Survival of Non-pathogenic Escherichia coli and Escherichia coli O157:H7 in Delmarva Field Plots Amended with Animal Manure

Introduction: 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 grown on the soils.  The presence of pathogens in fresh produce has resulted in several outbreaks of human gastroenteritis that have been linked to the consumption of contaminated fresh produce.

Purpose: Evaluate and compare the survival and persistence of a multi-strain cocktail 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.

Methods: A field experiment comprising eight treatments with four replications each was conducted in Othello soil at the University of Maryland Eastern Shore Ag Research Farm.  Soil plots were individually amended with or without PL or DM in late fall.  Individual plots, except controls, 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.  Soil samples collected for 30 days post-inoculation were analyzed for viable E. coli by direct plating and/or mini-MPN.

Results: Survival of Ec and attO157 populations declined more rapidly in DM compared to PL treatments. By 30 days, at high inoculum densities, Ec counts on PL and DM declined to 4.6 and 2.43 log CFU/g, respectively, and attO157 counts declined to 3.83 and 0.46 log CFU/g, respectively.  No Ec or attO157 were found in control soil.

Significance: Manure type influenced survival of E. coli in soil, with attO157 declining more rapidly than Ec.  These findings should be taken into consideration when setting guidelines for fresh produce safety relative to PL and DM application to soil.