Long-term impacts of untreated dairy manure on the microbiome and Shiga toxin-producing persistence in agricultural soil.

Biological soil amendments of animal origin (BSAAO) improve the soil health of agricultural fields for plant growth. However, as natural reservoirs for bacterial foodborne pathogens, BSAAO application can introduce and support microbes of public health concern, such as pathogenic Shiga toxin-producing (STEC), in agricultural soils. Using shotgun metagenomic sequencing, this project investigated the microbiome of soil with and without BSAAO, focusing on STEC and the population over time alongside changes in the soil microbiome and soil abiotic properties. Two farms in Ohio, one using an untreated dairy manure amendment and one that does not use a BSAAO, were sampled for over a year for metagenomic analysis of the soil microbiome. All manure samples were positive for genes, indicating the presence of STEC. Impacts of the manure on the soil lasted four weeks by several measures including higher diversity and more frequent STEC detection. Outside of these four weeks post-amendment, Shiga toxin genes () were identified periodically in both fields throughout the year. STEC detection significantly correlated with higher O-serogroup diversity, as well as lower soil cation exchange capacity and concentrations of calcium, magnesium, and organic nitrogen. Differential abundance analysis of the soil metagenomes identified several taxa influenced by amendment but did not identify taxa correlated with detection of genes. This work provides insights into the timing of and ecological factors associated with STEC persistence in the agricultural environment.IMPORTANCEShiga toxin-producing (STEC), including O157:H7, is a major etiological agent of foodborne human disease outbreaks associated with fresh produce and can be transferred to produce via contaminated agricultural soil. Given the devastating impacts of foodborne STEC outbreaks on public health and growers, it is necessary to understand the longevity of the impacts of manure application on the pathogen risk in the soil as well as better understand the ecological and environmental conditions that contribute to STEC survival in the agricultural soil environment. This work expands upon the knowledge of conditions that support STEC persistence in the produce-growing environment and its longevity following amendment in commercial fields with naturally occurring STEC contamination.