Survival of Generic E. coli and Naturally Occurring Listeria spp. in Manure-amended in Loamy and Sandy Soils in the Northeastern United States

Introduction: Enterohemorrhagic Escherichia coli and Listeria monocytogenes can contaminate leafy greens through inappropriately managed raw manure applied to soils.  Current FDA regulations call for a 90-120 day interval between application of manure and harvest of crops grown in manure-amended soils.

Purpose: To determine if addition of dairy-manure solids (DS) affects persistence of E. coli and Listeria spp. in soils.

Methods: A three-strain inoculum (TVS353,TVS354, and TVS355) of rifampicin-resistant E. coli (rEc) (6 log CFU/ml) was sprayed onto field plots (2m²) of either loamy sand (L) or sandy (S) soils amended with DS or no manure (NM). Manure was either tilled into soil or spread on the surface, and each plot was seeded with spinach (‘Space’ variety). Survival of inoculated rEc, naturally present E. coli (nEC) and Listeria spp. in DS in untilled and tilled plots were quantified over 80 days-post-inoculation (dpi) by colony count or MPN.

Results: By 80 dpi, in untilled plots, rEc populations declined by 2.6 – 3.9 and 2.4 – 2.7 log CFU/g in NM and DS plots, respectively. In tilled plots, rEc populations declined by 0.9 – 1.1 log CFU/g and1.1 – 1.2 log CFU/g in NM and DS plots, respectively, at 80 dpi. Listeria spp. populations declined by 0.62 - 1.3 log CFU/g in tilled DS plots, and by 1.2- 2.6 log CFU/g in untilled NM by 80 dpi. In tilled DS plots, nEC populations declined by 0.6 – 0.7 log CFU/g by 28 dpi, compared to declines of rEC of 0.2 – 0.3 log CFU/g by 28 dpi. BOX-PCR-typing revealed that 72.5% of the rEc isolates recovered at 65 dpi from the rhizosphere, were strain TVS355. 

Significance: Tilling, with or without DS, slowed declines of rEc populations in soils. However, the presence of DS slowed the decline of Listeria spp. in tilled soils. Population declines of E. coli and Listeria spp. were not affected by soil type.