Biotransformation of chromium (VI) by Bacillus sp. isolated from chromate contaminated landfill site

A potent Cr (VI) resistant bacterial strain DHS-12(7) was isolated from tannery waste landfill site and identified as Bacillus sp. based on pheno- and genotypic methods. The maximum tolerable concentration (MTC) of Cr (VI) was found as 1000 and 250 ppm in nutrient-rich and minimal salt medium, respectively while significant variation of MTC and Cr (VI) reduction efficiency was observed in the solid and liquid medium. The complete reduction of 10 and 25 ppm Cr (VI) was recorded within 18 h and 24 h., respectively in the minimal salt medium and the reduction trend of the strain was bacterial growth-associated. The detection of almost invariable total Cr concentration and the presence of NADH-dependent chromate reductase activity in the cell-free extract, indicated the extracellular Cr (VI) reduction rather than intracellular uptake. Furthermore, chromate reductase (ChrR) gene was amplified, sequenced and the translated amino acid sequence of the ChrR gene showed a high degree of homology with chromate reductase of different organisms. Conclusively, the findings of the present study highlight that Bacillus sp. strain DHS-12(7) can be a potent bioremediation agent for detoxification of Cr (VI) from the contaminated environmental sites for ecological restoration.