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Unorganized development all over the world has resulted in the deterioration of the environment. The Mithi river located in Mumbai area is also one of the victims of this unorganized development. It has become a dumping site for industrial effluents, which contains various toxic materials including heavy metals such as mercury, chromium, arsenic. Physicochemical analysis of the Mithi river water samples has confirmed the deterioration of river water. Various parameters of Mithi river water such as electrical conductivity (EC), total dissolved salts (TDS), salinity, chemical oxygen demand (COD) and biological oxygen demand (BOD) were found to be higher than the normal values prescribed for the river water. In the present study, mercury-resistant bacteria were isolated from Mithi river and identified by biochemical and molecular analysis. The molecular identification of the mercury-resistant bacteria was done by 16S rDNA identification, which were identified as Bacillus sp. strain CSB_B078, Klebsiella pneumoniae strain FY2, Klebsiella pneumoniae isolate 23, Enterobacter sp. strain Amic_7, Enterobacter sp. strain 08, Acinetobacter seohaensis strain S34, Acinetobacter sp. 815B5_12ER2A. Mercury-resistant bacteria were isolated from both low-and high-salinity sites of Mithi river. The minimum inhibitory concentration of bacterial isolates against mercury was determined. Bacterial isolates could tolerate and grow in the presence of 700 ppm mercury and could also tolerate a high salinity of 35 ppt of NaCl. Resistance of bacterial isolates to high concentration of mercury and high-salinity stress suggests that the bacterial isolates could be an efficient candidate for bioremediation of mercury.

The contamination of natural resources with mercury has gained attention due to its high toxicity to all life forms. Bioremediation of mercury using bacteria is a useful technique to remediate mercury contamination. In the present study mercury resistant bacteria (MRB) were isolated from Mithi River water samples. Total heterotrophic bacteria (THB) and MRB present in the Mithi river water samples were enumerated. The count for THB was found to be 3.7 × 106 CFU/ml. MRB enumerated in the nutrient agar medium with mercury concentrations of 50, 100 and 150 ppm had counts of 2.8 × 106, 9.1 × 105 and 5.8 × 104 CFU/ml, respectively. The minimum inhibitory concentration (MIC) of the isolated bacterium was found to be around 500 ppm of mercury, and it was selected for further analysis. The bacterial isolate was found to tolerate a wide range of salt concentrations from 5 to 35 ppt of NaCl. The bacterial isolate was characterized by using standard biochemical tests and identified by using the 16S rDNA technique. Homology analysis of the 16S rDNA gene has confirmed the identity of the bacterium as Bacillus thuringiensis strain RGN1.2 with NCBI accession no. KX832953.1. It could remove 96.72%, 90.67% and 90.10% of mercury in 48 hours at 10, 25 and 50 ppm of mercury.