Purpose: The objective of this study is to determine the fate of Shiga toxigenic Escherichia coli (STEC) and generic Escherichia coli in Central Florida agricultural surface water at different temperatures.
Methods: Five rifampicin (Rif) resistant STEC strains (O145, O104, O111, O103, O157) strains and naladixic acid (Nal) resistant generic E. coli (K-12) were prepared as one cocktail and inoculated into 100 ml agricultural water samples from two ponds to a final concentration of 105 CFU/ml. The same inoculation procedure was applied to sterilized surface water and EPA worst case water. Duplicate experiments with triplicate samples were performed at 15 and 25±1°C (n=6). Escherichia coli was enumerated by spread plating onto tryptic soy agar supplemented with Rif and Nal or a three-by-four tube MPN method (when no colonies were observed) for up to 168 days. The effect of protozoans was tested in a 14-day experiment in non-sterile agricultural surface waters with cycloheximide at all temperatures (n=3).
Results: STEC and generic E. coli concentrations decreased 5.4 log CFU/ml, on average, and reached ≤0.5 log CFU/ml in nonsterile surface water at all tested temperatures by day 168 (P>0.05); populations were significantly (P≤0.05) higher in sterile surface water and EPA worst case water at all sampling points beginning at day 28. Addition of cycloheximide to reduce protozoa grazing had no significant effect on behavior of E. coli populations.
Significance: Monitoring changes in the generic E. coli (represented by K-12) population is a reasonable indicator of STEC survival in agricultural water. Population reductions of E. coli in nonsterile agricultural water were not primarily driven by protozoa grazing, but rather by factors not evaluated here.