Biological soil amendments can support survival of pathogenic and non-pathogenic Escherichia coli in soils and sporadic transfer to Romaine lettuce

Authors: XIONG, ZIRUI - University Of Delaware; GABRIEL, ELLEN - Oak Ridge Institute For Science And Education (ORISE); GUTIERREZ, ALAN - Oak Ridge Institute For Science And Education (ORISE); East, Cheryl; KNIEL, KALMIA - University Of Delaware; DANYLUK, MICHELLE - University Of Florida; JAY-RUSSELL, MICHELE - University Of California, Davis; Sharma, Manan

Submitted to: International Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2025
Publication Date: 3/7/2025
Citation: Xiong, Z.R., Gabriel, E., Gutierrez, A., East, C.L., Kniel, K.E., Danyluk, M.D., Jay-Russell, M., Sharma, M. 2025. Biological soil amendments can support survival of pathogenic and non-pathogenic Escherichia coli in soils and sporadic transfer to Romaine lettuce. International Journal of Microbiology. 434. Article e111147. https://doi.org/10.1016/j.ijfoodmicro.2025.111147.
DOI: https://doi.org/10.1016/j.ijfoodmicro.2025.111147

Interpretive Summary: Treated biological soil amendments (BSAs) can be applied in large quantities while organic lettuce plants are actively growing, also known as “side-dressing”. Side-dressing with treated BSAs is common for organic lettuce growers. The investigation of lettuce outbreaks caused by E. coli O157:H7 prioritizes routes of contamination and survival of the pathogen in pre-harvest agricultural environments. Work completed here shows that treated BSAs used in side-dressing can support but do not enhance survival of non-pathogenic and pathogenic E. coli in soils. Transfer of E. coli from soils to lettuce was low and sporadic, indicating that wide-spread contamination of lettuce with E. coli O157:H7 may not be due to side-dressing with treated BSAs. This work helps farmers better understand the risks of common agronomic practices used in growing organic leafy green crops.

Technical Abstract: Biological soil amendments (BSA), including those of plant and animal origin, are essential agricultural inputs in organic leafy green production, providing critical nutrients such as nitrogen and phosphorus. However, untreated BSAs can introduce or transfer pathogens like Escherichia coli O157:H7 to lettuce in pre-harvest environments. Organic leafy green growers have been shifting away from untreated BSAs towards treated pelletized manure-based products, such as heat-treated poultry pellets (HTPP) and seabird guano (SBG), because of the reduced risk of microbial contamination and increased nutrient stabilization. Risks associated with side-dressing with treated BSAs have not been evaluated. Romaine lettuce was grown in soil with controlled light, temperature, and relative humidity in an environmental growth chamber. Soils were side dressed twice with different combinations of three types of treated BSAs: HTPP, SBG, and corn steep liquor (CSL). An unamended soil treatment was also included. Soils were co-inoculated with non-pathogenic, rifampicin-resistant E. coli and two chloramphenicol-resistant E. coli O157:H7 strains at the second of two side-dressing events. Survival of E. coli in soils over 28 days was evaluated. On day 28 post inoculation, two heads of Romaine lettuce from each planter were harvested, and the presence of E. coli on lettuce leaves was determined. Four nonlinear statistical models were fit to survival data of E. coli in soils. In all soils regardless of BSA treatment, E. coli TVS 353 declined by 4.08-4.51 log CFU/g soil over 28 days, and E. coli O157:H7 declined by 2.77-4.3 log CFU/g soil over 28 days. E. coli TVS 353 and O157:H7 were recovered from 13.3% (6/45) and 11.1% (5/45) of plants, respectively. The transfer of E. coli from soils to Romaine lettuce was low, sporadic, and could not be measured quantitatively. Treated BSAs used in organic lettuce production supported but did not enhance survival of E. coli in side-dressed soils.