr(VI) removal can be considered additive or synergistic with respect to the individual processes. Chromate removal was improved by increasing PAC concentrations in both PAC and AS-PAC systems. Cr(VI) removal using the AS-PAC system was higher than using AS or PAC. The increase of Cr(VI) caused longer lag phase and lower observed specific growth rate (μ_obs), biomass yield (Y_X/S), and specific growth substrate consumption rate (q_S) of activated sludge; additionally, PAC did not enhance the growth kinetic parameters (μ_obs, Y_X/S, q_S). Cr(VI) reduction in AS-PAC system was the result of the additive effect of each individual Cr(VI) removal process.     

Intracellular flavonoids as electron donors for extracellular ferricyanide reduction in human erythrocytes.

Reduction of extracellular ferricyanide [Fe(CN)(6)](-3) to ferrocyanide by intact cells reflects the activity of a trans-plasma membrane oxidoreductase that, in human red blood cells, utilizes ascorbic acid as an electron donor. We herein report that the flavonoids quercetin and myricetin, while inhibiting dehydroascorbic acid uptake-and thus the erythrocyte ascorbic acid content-effectively stimulate the extracellular reduction of ferricyanide. Other flavonoids such as rutin, acacetin, apigenin, and genistein do not show the same effect. The notion that quercetin or myricetin may serve as an intracellular donor for a trans-plasma membrane oxidoreductase is supported by the following lines of evidence: (i) they afford direct reduction of ferricyanide; (ii) extracellular reduction of ferricyanide was not mediated by direct effects of the flavonoids released by the cells and was abolished by the sulphydryl reagent parachloromercuribenzenesulfonic acid (pCMBS); (iii) the intracellular concentrations of quercetin or myricetin well correlate with increases in ferricyanide reduction; (iv) the intracellular concentration of the flavonoids dramatically declines after ferricyanide exposure. Taken together, the results presented in this study demonstrate that myricetin and quercetin, which accumulate in large amounts in red blood cells, act as intracellular substrates of a pCMBS-sensitive trans-plasma membrane oxidoreductase. This may represent a novel mechanism whereby these flavonoids exert beneficial effects under oxidative stress conditions.     

Characterization of enzymatic reduction of hexavalent chromium by Escherichia coli ATCC 33456.

Chromium reduction by Escherichia coli ATCC 33456 quantitatively transferred hexavalent chromium, Cr(VI), to trivalent chromium, Cr(III). The reduced chromium was predominantly present in the external medium. Supernatant fluids of cell extract, obtained by centrifugation at 12,000 and 150,000 x g, showed almost the same Cr(VI) reduction activity, indicating that Cr(VI) reduction by E. coli ATCC 33456 was a largely soluble reductase activity. In studies with respiratory inhibitors, no inhibitory effects on aerobic and anaerobic Cr(VI) reduction were demonstrated by addition of cyanide, azide, and rotenone into both intact cell cultures and supernatant fluids of E. coli ATCC 33456. Although cytochromes b and d were identified in the membrane fraction of cell extracts, Cr(VI) was not reduced by the membrane fraction alone. The cytochrome difference spectra analysis also indicated that these cytochromes of the respiratory chain require the presence of the soluble Cr(VI) reductase to mediate electron transport to Cr(VI). Stimulation of Cr(VI) reduction by an uncoupler, 2,4-dinitrophenol, indicated that the respiratory-chain-linked electron transport to Cr(VI) was limited by the rate of dissipation of the proton motive force.     

Hexavalent chromium reduction by a dichromate-resistant gram-positive bacterium isolated from effluents of tanneries

A gram-positive, chromium (Cr)-resistant bacterial strain (ATCC 700729) was isolated from effluent of tanneries. It was grown in media containing potassium dichromate concentration up to 80 mg ml⁻¹ of the medium. The dichromate reducing capability of the bacterium was checked by estimating the amount of Cr VI in the medium before and after introduction of bacterial culture. The influence of factors like pH of the medium, concentration of Cr, and the amount of the inoculum was studied to determine the ability of the bacterium to reduce Cr VI in the medium under various conditions. In a medium containing dichromate 20 mg ml⁻¹ more than 87% reduction of dichromate ions was achieved within 72 h. The feasibility of the use of this bacterial strain for detoxification of dichromate in the industrial wastewater has been assessed. The isolated strain can be exploited for specific environmental clean-up operations. Received: 7 April 1999 / Accepted: 12 September 1999     

六价铬还原细菌Bacillus cereus S5.4的筛选鉴定及还原特性研究

六价铬生物毒性极大,是造成环境污染的主要重金属之一,其生物治理策略已引起了广泛关注。已经发现许多微生物具有六价铬抗性和还原性,但能工业应用的还十分有限。从宝钢电镀污泥中分离得到一系列高六价铬抗性菌株,其中一株S5.4显示出高六价铬还原性,经形态和生理生化特征及16s rDNA序列比对,鉴定为Bacillus cereus。该菌株好氧生长,在固体LB培养基上培养48h能耐受40mmol/L Cr6 ,并对Mn2 、Ba2 和Mo6 也显出高抗性;在液体LB培养基中培养72h完全还原2mmol/L Cr6 ,并能在补充培养基和六价铬的条件下连续还原。该菌株还原六价铬时,最适浓度为2mmol/L Cr6 ,最适温度范围30~37℃,最适pH 7~9。     

Modeling simultaneous hexavalent chromium reduction and phenol degradation by a defined coculture of bacteria

Based on the kinetics of Cr(VI) reduction by Escherichia coli ATCC 33456 and phenol degradation by Pseudomonas putida DMP-1, a mathematical model is developed to describe simultaneous Cr(VI) reduction and phenol degradation in the coculture of the two species. The developed model incorporates the toxicity effects of Cr(VI) and phenol on phenol degradation and Cr(VI) reduction in the coculture. The model illustrates the inhibitory effects of phenol on Cr(VI) reduction and Cr(VI) toxicity toward phenol degradation. The model also reveals the recoveries of the activities of the repressed bacterial cells with continuous Cr(VI) reduction and phenol degradation in the coculture. The model is capable of predicting simultaneous Cr(VI) reduction and phenol degradation within a broad range of Cr(VI) and phenol concentrations and under an appropriate composition of populations. However, the model simulates lower concentrations of phenol than experimental observations once Cr(VI) is reduced to a low level (<7 mg/L). The model simulation for Cr(VI) also deviates from experimental data when P. putida is outnumbered by E. coli by a ratio of 1:5. (c) 1995 John Wiley & Sons, Inc.     

Bacterial reduction of hexavalent chromium by Enterobacter cloacae strain HO1 grown on sucrose

Chromium(VI) was reduced by Enterobacter cloacae strain HO1 grown with sucrose as a carbon source and nitrate as the initial terminal electron acceptor. Under excess substrate conditions, the Cr(VI) concentration, initially at 5 and 10 mg/l, was reduced to less than 100 mg/l.     

Enhancement of hexavalent chromium reduction and electricity production from a biocathode microbial fuel cell

Enhancement of Cr (VI) reduction rate and power production from biocathode microbial fuel cells (MFCs) was achieved using indigenous bacteria from Cr (VI)-contaminated site as inoculum and MFC architecture with a relatively large cathode-specific surface area of 340–900 m² m⁻³. A specific Cr (VI) reduction rate of 2.4 ± 0.2 mg g⁻¹VSS h⁻¹ and a power production of 2.4 ± 0.1 W m⁻³ at a current density of 6.9 A m⁻³ were simultaneously achieved at an initial Cr (VI) concentration of 39.2 mg L⁻¹. Initial Cr (VI) concentration and solution conductivity affected Cr (VI) reduction rate, power production and coulombic efficiency. These findings demonstrate the importance of inoculation and MFC architecture in the enhancement of Cr (VI) reduction rate and power production. This study is a beneficial attempt to improve the efficiency of biocathode MFCs and provide a good candidate of bioremediation process for Cr (VI)-contaminated sites.     

In vivo reduction of chromium (VI) and its related free radical generation

Chromium (VI) compounds are widely recognized as human carcinogens. Extensive studies in vitro and in model systems indicate that the reactive intermediate, Cr (V), generated by cellular reduction of Cr (VI), is likely the candidate for the ultimate carcinogenic form of chromium compounds. Here we review our current understanding of the in vivo reduction of Cr (VI) and its related free radical generation. Our results demonstrate that Cr (V) is indeed generated from the reduction of Cr (VI) in vivo, and that Cr (V) thus formed can mediate the generation of free radicals. Cr (V) and its related free radicals are very likely to be involved in the mechanism of Cr (VI)induced toxicity and carcinogenesis. These studies also illustrate that in vivo EPR spectroscopy and magnetic resonance imaging can be very useful and powerful tools for studying paramagnetic metal ions in chemical and biochemical reactions occurring in intact animals.