Diversity of chromium-resistant bacteria isolated from soils contaminated with dichromate

Intensive industrial applications of hexavalent chromium [Cr(VI)] have caused substantial soil contamination. Chromium-resistant bacteria (CRB) isolated from soils can be used to transform toxic Cr(VI) to less toxic Cr(III) in soils. The objective of this study was to assess the biodiversity of CRB isolated from soils contaminated with potassium dichromate and evaluate the community response to Cr(VI) toxicity. We isolated 16 CRB from soil samples obtained from Brazil and the U.S. and identified them by 16S rRNA gene sequencing. Ten of our isolates were bacterial species that had never been reported as CRB. Most of the isolates from the Brazilian and U.S. soils belonged to the Bacillaceae family and Actinomicetales order, respectively. Simpson's and Shannon-Weaver diversity indices suggested that the diversity of CRB species was greater in the U.S. soil samples. Bacterial growth and Cr(VI) reduction was affected by different Cr(VI) concentrations. The minimal inhibitory concentration of Cr(VI) that affected the growth of the isolates varied widely, showing different levels of Cr(VI) resistance. The CRB displayed substantial removal of Cr(VI) achieving 100% removal of 7.5 μM of Cr(VI) in 24 h. Results indicate the presence of a diverse array of CRB adapted to Cr(VI) stress in the two soils.