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The presence of heavy metals in aquatic environments poses a serious environmental risk. The usage of heavy metal-tolerant bacteria may serve as a cost-effective tool for bioremediation of polluted water bodies. Thus, the present study aims at the isolation and characterization of heavy metal tolerant gram-negative bacteria collected from the polluted water of the river Yamuna, Delhi, India. The water samples were collected from the downstream river and enriched separately in the nutrient broth. Appropriate dilutions were then plated in eosin methylene blue (EMB) agar supplemented with cadmium (10 to 4000 µg/ml) and nickel (10 to 3000 µg/ml). The colonies having different morphologies that were formed in the EMB agar were selected, and their metal tolerance concentration (MTC) was determined. Based on their MTC values, two isolates were identified by using morphological, biochemical, and molecular (16S rRNA gene sequencing) methods. Besides, their growth kinetics, co-metal tolerance, and antibiotic resistance were also determined. The identified isolates 2 and 8 were tolerant to cadmium (3000 µg/ml) and nickle (2000 µg/ml), respectively. The isolates were found to be closely related to Pantoea agglomerans JCM 1236 (sample 2) and Enterobacter asburiae JCM 6051 (sample 8). They were not only co-tolerant to cadmium (3000 µg/ml) and nickle (2000 µg/ml) but also showed resistance to various antibiotics. Scanning electron microscopy (SEM) showed changes in surface morphology of nickle and cadmium-treated samples 2 and 8, and the energy dispersive X-ray spectroscopy (EDX) studies revealed the presence of cadmium and nickel in both the isolates after metal treatment. The results conclude that the identified heavy metal tolerant bacteria could be useful for the bioremediation of contaminated wastewater and industrial effluents.       Key words: Heavy metal removal, gram-negative bacteria, 16S rRNA gene sequencing, maximum tolerance concentration, bioremediation.

Characterization of heavy metal (cadmium and nickle) tolerant Gram negative enteric bacteria from polluted Yamuna River, Delhi