Purpose: To perform an assessment of prevalence and spatial distribution, including the potential risk, based on indicators and molecular evidence, and confirmed presence of Escherichia coli O157:H7, including other Shiga toxin-positive E. coli, and Salmonella enterica, in a cantaloupe field located, at one edge, in immediate proximity to a dairy.
Methods: Evaluation of total coliforms, fecal indicators and detection of molecular markers for human pathogens was performed approximately 2 weeks before the estimated harvest date, at commercial maturity, and after field discing on fruit, soil and environmental samples including irrigation water, manure, and bedding.
Results: Quantification of total coliforms on melons provided evidence of greater populations in areas adjacent to animal corrals that corresponded with elevated soil organic matter and nitrogen content. Detection of the fecal indicators Enterococcus and E. coli was encountered in diverse environmental samples across substantial spatial distances and all time points. Molecular markers for E. coli O157:H7 and pathogenic E. coli, particularly eae and stx2 genes were observed on multiple dates. E. coli O157:H7 (stx1-) was isolated and confirmed from fruit and soil. Detection of pathogenic E. coli and Salmonella markers was positive on the harvest-decision date and the crop destroyed. After discing of the field, total coliforms and E. coli increased 2-3 log CFU/100 g along the entire field. Detection of pathogenic E. coli markers (eaeA and stx2) was still positive and broadly distributed. Following planting, growth and incorporation of a monocot cover crop the E. colipopulations and pathogen virulence markers were no longer detectable.
Significance: Proximity to animal operations can significantly alter the microbiological quality of adjacent crops and variable factors must be evaluated to establish a scientific basis for recommended buffer setbacks in produce guidance and audit schemes.