Cr(VI) resistance in Pseudomonas isolated from soil contaminated with chromate

Hexavalent chromium is a severe contaminant since, unlike trivalent chromium, it is highly water-soluble, toxic, mutagenic to most organisms, and carcinogenic for humans. Because of its structural similarity to sulfate, chromate [Cr(VI)] crosses the cell membrane via the sulfate transport system. Inside the cell, chromium toxicity is related to the process of the reduction of Cr(VI) to Cr(III). To persist in Cr(VI)-contaminated environments, microorganisms must have effective systems to counteract the negative effect of this form of chromium. There are several mechanisms for bacterial resistance to chromate and may be conferred by genes located on chromosomes or on plasmids. Chromate resistance in bacteria has been mainly examined in strains with a little chromate resistance. Here, Cr(VI)-hyper-resistant Pseudomonas strains [Cr(VI)- MIC higher than 40 mM], isolated from a soil artificially polluted with chromate, were used as a model for the study of Cr(VI) resistance. The results obtained from the analysis of mutant strains sensitive to chromate highlighted that bacterial Cr(VI) resistance depends on key processes such as: i) DNA repair mediated by helicase, ii) activation of the sulfur starvation response, required to alter the sulfur starvation response induced by oxidative stress in cells, iii) modulation of the sulfate ABC transporter by which chromate enters the cell, iv) supply of NADPH to the cell via activity of the malic enzyme, and v) the maintenance of the equilibrium of pyridine nucleotides by the activity of sth gene. Gene expression profile of cells exposed to Cr(VI) obtained by whole genome microarrays was also used in order to obtain a complete profile of genes involved in chromate resistance