Survival and Persistence of Non-pathogenic Escherichia coli and Escherichia coli O157:H7

Land application of raw animal manure to enhance soil productivity may pose a food safety risk from pathogenic microorganisms that survive and contact fresh produce. Two studies, conducted at the University of Maryland Eastern Shore, evaluated the survival and persistence of non-pathogenic E. coli (Ec) and attenuated E. coli O157:H7 (attO157) as influenced by the application of poultry litter (PL) or dairy manure (DM) on field plots in the Delmarva peninsula and in the greenhouse where two soil types, sandy loam (SL) and clay loam (CL), were used. Field plots were individually amended with or without PL or DM in late fall and received spray inoculum at either low 5x10⁶ CFU/m², or high, 5x10⁸ CFU/ m², cell densities; inocula contained three rifampicin-resistant (Rif^R) strains of Ec and two Rif^R strains of attO157 grown in dairy manure extract. Greenhouse pots (8.89 cm), filled each with 350g of SL or CL soil were amended with or without PL or dairy manure liquid (DML) and received either a low, 1.15x10⁴ CFU/m², or high, 2.07x10⁵ CFU/m², dose of a multi-strain inoculum. Soil samples were collected periodically over 150 days post-inoculation from the field and 56 days post-inoculation from the greenhouse.  All samples were analyzed for viable E. coli by direct plating and/or mini-MPN.  Greenhouse results show that by day 28 at high inoculum densities, Ec counts on DML in SL and CL soil declined to 2.94 and 3.01 log₁₀ CFU/g, respectively, and attO157 counts declined to 2.88 and 3.01 log₁₀ CFU/g, respectively.  By day 56 at high inoculum densities, Ec and attO157 counts on PL in both soils declined to 3.32 and 2.62 log₁₀ CFU/g, respectively. In comparison, by day 30, Ec counts in high inoculum dosed PL and DM field plots declined to 4.6 and 2.43 log₁₀ CFU/g, respectively, and attO157 counts declined to 3.83 and 0.46 log₁₀ CFU/g, respectively.  In both studies the survival of Ec and attO157 populations declined more rapidly in soil amended with DML and DM when compared to soil amended with PL. E. coli survived for longer durations in field plots compared to pots in the greenhouse. Manure type influenced the survival of E. coli in soil, and these findings should be considered when setting guidelines for produce safety relative to PL, DM and DML application to soil. Greenhouse studies may underestimate survival of Ec in manure-amended soils compared to field studies.