Population Dynamics of Generic E. coli and Naturally Occurring Listeria in Manure-Amended Soils in the Northeastern U.S

Introduction: Proposed U.S. FDA standards for use of manure as a biological soil amendment originally stipulated a 9-month waiting period between soil application and harvest to reduce the risk of pathogen contamination on fresh produce. Survival of bacterial pathogens and concomitant risk of produce contamination are impacted by many factors including climate, manure source, soil type, geographical location and bacterial populations.

Purpose: To compare population dynamics of non-pathogenic E. coli (gEc) and Listeria spp. in surface and tilled plots in two soil types amended with untreated dairy manure in the northeastern U.S.

Methods: A three-strain inoculum of rifampicin-resistant gEc (6 log CFU/ml) was sprayed onto field plots (2 m²) of either Loamy sand (L) or Sandy (S) soils amended with dairy-manure solids (DS) or no manure (NM). Manure was tilled into the soil in half the plots. Survival of inoculated gEc and native E. coli in DS in surface and tilled plots was determined over 56 days post-inoculation (dpi) by colony count or MPN.  The presence of Listeria spp. was assayed in selected plots.

Results: gEc and native E. coli populations in tilled L plots declined less slowly ( by 1.1 - 2.7 log MPN/gdw) compared to in surface L plots ( by 4.0 - 4.3 log MPN/gdw) at 56 dpi, . However, gEc populations increased in tilled S plots between 28 and 56 dpi while gEc populations declined ( 3.4 - 4.0 log MPN/gdw) in surface S plots.  In general, gEC and native E. coli populations increased after rain events between 0 - 14 dpi regardless of soil type or manure application. Listeria spp. was present in uninoculated NM and DS plots.

Significance: These results suggest that tillage and soil type may affect the survival of E. coli in soils as much as the presence of manure.