24 per L) were calculated. Furthermore, data of 280,000 water samples collected in France (with disinfectant residual), the Netherlands and Germany (both with and without disinfectant residual) were evaluated for E. coli incidence. In total, similar results were obtained for Germany and the Netherlands. In France, significantly higher incidences occurred as more small rural supply systems were included. The detailed data evaluation revealed a slight increase of mean E. coli concentrations during distribution in Germany and the Netherlands, for both disinfected and non-disinfected supply zones. This suggests that, if technical measures are taken to avoid contamination during distribution, non-disinfected supply zones can be regarded as being as safe as disinfected supply zones. Furthermore, the indicator principle of E. coli for faecal contaminations is valid in non-disinfected supply zones. In chlorinated systems, on-linemonitoring of chlorine residuals represents a good means to detect ingress of organic material.

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Monitoring of microbiological contaminants in water supplies requires fast and sensitive methods for the specific detection of indicator organisms or pathogens. We developed a protocol for the simultaneous detection of E. coli and coliform bacteria based on the Fluorescence in situ Hybridization (FISH) technology. This protocol consists of two approaches. The first allows the direct detection of single E. coli and coliform bacterial cells on the filter membranes. The second approach includes incubation of the filter membranes on a nutrient agar plate and subsequent detection of the grown micro-colonies. Both approaches were validated using drinking water samples spiked with pure cultures and naturally contaminated water samples. The effects of heat, chlorine and UV disinfection were also investigated. The micro-colony approach yielded very good results for all samples and conditions tested, and thus can be thoroughly recommended for usage as an alternative method to detect E. coli and coliform bacteria in water samples. However, during this study, some limitations became visible for the single cell approach. The method cannot be applied for water samples which have been disinfected by UV irradiation. In addition, our results indicated that green fluorescent dyes are not suitable to be used with chlorine disinfected samples.

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