Purpose: To determine the persistence of LM on romaine lettuce from a brief seed contamination event through the harvest period studying various LM strains and cultivar combinations, as well as different growth conditions. Additionally, to examine potential internalization of LM into the hypocotyl of romaine under simulated field conditions.
Methods: Persistence of 3 LM strains on 3 romaine lettuce cultivars was assessed independently by inoculating seeds with an initial inoculum of approximately 8 log CFU/ml. Seeds were grown on various soil types (e.g., standard potting mix) or sterile soft-top agar for up to 60 days. For internalization studies, seeds were inoculated with a LM strain expressing green fluorescent protein (GFP). Three plants were fixed, paraffin embedded, and sectioned; localization was studied using standard immunohistochemistry techniques.
Results: Plants grown on sterile soft-top agar maintained between 4.4 to 7.8 log CFU/g LM after a 60 day period, while LM dropped below the limit of detection (2 log CFU/g) by day 45 in plants grown in soil. Cultivar and LM strain differences were not significant. A total of 539 LM cells were internalized in all major tissue types with the majority localizing in the pith followed by cortex, xylem, phloem and epidermis.
Significance: Romaine lettuce can support growth and internalization of LM, which could serve as a vehicle of LM transmission to consumers warranting the need to improve current produce safety strategies.