Mixed cultures obtained from a full scale activated sludge system treating wastewater generating from Siriraj Hospital, Bangkok, Thailand were isolated and purified by repeatedly transferring the cells to fresh medium [3]. The different colonies morphology were isolated by picking a single colony and inoculated on thiosulfate agar by a streak plate technique. Its sulfur oxidation activity was tested prior to strain identification. The pure isolate was stored in 15% glycerol at -20 °C. Prior to use, it was activated by culturing in 200 ml thiosulfate mineral medium (TMN) at 30 °C, 180 rpm and transferred 10 % v/v to fresh medium every 5-7 days.