Isolation, identification and characterization of Cr(VI) reducing bacterium Microbacterium sp. BD6

Abstract:

[Objective] In order to provide high-efficient strains for bioremediation of Cr(VI) pollution, growth and reduction characteristics of a strain of highly efficient Cr(VI) reducing bacteria separated from sewer sludge of electroplating plant were studied. [Methods] Cr(VI) reducing strains were isolated and screened from sludge by enrichment culture method. The primary identification was carried out by physiological and biochemical analysis and 16S rRNA gene sequence analysis. Single factor experiments were used to determine the optimal culture conditions and the resistance characteristics to stress environment. In addition, the best electronic donor was screened out by investigating the effect of electronic donor on reduction ability of strain. [Results] An efficient Cr(VI) reducing strain was isolated and preliminarily identified as Microbacterium sp., named as BD6. Strain BD6 is suitable to grow in medium temperature and alkaline environment. Meanwhile, it can tolerate high-salt environment with NaCl concentration up to 50.0 g/L. The test showed that growth of the strain is strongly inhibited by Mn²⁺, slightly inhibited by Ni²⁺, Zn²⁺ and Cd²⁺ and promoted by Cu²⁺. The minimum inhibitory concentration of Cr(VI) on BD6 was 1700 mg/L. The reduction of Cr(VI) by BD6 is significantly improved by electronic donors, including glycerol, fructose, lactose, glucose and sodium pyruvate. Glycerol is considered as the best electron donor for the reduction of Cr(VI) by BD6. The reduction rate of 100 mg/L Cr(VI) is only 69.63% in 96 h without adding electron donor, however, it could reach 100% in 36 h after adding 2 g/L glycerol. The reduction of the strain to higher initial concentration of Cr(VI) can be enhanced by adding the amount of glycerol, but restrained by the toxicity of Cr(VI). Reduction test shows that the optimum reduction conditions and growth conditions of the strain are consistent. In the high-salt environment with NaCl concentration of 50.0 g/L and toxic environment with Cd²⁺ concentration of 50 mg/L, the reduction rate of the strain to 100 mg/L Cr(VI) are 96.79% within 72 h and 99.86% within 54 h respectively with presence of 2 g/L glycerol. [Conclusion] Microbacterium sp. BD6 is a potential candidate strain for bioremediation of Cr(VI) contamination.