Effect of Chromate Stress on Escherichia coli K-12

Abstract

The nature of the stress experienced by Escherichia coli K-12 exposed to chromate, and mechanisms that may enable cells to withstand this stress, were examined. Cells that had been preadapted by overnight growth in the presence of chromate were less stressed than nonadapted controls. Within 3 h of chromate exposure, the latter ceased growth and exhibited extreme filamentous morphology; by 5 h there was partial recovery with restoration of relatively normal cell morphology. In contrast, preadapted cells were less drastically affected in their morphology and growth. Cellular oxidative stress, as monitored by use of an H 2 O 2 -responsive fluorescent dye, was most severe in the nonadapted cells at 3 h postinoculation, lower in the partially recovered cells at 5 h postinoculation, and lower still in the preadapted cells. Chromate exposure depleted cellular levels of reduced glutathione and other free thiols to a greater extent in nonadapted than preadapted cells. In both cell types, the SOS response was activated, and levels of proteins such as SodB and CysK, which can counter oxidative stress, were increased. Some mutants missing antioxidant proteins (SodB, CysK, YieF, or KatE) were more sensitive to chromate. Thus, oxidative stress plays a major role in chromate toxicity in vivo, and cellular defense against this toxicity involves activation of antioxidant mechanisms. As bacterial chromate bioremediation is limited by the toxicity of chromate, minimizing oxidative stress during bacterial chromate reduction and bolstering the capacity of these organisms to deal with this stress will improve their effectiveness in chromate bioremediation.Chromate [Cr(VI)] is a widespread environmental pollutant, as it is a by-product of numerous industrial processes and nuclear weapons production (5). Because chromate is soluble, environmental contamination is difficult to contain. This solubility also promotes the active transport of chromate across biological membranes (7), and once internalized by cells, Cr(VI) exhibits a variety of toxic, mutagenic, and carcinogenic effects (34, 44). In contrast, most cells are impermeable to Cr(III), which is insoluble under typical environmental conditions (30); as measured by the Ames test, it is therefore some 1,000-fold less mutagenic than Cr(VI) (19). Thus, strategies for decontamination of environmental chromate focus on reducing it to Cr(III). Chemical methods for this are prohibitively expensive for large-scale environmental application and frequently have damaging consequences of their own (7), and so bacterial bioremediation is of considerable interest as an environmentally friendly and affordable solution to chromate pollution.Several bacteria, including Escherichia coli, Shewanella oneidensis, and numerous species of Pseudomonas and Bacillus, can reduce Cr(VI) to Cr(III) (17, 39); nonetheless, an effective bacterial system for in situ reduction has not yet been developed. One reason is that chromate is also toxic to the remediating bacteria (17). Our in vitro studies have strongly i...

Escherichia coli YqhD Exhibits Aldehyde Reductase Activity and Protects from the Harmful Effect of Lipid Peroxidation-derived Aldehydes

“…These results suggested that YqhD may play an antioxidant role by protecting E. coli from the oxidative damage caused specifically by ROS. This YqhD function would be of particular importance when cells encounter compounds that generate superoxide (tellurite) or hydroxyl radicals (chromate) (4, 5) since these toxics also affect cytoplasmic GSH levels (9,29,30). In this context, the hypersensitive phenotype to K 2 TeO 3 and K 2 Cr 2 O 7 observed in E. coli ⌬yqhD could be explained by a dual effect caused by ROS, namely the known direct damage on proteins, DNA, and membrane lipids and, indirectly, the inherent toxicity of peroxides and reactive aldehydes generated by membrane lipid peroxidation, which could not be scavenged in the absence of GSH.…”

Bioreduction of Cr (VI) by Heavy Metal Resistant Pseudomonas Species

“…Chromium occurs mainly in two forms in the environment i.e., trivalent and hexavalent (as chromate and dichromate) and is actively transported to cells (Ortegel et al, 2002). Among the different forms of chromium, the hexavalent chromium is the more toxic and carcinogenic due to its high solubility in water, rapid permeability through biological membranes and subsequent interaction with intracellular proteins and nucleic acids (Kamaludeen et al, 2003;Ackerley et al, 2006). Reduction of Cr (VI) leads to the formation of stables, less soluble and less toxic Cr (III).…”

Genome Sequencing Revealed Chromium and Other Heavy Metal Resistance Genes in E. cloacae B2-Dha

“…One of many possible ways to increase the effectiveness of chromium bioremediation by using bacteria is to alter the expression of these genes to minimize oxidative stress during chromate reduction. This approach has been proposed by several other researchers [12,48]. Previously, we have postulated that the B2-DHA is resistant to chromium and it can decrease chromium content significantly in the contaminated source by accumulating it in the cells [6].…”

“…However, chromium has multiple effects on bacteria including competitive inhibition of sulphate transport, DNA mutagenesis and protein damage [8]. Microorganisms have developed various mechanisms to survive chromium toxicity: (i) transmembrane efflux of chromate (ii) the ChrR transport system (iii) the reduction of chromate (iv) protection against oxidative stress and (v) DNA repair systems [3,[9][10][11][12][13][14][15]. In addition, chromate resistance is attributed to the functions of a series of chromosomal or plasmid encoded genes, including the chromium resistance (chr) operon comprising of either chrBAC or chrBACF in bacteria [9,16,17].…”