Bacterial mechanisms for Cr(VI) resistance and reduction: an overview and recent advances

Chromium pollution is increasing incessantly due to continuing industrialization. Of various oxidation states, Cr⁶⁺ is very toxic due to its carcinogenic and mutagenic nature. It also has deleterious effects on different microorganisms as well as on plants. Many species of bacteria thriving in the Cr⁶⁺-contaminated environments have evolved novel strategies to cope with Cr⁶⁺ toxicity. Generally, decreased uptake or exclusion of Cr⁶⁺ compounds through the membranes, biosorption, and the upregulation of genes associated with oxidative stress response are some of the resistance mechanisms in bacterial cells to overcome the Cr⁶⁺ stress. In addition, bacterial Cr⁶⁺ reduction into Cr³⁺ is also a mechanism of specific significance as it transforms toxic and mobile chromium derivatives into reduced species which are innocuous and immobile. Ecologically, the bacterial trait of reductive immobilization of Cr⁶⁺ derivatives is of great advantage in bioremediation. The present review is an effort to underline the bacterial resistance and reducing mechanisms to Cr⁶⁺ compounds with recent development in order to garner a broad perspective.