Cr(VI) reduction by Enterobacter sp. DU17 isolated from the tannery waste dump site and characterization of the bacterium and the Cr(VI) reductase
Graphical abstract
Introduction
Due to anthropogenic activities, elevated concentration of chromium (Cr) in environment has become a major concern for the scientists worldwide. Cr(VI) is considered to be highly toxic and classified as a Group ‘A’ human carcinogen because of its mutagenic, carcinogenic and teratogenic nature (Kathiravan et al., 2011, Mishra et al., 2012). Cr(VI) exists in oxoanionic form in solutions and shows structural similarity with phosphate/sulphate anions; hence enters the cells through membrane using phosphate/sulphate anionic transporters (O’Brien et al., 2003, Ramırez-Dıaz et al., 2008). Inside the cell, cysteine, ascorbate and glutathione like molecules and glutathione reductase and lipoyl dehydrogenase like metabolic enzymes vicariously stimulate reduction of Cr(VI) into Cr(III) via highly active intermediates of Cr(V) and Cr(IV) (O’Brien et al., 2003, Cheung and Gu, 2007, Eswaramoorthy et al., 2012). During this reaction, one of its intermediates, Cr(V) gets reconverted into Cr(VI) and releases electrons (e−) to substantially generate reactive oxygen species (ROS). The ROS is the main cause of oxidative damage of cell constituents like DNA, protein and lipid (Leonard et al., 2003, Cheung and Gu, 2007).
Certain plants and microbial isolates of various groups like bacteria, yeast, fungi, algae and protozoa have wide range of adaptability against Cr(VI) (Cervantes et al., 2001, Polti et al., 2011). Their mechanisms of resistance include enzymatic reduction (chromosome encoded), Cr(VI) efflux system (plasmid encoded), biosorption, bioacculmulation, SOS response, enzymatic DNA repair system, etc. (Ramirez-Diaz et al., 2008). Among them, some of the resistance mechanisms are being emphasized in an effective way to remove metal toxicity using bioremediation approaches.
To date, a variety of Cr(VI) resistant bacteria have been screened to clean up the Cr(VI) toxicity (Colin et al., 2012). Much studies have been carried out on enzyme based reaction, which brings about ‘safe’ reduction of Cr(VI) by mitigating the redox cycling (Eswaramoorthy et al., 2012). Moreover, the transformed product Cr(III) is found to be nontoxic on account of its insolubility at biological pH and less permeability to biological membrane (Poljsak et al., 2010). Various aspects are being considered to achieve maximum level of Cr(VI) reduction but biosafety of those microbial agents in terms of its antibiotic resistivity and infectious nature is not much studied, which may generate considerable medical problems and therapeutic hurdles. Those variants may attract by their high Cr(VI) removal capability but their biosafety, in terms of reducing health (by infectious diseases) in human being and animals is of great concern (Allen et al., 2010, Garcia-Armisen et al., 2011).
Therefore, in the present study, it was considered necessary to screen highly Cr(VI) reducing bacterial strain that would also be non-pathogenic. Bacterial Cr(VI) reducing capability was characterized under various factors to examine its efficacy for the bioremediation purpose.
Section snippets
Sample collection and isolation of Cr(VI) resistant bacteria
The soil sample along with healthy plant roots (of Malvaceae family) were collected from the rhizosphere of tannery waste dump site located at Jajmau, Kanpur, India and maintained at 4 °C until used for microbial analysis. Soil pH was slightly basic (i.e. pH 7.59). Furthermore, the soil sample was digested with HNO3 and HCl (3:1) followed by dilution with double distilled water to detect the heavy metals using atomic absorption spectroscopy (AAS) (AA-6300 Shimadzu, Japan). The concentrations of
Screening and selection of bacteria
A total 19 bacterial strains (DU1-DU19) resistant to Cr(VI) were isolated from tannery waste dump site. Different strains showed different MIC of Cr(VI) ranging from 100 to 1100 mg L−1. Cr(VI) reduction in LB broth was carried out in all screened variants. Only 10 strains reduced Cr(VI) and other 9 strains showed negative result for the same. Previous studies also reported that some bacteria resistant to Cr(VI) were unable to reduce Cr(VI), suggesting that Cr(VI) tolerance did not relate to its
Conclusions
Most of the earlier reports on Cr(VI) bioreduction have been focused only for its optimization, whereas variants biosafety is highly ignored. Considering this, Enterobacter sp. DU17 was isolated from the Cr(VI) contaminated tannery waste dump site and selected on the basis of its non-pathogenicity, for the first time. Resistivity against different co-metal ions also makes bacterium suitable for bioremediation in multiple metal contaminated sites. Enterobacter sp. DU17 reduced Cr(VI) completely
Acknowledgements
This work was supported by R & D grant of University of Delhi (DU) and fellowship to ZR by CSIR, New Delhi. The authors are also highly thankful to Mr. Siva P.K. Chetri and colleges of Lab no. 28 of Department of Botany, DU for technical helps.
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