Chromate Reduction by Chromium-Resistant Bacteria Isolated from Soils Contaminated with Dichromate

Extensive use of hexavalent chromium [Cr(VI)] in various industrial applications has caused substantial environmental contamination. Chromiumresistant bacteria isolated from soils can be used to remove toxic Cr(VI) from contaminated environments. This study was conducted to isolate chromium-resistant bacteria from soils contaminated with dichromate and describes the effects of some environmental factors such as pH, temperature, and time on Cr(VI) reduction and resistance. We found that chromiumresistant bacteria can tolerate 2500 mg L-1 Cr(VI), but most of the isolates tolerated and reduced Cr(VI) at concentrations lower than 1500 mg L-1. Chromate reduction activity of whole cells was detected in five isolates. Most of these isolates belong to the genus Bacillus as identified by the 16S rRNA gene sequencing. Maximal Cr(VI) reduction was observed at the optimum pH (7.0–9.0) and temperature (30°C) of growth. One bacterial isolate (Bacillus sp. ES 29) was able to aerobically reduce 90% of Cr(VI) in six hours. The Cr(VI) reduction activity of the whole cells of five isolates had a KM of 0.271 (2.61 mM) to 1.51 mg L-1 (14.50 mM) and a Vmax of 88.4 (14.17 nmol min-1) to 489 mg L-1 h-1 (78.36 nmol min-1). Our consortia and monocultures of these isolates can be useful for Cr(VI) detoxification at low and high concentrations in Cr(VI)-contaminated environments and under a wide range of environmental conditions.