Autoinducer-2–Like Activity on Vegetable Produce and Its Potential Involvement in Bacterial Biofilm Formation on Tomatoes

Quorum sensing employing autoinducer molecules is a strategy used by bacterial populations to coordinately modulate their response to environmental stresses and host defense mechanisms. The objectives of this study were to determine the levels of autoinducer-2 (AI-2)–like activity on selected vegetable produce and determine whether AI-2–like molecules can promote E. coli O157:H7 biofilm formation on tomatoes. Twelve different fruit and vegetable produce samples were screened for AI-2–like activity using autoinducer sensing V. harveyi biosensor strains. All samples except strawberries showed AI-2 activity albeit at varying levels, with eggplant having the highest levels. Tomatoes, when stored at 4°C for 9 days, showed increasing levels of heterotrophic bacterial populations as compared to AI-2–like activity levels, which fluctuated. Rinses from Roma tomato surfaces that were stored at refrigeration temperature for up to 9 days caused a significant increase (1.8–3.6-fold as compared to the negative controls) in biofilm formation by luxS mutant (non AI-2 producing) generic E. coli and E. coli O157:H7 strains using a micro-titer plate-based biofilm assay. These results suggest that AI-2–like activity, which is present on the surfaces of tomatoes, has the potential to enhance the production of bacterial biofilms.