Purpose: Accurate determination of environmental prevalence and on-farm sources of Listeria monocytogenes for fresh produce is essential for risk determinations.
Methods: Different base-nutrient concentrations of enrichment media were compared for detection of Listeria sp. and Lm after 48h incubation at 35°C. The sample matrix was leafy green residues incorporated into diverse soils. Detection by agar culture, conventional PCR, qPCR, and commercial kit PCR were assessed. Field plots of red chard were inoculated with log 7 CFU/l of rifampicin resistant L. innocua (TVS451) and soil-incorporated after 18h. Soil was sampled over 77 days and processed using the various methods. In commercial fields, soil was collected from 195 locations following incorporation of a leafy green crop. Fifteen 500g samples were collected 7-10 days after incorporation or following the first replant irrigation at the depth of residual moisture.
Results: Post-enrichment detection of Lm by PCR was greatly enhanced 2X Listeria Enrichment Broth with supplements and 1) detection using the Roka Atlas system for primary PCR; 2) plating on R&F LisLm agar; and 3) species and colony confirmation using probe-based qPCR. Red chard: clay loam soil samples were positive for TVS451 for at least 77 days depending on specific methods used for enrichment and detection. Application of the optimized protocol to diverse commercial leafy greens soils did not result in positive detection. One creek sediment sample was positive for Listeria spp.
Significance: Perceptions of the ‘ubiquitous’ prevalence of Lm in produce production environments can alter the prioritization of preventive controls validation and implementation. Although preliminary, this study indicates that focus on postharvest handling and processing environments may be a higher priority.