Escherichia coli O157:H7 Survives Longer than Fecal Indicator Bacteria in a Pond in an Agricultural Watershed.

The USEPA uses concentrations of fecal indicator bacteria, Esherichia coli and fecal enterococci, in surface waters to indicate the potential presence of pathogens such as E. coli O157:H7. The connection between concentrations of the indicator bacteria and actual bacterial pathogens in surface waters is based on the assumption that indicators and the pathogens behave similarly.  We tested whether death rates under natural conditions of a pond in an agricultural watershed were similar for indicator bacteria and E. coli O157:H7.  Known numbers of E. coli, Enterococcus faecium, Enterococcus gallinarum, and E. coli O157:H7 were inoculated into (i) raw pond water and (ii) filtered pond water (to eliminate protozoa predators), and 10-ml aliquots of each inoculated waters were pipetted into sentinel chambers with a 0.45µm-pore-size membrane secured at both ends of the chamber to allow equilibration with the exterior environment. Chambers were secured to a floating frame, and placed in the pond at a depth of 50 cm (the extinction depth of UV irradiation). Triplicate chambers were recovered at days 0, 1, 3, 7, 14, 25, and 33, and assayed with IDEXX (Colilert and Enterolert reagents) for indicator bacteria. Two of three chambers were assayed for E. coli O157:H7 with a combined MPN enrichment format and TaqMan assay specific for the virulence gene eaeA.  Preliminary results indicated that fecal enterococci died off quicker than E. coli, and E. coli O157:H7 survived longer than both indicator bacteria.  These results demonstrated differential rates of inactivation between indicator bacteria and E. coli O157:H7, and underscores the need to develop methods for detecting and enumerating the pathogens themselves.