BIOLOGICAL TREATMENT OF CHROMIUM FROM INDUSTRIAL WASTEWATER USING RECOMBINANT ALCALIGENES EUTROPHUS AE104 (pEBZ141

Abstract

Most heavy metals are well-known toxic and carcinogenic agents and when discharged into the wastewater represent a serious threat to the human population and the fauna and flora of the receiving water bodies. The treatment of the industrial wastewater can be particularly difficult in the presence of toxic compounds. Excessive concentration of Chromium in soluble form is toxic to a wide variety of living organisms, from bacteria to humans. Chromium is a known mutagen, with Cr(VI) causing mitotic inhibition, reduction of cell growth and cell death. The search for biological treatment for efficient and particularly cost-effective remedies promises to fulfill these requirements. This novel approach is competitive, cost-effective and cheap. Biological removal of heavy metals using natural and genetically engineered microorganisms has aroused great interest because of its lower impact on the environment. Ralstonia metallidurans, formerly known as Alcaligenes eutrophus and thereafter as Ralstonia eutropha, is a L-Proteobacterium colonizing industrial sediments, soils or wastes with a high content of heavy metals. The type strain CH34 carries two large plasmids (pMOL28 and pM0L30) bearing a variety of genes for metal resistance. R. metallidurans strain contains at least eight determinants encoding resistance to heavy metals, located either on the bacterial chromosome or on one of the two indigenous megaplasmids pMOL28 and pM0L30. Chromate resistance in R. metallidurans is based on chromate efflux catalyzed by ChrA efflux pumps. The bacterium harbours two chromate resistance determinants, the previously known chrj on plasmid pMOL28 (genes chrl, chrBj, chrAl, chrC, chrE, chrFi) and chr2 on the chromosome (genes chrB2, chrA2, chrF2). R. metallidurans strains AE128 (pM0L30) and AE126 (pMOL28) contain only one of the two megaplasmids, respectively and strain AE104 is plasmid free. The bacterial strains and the plasmid used in this study have been mentioned. Tris-buffered mineral salt medium was used for growing Alcaligenes eutrophus AE104 (pEBZ141). The cells were cultivated for 18 h at 30 °C in Tris-buffered mineral salt medium containing 3 mM disodium sulphate and 46 mM sodium gluconate as the carbon source. The cells were harvested by centrifugation, washed, and suspended in 10 mM Tris HC1, pH 7.0, containing 46 mM sodium gluconate, and 5 mM Chromium. Interaction among induction of chr resistance determinant, and chromate reduction have been demonstrated.