Validation and use of the dead-end ultrafiltration method for the capture and recovery of Shiga toxin-producing Escherichia coli from surface water.

Root cause analysis following microbial water contamination events typically relies upon the collection of large sample volumes. Dead-end ultrafiltration (DEUF) is an efficient method for sampling and transporting up to 100 L of water from field environments. A single-lab validation (SLV) using DEUF to recover Shiga toxin-producing Escherichia coli (STEC) was recently completed using 100 L surface water samples ranging in turbidity from 3.7 to 12.8 NTU (nephelometric turbidity unit). The SLV trials using E. coli O157:H7 and O121:H19 indicated that a positive recovery rate (25%-85%) from eluates was achievable after inoculating ultrafilters with low-level contamination concentrations of ≤6 CFU/filter (where CFU is colony forming unit). Higher inoculum levels, >27 CFU/filter, resulted in positive detection of STEC in all eluates. To assess interchangeability of ultrafilter brands, three different products with similar fiber and pore size construction were evaluated. Equivalency was demonstrated amongst the brands tested, with no discernible difference noted in STEC detection and recovery. Since processing delays can occur, stability of E. coli O157:H7 in ultrafilters and ultrafilter backflush eluates (UBEs) was examined. Processing of ultrafilters held at 4°C for up to 6 days did not have substantial effects on detection and recovery of E. coli O157:H7. Stored UBE yielded an overall reduction of 0.72 log MPN mL (where MPN is most probable number) from Day 0 to Day 6 and an average estimated reduction of 0.07 log MPN mL (95% CI: 0.019-0.125) per day. The effect of days of UBE storage was statistically significant, supporting a recommendation to store UBE for the least number of days possible.