Purpose: A longitudinal study was conducted, using five produce farms in New York State, to characterize prevalence, persistence, and diversity and to determine landscape and meteorological factors that predict the likelihood of L. monocytogenes in the preharvest environment.
Methods: Over the two-year study, farms were sampled four times a year and 588 samples were collected. Soil, water, fecal, and drag swab samples were collected from four fields per farm on each visit; GPS coordinates for each site were recorded and revisited. Samples were cultured for L. monocytogenes; isolates were confirmed by PCR, and sequenced in the sigBgene.
Results: The prevalence for L. monocytogenes was 15% (88/588). Farm, season and sample type were found to be significantly associated with the frequency of L. monocytogenes positive samples (P = 0.0008, 0.0010, 0.0001). The frequency of L. monocytogenes-positive samples was highest amongst water samples (48/174). Analysis of sigB sequences showed a high diversity of L. monocytogenes allelic types amongst farms (D = 0.80 ± 0.10). There was evidence of persistence on one farm, demonstrated by the recovery of L. monocytogenes isolates with the same allelic type at least three times from the same location. Sample sites were partitioned into high and low prevalence clusters according to classification tree (CT) models based on landscape and meteorological data. Proximity to water (> 37.5m) was identified as the most important factor influencing the likelihood of a L. monocytogenespositive sample, but temperature, soil properties, and proximity to livestock production and urban development were also important.
Significance: This information can be used to develop risk maps that indicate locations where the prevalence of L. monocytogenes is higher, which consequently may pose a greater risk for contamination of produce. These findings will support the development of science-based mitigation strategies for growers.