Isolation and Identification of Chromium (VI)-Resistant Bacteria From Soltan Abad River Sediments (Shiraz-Iran)

Abstract

Background: Chromium (Cr) is an important heavy metal widely used in industries. It is one of the seventeen chemicals with the highest danger to human health according to the United States environmental protection agency (USEPA). Objectives: The aim of this study was to isolate and identify Cr (VI)-resistant bacteria from Soltan Abad River sediments (Shiraz-Iran) and evaluate their potential for the detoxification of Cr (VI) to Cr (III) under environmental conditions. Materials and Methods: Bacterial isolates were identified on the basis of colony morphology, Gram staining, and biochemical tests using standard microbiological methods. The minimum inhibitory concentrations (MICs) of chromium were determined by the broth agar dilution method. Cr (VI) reduction assay was determined by measuring the absorbance of the purple complex of Cr (VI) with 1,5-diphenylcarbazide. The growth of Cr (VI)-resistant bacteria was determined at different concentrations of Cr (VI). Finally, the effects of temperature and pH on the growth of selected bacteria and Cr (VI) reduction were investigated using an Luria-Bertani (LB) medium containing 50 mg/L Cr (VI). Results: The results indicated that Pseudomonas aeruginosa and Serratia marcescens were the most resistant bacteria with MICs of 200 and 150 mg/L, respectively. Both bacteria completely reduced 25 and 50 mg/L of Cr (VI) in 36 hours and 48 hours, respectively. The growth rate of both bacteria decreased with increases in the Cr (VI) concentration, and the chromate reduction was directly correlated with the growth of the bacteria. These bacteria were capable of reducing Cr (VI) at a wide range of temperatures (25 to 45°C) and pH levels (5 to 9). The optimum medium conditions for Cr (VI) reduction and the growth of the isolates were temperatures between 30 and 40°C and a pH of 7.0 to 8.0. Conclusions: P. aeruginosa and S. marcescens can be good candidates for the detoxification of Cr (VI) in Soltan Abad River.