Differential persistence of Salmonella spp. and Listeria monocytogenes in soilless substrates for hydroponic lettuce production.

Soilless substrates used in hydroponic lettuce systems may differentially support the persistence of human pathogens. This study evaluated the impact of soilless substrate type with and without plants on the persistence of Salmonella and Listeria monocytogenes in an indoor hydroponic lettuce production system over 14 days. On day 0, rockwool (RW), low-density foam (LDF), peat moss (PM), and coco coir (CC) substrates in sub-irrigated nutrient solution were inoculated with Salmonella or L. monocytogenes cocktail at an initial concentration of ∼6 log colony forming units (CFU) per milliliter. Treatments included planted and non-planted substrates in a factorial randomized complete block design with three biological replicates. Samples were collected on days 0, 3, 5, 7,10, and 14. Samples were processed, and bacteria were enumerated on selective media. A generalized linear mixed model with negative binomial distribution was used to assess the effect of planting status, substrate type, bacteria type, and their interactions, with nutrient solution temperature, pH, and electrical conductivity as covariates. Bacteria concentration (log10 CFU/substrate) reduced significantly over time (p < 0.001). Substrate type had a substantial effect on pathogens, with higher concentrations in CC compared to the other substrate types (p < 0.001). L. monocytogenes and Salmonella also show distinct persistence patterns (p < 0.001). Significant interactions (p < 0.001) were observed between substrate type and bacteria type; and substrate type and planting status, indicating differential persistence dynamics across substrates over time under controlled indoor conditions for hydroponic lettuce production.