The fate of Escherichia coli O157 in soil and its potential to contaminate drinking water.

The survival and transport of Escherichia coli and E. coli O157 after cattle slurry application were studied on drained plots in both grassland and arable stubble at three sites in Scotland. Leaching losses were between 0.2% and 10% of total E. coli and were dependent on rainfall. Recovery of E. coli in grass and soil declined with approximately first order kinetics. Residual numbers, in excess of background declined more slowly. The pattern was similar for both grass and arable plots. Laboratory incubations of soil cores, with applied slurry containing E. coli and E. coli O157 were performed in soils with different moisture contents at two temperatures for clay loam and sandy loam soils. Both E. coli populations were measured over a 4-week period. Using a dual population approach, the die off of the susceptible pool was linear with a half-life of 3-4 days, and was faster at the higher temperature and lowest moisture content. The resistant pool was not strongly affected by temperature or moisture and had a half-life for die off of between 18 and 24 days. After a 4-week period, < 100 cfu g/soil of E. coli and E. coli O157 remained. The die off rate of E. coli O157 was the same or slightly faster than that of the commensal E. coli population, indicating that the field behaviour of E. coli O157 can be studied by monitoring the total population of E. coli applied with slurry. The risk of significant pollution of water by E. coli is highest immediately after application of slurry, and the first increments of drainflow carry significant concentrations. Thereafter, the risk of pollution is very low. If weather conditions are dry after application on well-drained sandy soils, it is unlikely that any significant losses of organisms to drains will occur. Such data can be used to control and minimise the risk of E. coli O157 contaminating drinking water.