Escherichia coli O157:H7 in drinking water from private water supplies in the Netherlands

Schets, F.M., During, M., Italiaander, R., Heijnen, L., Rutjes, S.A., van der Zwaluw, W.K. and de Roda Husman, A.M. (2005) Water Research, 39 (18); 4485-93.

Escherichia coli O157:H7 is a known cause of human disease and has become increasingly recognised as a health problem. Clinical manifestations can range from asymptomatic excretion to mild non-bloody diarrhoea to hemorrhagic colitis, and to severe complications such as haemolytic uremic syndrome (HUS) with acute renal failure sometimes resulting in death. Those at particular risk of HUS are children under 5 years, pregnant women and the elderly.

The main reservoir of E. coli O157 is ruminants, particularly cattle. Water supplies can be potentially contaminated with E. coli O157 when farmed or wild animals graze in water catchment areas. The microbiological quality of drinking water from private water supplies in the Netherlands was tested to examine compliance with Dutch drinking water legislation (with respect to faecal indicators identical to the European Drinking Water Directive 98/83/EG) and the presence of total coliforms, E. coli and enterococci were studied as well as the occurrence of E. coli O157.

Samples of drinking water from the consumer's tap were taken from 144 private water supplies located at camp-sites, offices, hospitals and breweries throughout the Netherlands during the summers of 2002 and 2003. All supplies tested were from groundwater sources. Standard membrane filtration methods were used to enumerate faecal indicators. A specific enrichment method was used to determine the presence of E. coli O157.

There were 147 samples analysed. Most of the samples (86.4%) did not contain intestinal enterococci, total coliform bacteria, E. coli or E. coli O157. However 10.9% of drinking water samples contained one or more of the faecal indicators total coliforms, E. coli and intestinal enterococci and therefore did not comply with Dutch legislation requiring the absence of faecal indicators, in 100ml samples. For 50% of the positive water supplies no water treatment was normally used by the householders. There were 4 private water supplies (2.7%) in which E. coli O157:H7 was detected, the water from these supplies otherwise met microbiological water quality standards (ie negative for total coliforms and E. coli in 100ml sample).

All of the E. coli O157 strains were isolated from private water supplies on camp-sites in agricultural areas with average to large densities of dairy herds and other grazing animals. Pulsed field gel electrophoresis analysis of bacterial isolates indicated that they were similar to strains previously isolated from cattle in the Netherlands . No testing was performed on animal herds for the current study.

The usefulness of water quality testing of these private water supplies is confirmed by this study. It was found that routine monitoring of standard 100ml sample volumes with standard membrane filtration methods for total coliforms and E. coli that is required by European drinking water legislation may not elucidate the presence of E. coli O157. When pathogens such as E. coli O157 are found in drinking water there may be resulting health consequences, however a risk assessment needs to be undertaken as the monitoring of both faecal indicator parameters and pathogens does not predict the health effects of microbiologically contaminated drinking water on a population.

Comment The authors note that differences in sample volume (100ml for standard E. coli versus 100ml and 1000 ml for E. coli O15) and test methods (membrane filtration + plating for the standard E. coli test versus membrane filtration + enrichment culture + immunomagnetic separation + plating for E. coli O157) may account for discrepancies in the results. There was only one sample where a confirmed E. coli O157 was detected in a 100ml volume. The rate of positive detections varied on different growth media.