Microbial Cr(VI) reduction by indigenous activated sludge bacilli and pure culture of escherichia coli ATCC 33456

Abstract

Cr(VI) reducing microbial species Bacillus thuringiensis, Bacillus cereus and Bacillus mycoides were isolated from dried sludge collected from sand drying beds at a local wastewater treatment plant in Brits (South Africa). The plant received high periodic loadings of Cr(VI) contaminated effluent from an abandoned chrome processing foundry within the chrome mining town of Brits. Other Cr(VI) producing organisms in the sludge sample were identified as Microbacterium foliorum and Lycinobacillus sphaerococcus. Among the five confirmed Cr(VI) reducers, B. thuringiensis was determined to have the highest performance in reducing Cr(VI) to Cr(III). The Cr(VI) reducing capability of organisms isolated at the site has not diminished over a 15 year period (2006- 2021). In the current study, the performance and pathway structures responsible for Cr(VI) reduction are analysed and compared with the activity of a metabolically versatile Escherichia coli ATCC 33456. The observations from this study showcase advanced enzymatic Cr(VI) reductase structures in B. thuringiensis never before observed in other microbial species. Blocking of electron carrier enzymes suggested the involvement of dissolved thioredoxin in the cytosol and bulk media as possible biocatalysis activators for Cr(VI) reduction in resting cells.