Abstract
Chromium of anthropogenic origin contaminates the environment worldwide. The toxicity of chromium, a group I human carcinogen, is greatest when it is in a hexavalent oxidation state, Cr(VI). Cr(VI) is actively transported into the cell, triggering oxidative damage intracellularly. Due to the abundance of unspecific intracellular reductants, any microbial species is capable of bio-transformation of toxic Cr(VI) to innocuous Cr(III), however, this process is often lethal. Only some bacterial species are capable of sustaining the vegetative growth in the presence of a high concentration of Cr(VI) and thus operate as self-sustainable bioremediation agents. One of the successful microbial Cr(VI) detoxification strategies is the activation of chromate efflux pumps. This work describes transplantation of the chromate efflux pump from the potentially pathogenic but highly Cr resistant Bacillus pseudomycoides environmental strain into non-pathogenic but only transiently Cr tolerant Bacillus subtilis strain. In our study, we compared the two Bacillus spp. strains harboring evolutionarily diverged chromate efflux proteins. We have found that individual cells of the Cr-resistant B. pseudomycoides environmental strain accumulate less Cr than the cells of B. subtilis strain. Further, we found that survival of the B. subtilis strain during the Cr stress can be increased by the introduction of the chromate transporter from the Cr resistant environmental strain into its genome. Additionally, the expression of B. pseudomycoides chromate transporter ChrA in B. subtilis seems to be activated by the presence of chromate, hinting at versatility of Cr-efflux proteins. This study outlines the future direction for increasing the Cr-tolerance of non-pathogenic species and safe bioremediation using soil bacteria.
Highlights
Chromium, as a transition metal, is found in a variety of oxidation states, out of which the most stable are trivalent Cr(III) and hexavalent Cr(VI)
We found that the PchrA promoter is recognized in B. subtilis, and that the expression of GFP-ChrA under its control seems to be partially dependent on the concentration of chromium in the environment
The amount of Cr accumulated by the cells of B. subtilis strain after 18 h remained approximately at the same level, 0.3 ± 0.23 (Figures 1A,C), suggesting that this strain could already have received a lethal dose of Cr after 4 h of exposure and prolonged incubation had no further effect
Summary
As a transition metal, is found in a variety of oxidation states, out of which the most stable are trivalent Cr(III) and hexavalent Cr(VI). Cr(III) is mostly present in minerals in the form of [Cr(OH)3] or hydrated oxides (Ehrlich, 2002), which due to their inability to cross cell membranes are considered to be of low toxicity. Bacillus pseudomycoides Chromate Transporter highly mobile and extremely toxic (Cervantes et al, 2001). The toxicity of Cr(VI) is caused by the induction of massive oxidative stress in the cell, triggered by the generation of the highly reactive intermediates Cr(V) and Cr(IV) during the intracellular reduction of Cr(VI). These tend to re-oxidize back to Cr(VI), or give rise to reactive oxygen species (ROS) after reaction with intracellular oxygen. Generated ROS target cytoplasmic membranes, proteins and DNA, which eventually leads to the inhibition of vital cellular processes (Kanmani et al, 2012)
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