Aerobic Cr(VI) Reduction by an Indigenous Soil Isolate Bacillus thuringiensis BRC-ZYR2

Chromium (Cr) may cause losses in the yield of field plant, which is one of the favorite habitats of Bacillus thuringiensis (Bt). The purposes of our study were to assess the Cr(VI)-resistance and Cr(VI)-reducing abilities of an indigenous soil isolate of Bt and to determine the factors governing Cr(VI) reduction. Towards this end a novel dichromate-reducing Bt BRC-ZYR2, characterized with insecticidal crystal proteins (ICPs), was isolated from a uranium deposit. Minimum inhibitory concentrations (MICs) of Cr(VI) were determined by broth dilution method and the concentrations of Cr(VI) and total Cr in the supernatant were quantified colorimetrically using 1,5-diphenylcarbazide (DPC) reagent and a mixture of sulfuric-nitric acids, respectively. The isolate contained five ICP genes (cry1Ba, cry1Bb, cry1Be/cry1Bf, cry9Ca and cry9Da) and exhibited a high level of Cr(VI) resistance with MICs of 150 mg L−1 at pH 7.0 and 30 °C, and 500 mg L−1 under optimal conditions (pH 9.0 and 40 °C). The total Cr concentration was similar to initial concentration of Cr(VI) under the optimal condition, suggesting that the essential removal of the Cr(VI) was dependent on Bt reduction. Under optimal conditions, the initial Cr(VI) concentrations from 25 to 75 mg L−1 significantly decreased in 24 h after incubation. Addition of Mn2+, Co2+, Mo2+ and Cu2+ activated Bt-mediated Cr(VI) reduction, while Zn2+, Ni2+ and glucose were found to inhibit the reduction. Our results indicated that this isolate could be a promising biopesticide with the potential for both insect biocontrol and Cr bioremediation in the field.