Purpose: The purpose of this study was to examine the prevalence, persistence and diversity of Listeria monocytogenes in two spinach fields from planting to harvest.
Methods: Spinach fields were sampled nine times over the 48 day crop. A total of 711 samples (486 soil, 162 spinach leaf, 36 drag swab and 27 water samples) were cultured for L. monocytogenes and isolates were analyzed by PFGE. Remotely-sensed meteorological factors were evaluated by logistic regression analysis for their association with the likelihood of an L. monocytogenes positive sample.
Results: L. monocytogenes was detected in 7.2%, 1.8%, 19% and 77% of soil, leaf, drag swab, and water samples, respectively. Analysis of PFGE-types showed persistence in one field, demonstrated by the recovery of L. monocytogenes isolates with the same PFGE-type at least three times over the 48 day crop. A high PFGE-type diversity (each D > 0.90) was observed among L. monocytogenes isolates from soil, drag swab and water samples. Of the 39 remotely-sensed meteorological factors assessed, average precipitation (within 3 d of sample collection) and average humidity (within 3 d of sample collection) were identified as important meteorological factors increasing the likelihood of an L. monocytogenes positive sample. On the other hand, average solar radiation was shown to decrease the likelihood of an L. monocytogenes positive sample.
Significance: This information can be used to develop models that predict meteorological conditions when the prevalence of L. monocytogenes may be high. These findings will support the development of science-based mitigation strategies for growers, such as modifications to personnel hygiene and or equipment sanitation to limit potential L. monocytogenes contamination transfer from field to packinghouse.