Persistence of Shiga Toxin-Producing Escherichia coli in Agricultural Soils after Application of Dairy Manure
Application of bovine manure to agricultural soils is a vital part of crop nutrient management, but carries the risk of introducing enteric pathogens that can contaminate fresh produce with Shiga-toxin producing E. coli (STEC) if manure is not properly managed. In Canada, the 120-day post-manure interval (PMI) ensures the reduction of common pathogens to safe levels prior to harvest of fresh produce. This practice reduces O157:H7 STEC, but manure management regulations need further examination to determine whether they adequately prevent fresh produce contamination by non-O157 STEC. This thesis investigated the die-off and persistence of shiga toxin-producing E. coli (STEC) O45, O111, O145, strains in comparison to O157:H7 strain, in order to verify non-O157 STEC under the 120 day PMI in soil amended with dairy manure. This study used 1 L microcosms under controlled conditions (moisture content 60 % field capacity, 66 % humidity, 22-25 °C, and 12-hour photoperiod) to assess the impact of dairy manure amendments (no amendment, liquid dairy manure, anaerobically digested dairy manure, sterilized liquid dairy manure at 5.61 L m-2) on O111:NM and O157:H7 STEC inoculated in soil mix. Die-off was independent of initial inoculum levels and O111:NM and O157:H7 persisted at similar levels after 7 days and were undetectable after 28 days. Differences in levels of NO3--N, NH4+-N, and total bacterial load did not impact die-off and persistence between treatment combinations. Overall, O111:NM and O157:NM reacted similarly under controlled conditions. In order to study STEC under field conditions a filter bag biocontainment system was developed to compare the persistence of emerging strains O45:H2, O111:NM, and O121:H19 STEC to O157:H7 STEC in soil after the application of raw dairy manure (5.61 L m-2). Under field conditions, all STEC serotypes tested survived above detection levels (475 CFU g-1 dry soil) for at least 63 days. The ability of O45:H2, O111:NM, and O121:H19 strains to persist at similar levels to O157:H7, suggests that they could be treated similarly under the 120-day PMI. Verifying this regulation would give consumers and producers more confidence in the current food safety regulations and their ability to ensure safe produce for consumption.