Introduction The contamination of drinking water sources with arsenic (As) poses a potential threat to human health 1 . Inorganic As, including the highly toxic trivalent form [arsenite; As(III)] and less toxic pentavalent As [arsenate; As(V)], is associated with increased cancer risk in a number of geographic areas 2 . The toxicity of As is attributed to the substitution of As (V) for phosphate, affinity of As (III) for protein thiol groups, and protein-DNA and DNA- DNA cross-linking 3 . Many bacteria exhibit resistance to lethal concentrations of arsenic (greater than 5 mM sodium arsenite) 4 , yet little is known about the genetics involved in As resistance in environmental bacteria. Plasmids have been detected in Rhodococcus sp., Acidiphilium multivorum, Staphylococcus aureus exhibiting high levels of resistance to arsenic species 5-7 . The most well- characterized genetic system for resistance to arsenicals is known as the ars operon 8 . According to Environment Protection Agency (EPA), Escherichia coli is used as indicator bacteria due to its ability to grow and persist in natural tropical environments to check the level of contamination and predict potential risks to human health 9 . This organism develops some complex mechanism of resistance in tropical environment for survival and adaptation. Some environmental strains of E. coli can change their properties according to different environmental signals such as fluctuation of temperature, pH, osmolarity etc. because they have the ability to carry some genes specific for regulatory proteins and mobile genetic elements such as plasmid and transposon 10 . In the well-studied ars-containing plasmid R773 (491 kbp), isolated from E. coli, the operon consists of five genes that are controlled from a single promoter located upstream of the first cistron (arsR) 11 . These cistrons, arsRDABC (in that order), encode an arsenic-inducible repressor (arsR), a negative regulatory protein that controls the upper level of transcription (arsD), an ATPase plus membrane-located arsenite efflux pump (arsA and arsB, respectively) and an arsenate reductase (arsC) 12 . Therefore, the central hypothesis of the present study is that E. coli adapted in arsenic polluted environment of Bangladesh might have developed resistance to arsenic and might possess inherent mechanism to resist this toxic metalloid. Materials and Methods Isolation of arsenite resistant presumptive Escherichia coli Ground water sample (SNGW-1) was collected from the Tubewell (90 m depth) of Madhabpur Union Parishad under Singair Upazilla Manikganj District of Bangladesh. A 100 μl of sample was directly spread onto minimal salts enrichment agar [2% (w/v)] medium described previously 13 supplemented with sodium arsenite, NaAsO 2 (2 mM). From 17 colonies, Gram-negative colonies were selected from the arsenite supplemented media and subcultured onto MacConkey agar to differentiate lactose fermenter from non- lactose fermenter. Lactose fermenting pink colonies from MacConkey agar was further subcultured on eosine methylene blue (EMB) agar to detect the presence of presumptive Escherichia coli in our studied sample. One of the isolate Sn26 was selected for further investigation. Characterization of Arsenite Resistant Escherichia coli Isolated from Tubewell Water of Singair, Manikganj, Bangladesh Santonu Kumar Sanyal, Ram Prosad Chakrabarty, M Anwar Hossain and Munawar Sultana * Department of Microbiology, University of Dhaka, Dhaka 1000, Bangladesh Arsenic is a toxic metalloid present in the natural environment and arsenite is more toxic form than arsenate. Bacterial species have evolved multiple defense mechanisms to tolerate and transform the toxic forms of arsenic. In this study, arsenic contaminated tubewell water (90 meter depth with arsenic concentration of 47 μg/l) was collected from Singair Upazilla under Manikganj District in Bangladesh. Among a total of 17 arsenite resistant bacteria, a lactose fermenting isolate Sn26 showed high resistance to sodium arsenite up to 25 mM under our laboratory conditions. Sn26 possesses arsB gene conferring arsenite transporter pump mediated arsenic resistance, confirmed by PCR and sequencing. PCR and sequencing of 16S rRNA gene (approximately 1450 bp) confirmed Sn26 isolate as Escherichia coli. The isolate contained a large plasmid of ~34 kb. It is necessary to analyze further the affiliation of this single plasmid with high arsenite tolerance. Keywords: Escherichia coli, arsB gene, Plasmid, Arsenite resistance Original Article Bangladesh J Microbiol, Volume 31, Number 1&2, June-December 2014, pp 13-17 *Corresponding author: Munawar Sultana, Department of Microbiology, University of Dhaka, Dhaka 1000, Bangladesh Tel: +880 (02) 9661920; E-mail: munawar@du.ac.bd