The finding that Escherichia coli can grow in water with low carbon concentration and compete with natural microbiota has challenged the applicability of this bacterium as an indicator for recent fecal contamination. With molecular tests E. coli can be frequently detected in drinking water biofilm but not in the water. At the same time the water usually complies with all quality standards. These observations enhance the questions about true fate of these bacteria in drinking water supply systems. The aim of this work was to determine if indigenous (naturally found) E. coli are able to colonize drinking water biofilm and show response to favourable growth conditions. Therefore, no inoculation of culture-isolated E. coli was performed throughout the studies. 5 week long monitoring on concentration dynamics was performed in fu ll scale drinking water distribution system (artificially recharged groundwater, 28 hours of residence time, < 0.2 mg/L free available chlorine) and showed that even if no cultivable E. coli are found in the biofilm, as many as 240 E. coli per cm2 can be detected with Fluorescent in situ hybridization. Subsequent increase in the nutrient availability (AOC ~ 500 μg/L), increased E. coli concentration in the biofilm for more than 30 times. The results showed a trend of E. coli accumulation in the biofilm of a water supply system and that the addition of nutrients (often accounted in biologically unstable water) significantly increase E. coli concentration.