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Aims: We investigated the fate of Salmonella in lettuce seeds grown in a hydroponic system and the potentials of applying photodynamic inactivation (PDI) to enhance microbial safety of hydroponic farming systems. Methods and Results: Lettuce was grown from Salmonella‐contaminated seeds, and rose bengal‐mediated PDI was applied. Without intervention, Salmonella could persist in plants and hydroponic farming environment throughout 6 weeks of lettuce growth. Cross‐contamination from Salmonella‐inoculated to noninoculated seedlings was observed. PDI significantly decreased Salmonella from 3.90 ± 0.31 log colony‐forming unit (CFU) per plant to 2.77 ± 0.49 log CFU per plant without extra illumination needed (p < 0.01) by week six. Conclusions: Salmonella from contaminated seeds could survive for an extended period in lettuce and hydroponic farming environment and posed serious cross‐contamination risks. Rose bengal‐mediated PDI showed promise in controlling Salmonella contamination in lettuce in a hydroponic farming setting. Significance and Impact of the Study: This study shed light on the serious food safety implications that Salmonella‐contaminated lettuce seeds might entail in a hydroponic farming environment and demonstrated rose bengal‐mediated PDI as a potential mitigation strategy. These findings contribute to the increasingly relevant field of urban farming systems and their associated food safety concerns. [ABSTRACT FROM AUTHOR]

Singapore in recent years is dedicating great efforts in developing urban agriculture in production of leafy green vegetables. Considering the small scales and diverse set-ups of local farms, it is necessary to collect baseline data to evaluate the microbial safety of local leafy green production. In this study, we collected lettuce crops and agriculture water used in lettuce production from ten different farms (five soil-based farms and five hydroponic farms, 100 samples over a period of five months) and tested with multiple microbial parameters. As a result, 16 out of the 50 agriculture water samples were enumerated with Escherichia coli (4.15 ± 0.81 log CFU/100 ml from 9 samples collected at soil-based farms, 3.79 ± 0.91 log CFU/100 ml from 7 samples collected at hydroponic farms). For the lettuce crops, 7 out of 25 samples from the soil-based farms (with overhead irrigation) were enumerated with an average of 3.28 ± 1.45 log CFU E. coli /g fresh matter, while all of the 25 samples from the hydroponic farms were below 10 CFU E. coli /g fresh matter. Within the 11 Salmonella strains isolated in this study, six strains were from soil-based agriculture water, four strains were from hydroponic agriculture water, only one strain was isolated from lettuce crops and it was from a hydroponic farm. When tested with lettuce crops grown in the laboratory, the five Salmonella isolates from the hydroponic farms showed a higher likelihood of internalization and translocation from the roots to the aerial parts (6/40 [15%] plants tested positive) than the six Salmonella isolates from the soil-based farms (2/48 [4%] plants tested positive). The opportunistic pathogen Stenotrophomonas maltophilia was detected with higher presence at hydroponic farms than the soil-based farms. Antimicrobial resistance (AMR) analysis were performed over all of the Salmonella and S. maltophilia isolates and no AMR strain has been identified so far. Taken together, these results suggested the great needs of both soil-based and hydroponic farms to improve their good agriculture practices such as to implement effective agriculture water treatment and to avoid using overhead irrigation with sprinkler systems. [ABSTRACT FROM AUTHOR]