Persistence of Escherichia coli in the soil of an organic mixed crop-livestock farm that integrates sheep grazing within vegetable fields.

Mixed crop-livestock farms (MCLF) integrate livestock and crops using their animals to graze crop residues and/or cover crops. MCLF are considered sustainable because grazing and the manure deposited by livestock enhance soil fertility and recycles farm nutrients. However, livestock manure may introduce enteric foodborne pathogens to the soil, which could contaminate fresh produce. Organic farmers in the United States follow the USDA National Organic Program (NOP) standards, which require 90 or 120 days between incorporating raw manure into the soil and harvest. Although not specifically addressed in NOP, organic farmers using grazing within production fields may also use this standard. The objectives of this study were to generate preharvest data to assess the die-off of generic Escherichia coli (E. coli) in the soil, after cover crops were grazed by sheep; and assess the genetic relatedness of generic E. coli isolates between soil and sheep faecal samples. We conducted a repeated observational study to evaluate the persistence of generic E. coli, as an indicator of faecal contamination and surrogate for STEC, in the soil of two fields (A and B) on an organic MCLF. Results showed a 3.70 log10 reduction in mean generic E. coli concentration MPN in the soil of field A from the highest of 3.70 log10 MPN/g on 48 day postsheep grazing (DPS) to -0.70 log10 MPN/g on 139 DPS. Field B showed a 3.51 log10 reduction in mean generic E. coli concentration in the soil from the highest mean of 3.51 log10 MPN/g on 14 DPS to the lowest mean -0.35 log10 MPN/g on 112 DPS. STEC prevalence in the sheep flock was 4.17% (1/24). Closely related generic E. coli strains were found between soil and faecal samples. Developing research-based waiting periods between grazing and harvest is important to inform best practices for farmers and food safety regulators.