INTEGRATING FOOD SAFETY AND SUSTAINABILITY: INVESTIGATING LETTUCE AND KALE INTERACTIONS WITH SALMONELLA AS INFLUENCED BY STRESS AND PLANT GROWTH- PROMOTING RHIZOBACTERIA

Abstract

As the human population grows and living standards improve, the demand for safe and nutritious food is increasing. Fresh produce provides essential nutrients and lowers the risks of chronic diseases, including cancer. Increased consumption of fresh produce, especially raw leafy greens, is suggested as a restorative practice to maintain and optimize human health. However, leafy greens and raw produce are frequently associated with foodborne pathogens, causing foodborne illnesses and economic loss. Contamination of foodborne pathogens could happen at any stage from the farm to fork continuum, becoming impossible to remove completely once contaminated. An in-depth understanding of the association between leafy greens and foodborne pathogens could provide insights into this complex association and allow the more efficient restriction of foodborne pathogens on leafy greens. This study investigated the Salmonella and human pathogen association from the plant nutrient connection using lettuce and kale as plant material. The impact of plant cultivar, drought, and plant development on Salmonella association on leaf surface was evaluated. Our results identified that red loose-leaf lettuce ‘Mascara’ with high contents of secondary metabolites were less favorable to Salmonella compared to green Romaine lettuce. Drought restricted the Salmonella association on surfaces of ‘Mascara’ lettuce and caused a more prolonged lag phase of Salmonella in lettuce washes. Leaf and wash phytochemical profiles of ‘Mascara’ lettuce diverged from Romaine lettuce and shifted in response to drought. The decline of Salmonella was positively associated with total flavonoids, phenolics and anthocyanins levels in lettuce tissues. Kale plants from 20 days (baby kale) to 59 days (ready-to-harvest mature kale) post-germination were recruited in this study to identify the impact of drought and plant development on Salmonella association. Drought impaired the Salmonella association on the baby kale surface and in washes, and shifted the phytochemical profiles, but not on mature kale plants. Baby kale plants supported higher levels of Salmonella than mature kale plants. Accumulation of flavonoids, possibly phenolics, but not glucosinolates in kale tissues, are of future interest as Salmonella restrictive compounds. To ensure food security under climate change, the impact of two strains of plant growth-promoting rhizobacteria (PGPR) - Pseudomonas spp., on plant growth promotion under environmental stresses and epiphytic Salmonella restriction were studied. For 26 days old kale plants, root inoculation of Pseudomonas spp. showed no growth-promoting effects but limited the Salmonella population on kale surface without changing the contents of secondary metabolites under regular wateringcondition. Salinity reduced kale plant biomass accumulation and limited Salmonella growth on surfaces, with higher antioxidant capacity detected in kale tissues. For both 20 days (baby) and 59 days (mature) kale, the inoculation of Pseudomonas spp. protected plants from biomass reduction due to drought and limited the surface survival of Salmonella under regular watering condition, with limited changes in overall phytochemical profiles and no changes in secondary metabolites detected. In summary, this study advanced our understanding of the impact of phytochemicals and plant growth-promoting rhizobacteria on the Salmonella and leafy green association. Our data suggested that the selection of specific cultivars, regulated abiotic elicitation of secondary metabolites and application of beneficial rhizobacteria are promising strategies to maintain plant growth and enhance food safety and nutritional quality under the great challenge of climate change.