Intracellular chromate-specific reductases minimize the number of electron transfers necessary for reduction of Cr(VI) to Cr(III), thus diminishing the amount of ROS generated and the level of oxidative stress ( Ackerley et al., 2004). Other response mechanisms are chromium-specific and include the shutdown of sulfate ABC transporters responsible for the CrO 4 2– entry ( Viti et al., 2014) or the intra- or extracellular enzymatic reduction of Cr(VI) ( Ramírez-Díaz et al., 2008). Some of them are not specific to chromate exposure but hinder the entry of Cr(VI) into the cell, such as passive sequestration of chromium ions by the cell surface, spores or intracellular matrix ( Fein et al., 2002 Nancharaiah et al., 2010 Sturm et al., 2018). Prokaryotes employ several strategies to combat chromate stress. Generated ROS target cytoplasmic membranes, proteins and DNA, which eventually leads to the inhibition of vital cellular processes ( Kanmani et al., 2012). These tend to re-oxidize back to Cr(VI), or give rise to reactive oxygen species (ROS) after reaction with intracellular oxygen. 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). Due to the structural similarity to the sulfate anion SO 4 2–, Cr(VI) is actively transported across cell membranes ( Cervantes and Campos-García, 2007). Chromium speciation and mobility in a given environment depend on its biogeochemical cycling. In contrast, Cr(VI) is soluble in water, therefore highly mobile and extremely toxic ( Cervantes et al., 2001). Cr(III) is mostly present in minerals in the form of or hydrated oxides ( Ehrlich, 2002), which due to their inability to cross cell membranes are considered to be of low toxicity. This study outlines the future direction for increasing the Cr-tolerance of non-pathogenic species and safe bioremediation using soil bacteria.Ĭhromium, 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). subtilis seems to be activated by the presence of chromate, hinting at versatility of Cr-efflux proteins. pseudomycoides chromate transporter ChrA in 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. Further, we found that survival of the B. pseudomycoides environmental strain accumulate less Cr than the cells of B. We have found that individual cells of the Cr-resistant B. strains harboring evolutionarily diverged chromate efflux proteins. In our study, we compared the two Bacillus spp. 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. One of the successful microbial Cr(VI) detoxification strategies is the activation of chromate efflux pumps. 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. 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. Cr(VI) is actively transported into the cell, triggering oxidative damage intracellularly. The toxicity of chromium, a group I human carcinogen, is greatest when it is in a hexavalent oxidation state, Cr(VI). 4Environmental Microbiology Laboratory, Ecole Polytechnique Fédérale de Lausanne, Lausanne, SwitzerlandĬhromium of anthropogenic origin contaminates the environment worldwide.3Department of Biology and Ecology, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia.2Department of Chemistry, Faculty of Sciences, Biochemistry and Environmental Protection, Novi Sad, Serbia.1Department of Microbial Genetics, Institute of Molecular Biology, Slovak Academy of Sciences, Bratislava, Slovakia.Zuzana Chromiková 1*, Romana Kalianková Chovanová 1, Dragana Tamindžija 2,3, Barbora Bártová 4, Dragan Radnović 3, Rizlan Bernier-Latmani 4 and Imrich Barák 1*
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