Survival and Persistence of Nonpathogenic Escherichia coli and Attenuated Escherichia coli O157:H7 in Soils Amended with Animal Manure in a Greenhouse Environment

What is it about?

Biological soil amendments (BSA`s), including manure from horses, dairy or beef cattle, and poultry litter, provide numerous benefits in conventional and organic agriculture but may also introduce enteric pathogens to the food supply. The benefits of using BSA’s must be balanced against the potential risks associated with use of raw or inadequately treated manure because of the potential transfer of fecal pathogens from soil to growing fruits and vegetables and possible resulting microbial contamination contributing to human illness. Proposed “Standards for the Growing, Harvesting, Packing and Holding of Produce for Human Consumption - Supplemental to the Proposed Rule” from the U.S. Food and Drug Administration (FDA) propose either a 90- or 120-day intervals between application of manure to the field and the harvest of crops, with direct contact between the crop and the soil requiring the longer interval, to minimize the risk of contamination of fruits and vegetables consumed raw. Exceptions to this 90/120 day interval period require may be obtained from FDA if scientific data documenting efficacy of the proposed change or showing that public health protection provided by the change is equivalent to the minimum interval needed for the application of untreated BSAs. Performing bacterial persistence studies with non-pathogenic surrogates in different soils amended with different sources of BSAs in greenhouses could be a practical and economical option that could assist researchers and growers in complying with regulatory requirements when seeking exceptions. Assessments of the effect of specific parameters (soil type, manure type, pot size) used in greenhouse studies on the survival of bacterial pathogens from manure currently are not well documented. This research report comprises three separate, replicated, greenhouse experiments focused on the impact of two different sizes (small and large) of containers (pots) on the survival of non-pathogenic E. coli (gEc) and attenuated E. coli O157:H7 (attO157) in three different soils types: sandy loam (SL), silt loam soil (SIL), and clay loam (CL); along with four manure amendments, either unamended (UN) or amended with poultry litter (PL), dairy manure liquid (DML), or horse manure (HM), in small (398 cm3) and large (2552.5 cm3) pots. In the small pot study (study 1), results show that soil types (SL and CL) did not influence the survival of E. coli in manure-amended soils, but the PL amendment enhanced survival of all E. coli compared to their survival in unamended and dairy manure liquid-amended soils at a statistically significant level. In the large pot study (study 2), unlike the small pot study, there were significant differences in the survival of gEc or attO157 populations in unamended SIL compared to unamended CL soils. However, as in study 1, gEc and attO157 populations in large pots survived at higher populations in PL-amended soils compared to HM- or UN-amended soils. In study 2, all E. coli populations in all manure-amended soils were significantly lower by week 8 than in week 1, indicating that E. coli populations declined over time in large pots. In a comparison between the small and large pot study (study 3), the effect of manure amendments was the same: PL-amended soils supported higher populations of E. coli then HM- or UN-amended soils in both small and large pots, and in most cases both attO257 and gEC populations were significantly lower by week 8 than in week 1. At the end of the comparative study (week 8), there were no differences in the populations of gEC in small pots compared to large pots, and no differences were observed in gEC survival in clay loam and silt loam soils in this study. However, attO157 populations in small pots were significantly greater than those in large pots during this study, perhaps due to the greater moisture availability in the small pots, which had limited drainage compared to the large pots. Clay loam soils supported higher populations of attO157 than silt loam soils as well. Intrinsic characteristics of the attenuated E. coli O157:H7 strains may make this population more sensitive to moisture fluctuations than the non-pathogenic E. coli strains used, and also may have contributed to the differences in survival of attO157 and gEC observed between small and large pots and soil types. This report represents the first published experiments comparing bacterial survival and persistence in manure-amended soils in various size containers in greenhouse studies with different soil types. The results shows that small pots and large pots can consistently assess the effect of manure-amendments on E. coli populations, but results from small and large pot studies may be dependent on the bacterial strains used in the study. These findings provide a unique contribution to development of guidelines for approaches to container studies conducted in greenhouses to meet FDA regulatory requirements for exceptions to standard practices relative to biological soil amendments and fresh produce safety.

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