Genetic correlation between chromium resistance and reduction in Bacillus brevis isolated from tannery effluent

Aims:  To investigate the genetic basis of Cr(VI) resistance and its reduction to Cr(III) in indigenous bacteria isolated from tannery effluent. Methods and Results:  Four bacteria resistant to high Cr(VI) levels were isolated and identified as Bacillus spp. Their Cr(VI) reduction ability was tested. To assess the genetic basis of Cr(VI) resistance and reduction, plasmid transfer and curing studies were performed. Among all, B. brevis was resistant to 180 μg Cr(VI) ml−1 and showed the greatest degree of Cr(VI) reduction (75·8%) within 28 h and its transformant was resistant to 160 μg Cr(VI) ml−1 and reduced 69·9% chromate. It harboured a stable 18 kb plasmid DNA. Transfer and curing studies revealed that both the chromate resistance and reduction were plasmid mediated. The presence of other metal cations did not have any significant effect on Cr(VI) bioreduction. Conclusions: Bacillus brevis was resistant to elevated Cr(VI) levels and may potentially reduce it in short time from an environment where other metal ions are also present in addition to chromium ions. The strain tested shows a positive correlation between genetic basis of Cr(VI) resistance and reduction. Significance and Impact of the Study:  To our knowledge, this is the first study on the genetic correlation between chromium resistance and reduction in bacteria. Such strains may potentially be useful in biotechnological applications and in situ Cr(VI) bioremediation.