Creek to Table: Tracking fecal indicator bacteria, bacterial pathogens, and total bacterial communities from irrigation water to kale and radish crops.

The impact of microbially contaminated irrigation water on risks to produce safety and public health is a complex issue that is not well understood. This study tracked fecal indicators, pathogenic bacteria, and total bacterial communities from a creek water irrigation source to irrigated produce to assess the impact of irrigation events on soil and produce-associated microbiota. Kale and radishes were drip-irrigated using Mid-Atlantic creek water in October 2017. Plant and soil samples were collected immediately before and after irrigation, and for 3 consecutive days thereafter. All samples (n = 134), including irrigation water, were tested for generic Escherichia coli and total coliforms (TC) using standard membrane filtration or direct plating, and for Salmonella enterica and Listeria monocytogenes by selective enrichment. DNA extracted from all samples was PCR-amplified for the V3-V4 region of the 16S rRNA gene for bacterial community profiling. In soil, TC levels were significantly higher immediately and 3 days post-irrigation compared to pre-irrigation (p < 0.01). E. coli levels in soil increased after irrigation, but the difference was not significant (p = 0.31), and die-off was not observed. No E. coli were detected on kale leaves. TC increased over the study period on radish roots (p < 0.01) but not kale leaves (p = 0.43). Although target pathogens were detected in irrigation water, S. enterica was detected from only one post-irrigation kale sample and L. monocytogenes was not detected in the field. The 16S rRNA gene sequencing data revealed differences in bacterial community structure and composition across sample types and showed that radish soil and root surface bacterial communities were more strongly influenced by irrigation compared to kale samples. This study provides insights into the impact of irrigation water on fresh produce microbiota, revealing that, although irrigation did influence crop-associated microbiota (especially below ground) in the field, bacterial pathogens were not likely transferred to the crop.