Numbers and transported state of Escherichia coli in runoff direct from fresh cowpats under simulated rainfall.

AIMS To investigate the number of Escherichia coli in runoff derived directly from fresh cowpats and to determine if the E. coli are attached to dense particles, in flocs or as individual cells. METHODS AND RESULTS Three cowpats were collected monthly from the same farm for 13 months and the number of E. coli in them estimated. A rainfall simulator was used to generate runoff from the individual cowpats, which was fractioned to determine the transported state of any E. coli present. The number of E. coli in the cowpat runoff was highly variable and was strongly correlated with the number of E. coli in the cowpat. Only a small percentage (approx. 8%) of the E. coli in runoff were attached to dense (>1.3 g ml(-1)) particles and there was no evidence of flocculation of the cells. CONCLUSIONS Escherichia coli in runoff from cowpats are transported predominantly as individual cells. SIGNIFICANCE AND IMPACT OF THE STUDY Mitigation strategies to reduce the number of faecal bacteria in overland flow from agricultural land need to be designed to trap single bacterial cells.