Bioremediation of hexavalent chromium by transformation ofEscherichia coliDH5α with chromate reductase (ChrR) genes ofPseudomonas putidaisolated from tannery effluent

Abstract Aims Hexavalent chromium (Cr(VI)), a toxic heavy metal, is a serious pollutant from tannery effluent, and its accumulation in soil and water causes severe environmental concerns and increasing public health issues. The present study focuses on the isolation and identification of chromium-reducing bacteria collected from the tannery industry in Dindigul, Tamil Nadu. Chromium-reducing bacteria Pseudomonas putida were identified by 16S rRNA sequencing followed by BLAST search. The plasmid with Cr(VI) reductase gene was isolated from Ps. putida and transferred to Escherichia coli DH5α for further studies. Methods and Results The bacterial cultures were kept under controlled conditions for 72 h to observe the growth rates and bacterial resistance to chromium. When strains wild-type and transformant E. coli DH5α were grown in chromium-supplemented media, they revealed significant growth, but strains cured type Ps. putida and E. coli DH5α recorded minimum growth. The Cr(VI) reduction employed by transformant E. coli DH5α and wild Ps. putida was 42.52 ± 1.48% and 44.46 ± 0.55%, respectively. The culture supernatant of the wild Ps. putida and transformant E. coli DH5α showed an increased reduction of Cr(VI) compared with cell extract supernatant and cell debris due to the extracellular activity of chromium reductase being responsible for Cr(VI) reduction. Besides, the chromium reductase gene was confirmed in the isolated Ps. putida and transformant E. coli DH5α. Conclusions Transformant bacteria could employ an alternative method for heavy metal detoxification in contaminated environments like tannery effluent and mining processes. Significance and Impact of Study High Cr(VI) concentration resistance and high Cr(VI) reducing the strain's ability make it suitable for bioremediation. These possible horizontal gene transfer events indicated in this study may have enabled transformant E. coli DH5α as a good candidate for reducing the heavy metal pollution.