Diversity of bacterial communities inhabiting soil and groundwater of arsenic contaminated areas in West Bengal, India

Soil and water contaminated with arsenic (As) through natural or anthropogenic inputs are commonly considered as native source of tolerant bacterial strains. The present study was successful in characterizing 12 hyper-tolerant bacteria, satisfying maximum tolerable concentration (MTC) for arsenate (As5+) ae<yen> 300 mM and arsenite (As3+) ae<yen> 30 mM, isolated from As affected North 24 Parganas and South 24 Parganas districts of West Bengal, India. Most of the bacteria showing higher level of tolerance to As5+ and As3+ were found as gram-positive and bacilli in shape. Positive responses to different biochemical tests indicated that some of these bacteria could be potent sources of various biotechnologically important enzymes. Some of the hyper-tolerant bacteria could reduce As5+ to As3+ while all others could oxidise As3+ to As5+. Phylogenetic analysis revealed that those hyper-tolerant bacterial strains were distributed among three phyla such as Actinobacteria, Firmicutes, and gamma-Proteobacteria. The Firmicutes were well represented in this study with more than half of the hyper-tolerant strains corresponding to members of this group. Moreover, majority of the isolates except SR10 belonging to this phylum were affiliated to different species of the genus Bacillus and showed different tolerance capability to As3+ and As5+. We present the first report of the genus Paenibacillus as being involved in arsenite oxidation with hyper-tolerance property to As. Four isolates named as SDe5, SDe12, SDe13, and SDe15 belonging to genera Bacillus and Rhodococcus exhibited highest tolerance to As and therefore represented as good candidates for bioremediation processes of native polluted soil and ground water.