The aim of this study was to develop a rapid method for the specific detection of respiring Escherichia coli (an indicator of fecal contamination) in potable water. Fluorescence in situ hybridization (FISH) with a rRNA-targeted oligonucleotide probe was used to detect E. coli cells and bacterial respiratory activity was estimated using 5-cyano-2,3-ditolyl tetrazolium chloride (CTC). Fluorescent signals from hybridized cells were increased by optimized tyramide signal amplification (TSA). Respiring E. coli in potable ground water with low rRNA content were enumerated within 8 hours using signal-amplified in situ hybridization following formazan reduction (TSA-CTC-FISH), whereas these starved E. coli cells could not be detected by conventional FISH (FISH without signal amplification) which generated weak fluorescence. TSA-CTC-FISH can be used for simultaneous identification in situ based on phylogenetic information and the activity of individual bacterial cells in potable water. This method would be useful in the rapid monitoring of harmful or fecal indicator bacteria in potable water.Key words: potable water, microbial monitoring, fecal indicator, respiratory activity, fluorescence in situ hybridization A continuous supply of microbiologically wholesome potable water is essential for life. While most potable water has been chlorinated for bacterial sterilization, non-chlorinated and dechlorinated waters have also been generally utilized for consumption, such as natural mineral water and tap water filtered through a household water purifier. The numbers of bacterial cells in non-chlorinated water sometimes increase after an environmental change (e.g. after bottling) and storage (19). Therefore, the abundance of bacteria in potable water should be routinely monitored to maintain microbiological quality control, and methods of monitoring harmful or fecal indicator bacteria in potable water are required for microbiological quality assurance. Several methods are currently applied to detect viable bacteria in potable water, and the most popular and simple techniques include culture-based ones. However, the required incubation times can be quite lengthy and the numbers of bacteria can be underestimated because most viable bacteria in oligotrophic environments are difficult to culture under conventional conditions. Therefore, more rapid, sensitive and reliable methods are desirable. Fluorescence staining can quickly detect viable bacteria without the need for culture. For example, carboxyfluorescein diacetate (CFDA) can detect esterase-active bacteria (6, 33) and the dimeric cyanine dye, BOBO-3, can be used to evaluate bacterial viability based on membrane integrity (3, 16). The redox dye, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), is widely used to detect respiring bacteria (7) in freshwater such as groundwater (26) and drinking water (28).However, CTC staining alone cannot reveal phylogenetic information regarding targeted bacteria and several species of harmful bacteria must be detected to ensure the quality of p...