Research – Small produce farm environments can harbor diverse Listeria monocytogenes and Listeria spp. populations

Journal of Food Protection

A comprehensive understanding of foodborne pathogen diversity in pre-harvest environments is necessary to effectively track pathogens on farms and identify sources of produce contamination. As such, this study aimed to characterize Listeria diversity in wildlife feces and agricultural water collected from a New York State produce farm over a growing season. Water samples were collected from a pond (N=80) and stream (N=52). Fecal samples (N=77) were opportunistically collected from areas <5m from the water sources; all samples were collected from a <0.5km2 area. Overall, 41% (86/209) and 24% (50/209) of samples were positive for Listeria monocytogenes and Listeria spp. (excluding L. monocytogenes), respectively. For each positive sample, one L. monocytogenes or Listeria spp. isolate was speciated by sequencing the sigB gene, which allowed for additional characterization based on the sigB allelic type (AT). The 86 L. monocytogenes and 50 Listeria spp. isolates represented 8 and 23 different ATs, respectively. A subset of L. monocytogenes isolates (N=44) from pond water and pond-adjacent feces (representing a ~5,000m2 area) were further characterized by PFGE; these 44 isolates represented 22 PFGE types, which is indicative of considerable diversity at a small spatial scale. Ten PFGE types were isolated more than once, suggesting persistence or re-introduction of PFGE types in this area. Given the small spatial scale, the prevalence of L. monocytogenes and Listeria spp., as well as the considerable diversity amongst isolates, suggests traceback investigations may be challenging. For example, traceback of finished product or processing facility contamination with specific subtypes to pre-harvest sources may require collection of large sample sets, and characterization of a considerable number of isolates. Our data also support the adage, “absence of evidence does not equal evidence of absence” applies to L. monocytogenes traceback efforts at the pre-harvest level.

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