A facultative oligotrophic strain from the water sample of River Mahananda, Siliguri India was selected for its property to produce exopolysaccharide (EPS) in nutrient-poor (oligotrophic) medium. Viability assay of the strain was performed in sterile liquid LB, R2A, river water and diluted (10(-3)) LB at 30 degrees C and pH 7 to understand oligotrophy. The selected strain was identified by 16S rRNA gene sequencing and designated as Klabsiella sp. PB12. Phylogenetic analysis showed its closest relationship with Klebsiella variicola ATCC BAA-830(T). Purification of EPS was performed by ethanol precipitation, dialysis and freeze-drying. Chemical analysis revealed that purified EPS was mainly composed of 72.32% (w/w) neutral sugar and 14.12% (w/w) uronic acids. Fourier transform infrared (FT-IR) spectroscopy indicated the presence of hydroxyl, carboxylic and methoxyl functional groups. The optimal dosages for flocculation of activated carbon suspension were 17 mg/l EPS and 4 mM CaCl2. EPS showed flocculating rate of above 80% over a wide range of pH (pH 3-10) whereas, more than 90% rate was noted in the temperature range (10-50 degrees C) tested in presence of CaCl2. Moreover, EPS showed characteristic emulsifying activity with toluene (66.6%), n-hexadecane (65%), olive oil (633%) and kerosene (50%). The apparent molecular weight of the EPS was similar to 2 x 10(5) Da. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.