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Bacillus subtilis (BS) is a well‐known beneficial microorganism for plants but is not competitive in the plant rhizosphere microbiome. We report the selective support of Brassica rapa subsp. Chinensis (Xiao Bai Cai) juice (XBCJ) on BS both in hydroponic nutrient solution and the plant rhizosphere of lettuce. After 2 weeks of being inoculated in the lettuce rhizosphere, the Bacillus population was enumerated at 3.30 ± 0.07 log CFU/unit in the BS group and at 5.20 ± 0.39 log CFU/unit in the BS + XBCJ group (p < 0.05). Accordingly, lettuce crops from the BS + XBCJ group were significantly higher than the control group for all of the tested biomass‐related parameters (p < 0.05). The treatment did not significantly affect the texture, colour, moisture contents, total phenolic contents, or antioxidant activities of the lettuce crops (p > 0.05). Non‐target ultra‐performance liquid chromatography‐quadrupole time‐of‐flight mass spectrometry (UPLC‐Q‐TOF‐MS) suggested that phenolic compounds could be the key class of phytochemicals being responsible for the selectivity. High‐throughput RNA‐based 16S rRNA gene sequencing and analysis were performed to depict the influence of BS and XBCJ over the global microbiome compositions of plant rhizosphere. [ABSTRACT FROM AUTHOR]

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]

This study performed microbial analysis of nutrient film technique (NFT) hydroponic systems on three indoor farms in Singapore (the "what"). To justify the necessity of sanitizing hydroponic systems, strong biofilm-forming bacteria were isolated from the facility and investigated for their influence on Salmonella colonization on polyvinyl chloride (PVC) coupons in hydroponic nutrient solutions (the "why"). Finally, sanitization solutions were evaluated with both laboratory-scale and field-scale tests (the "how"). As a result, the microbiome composition in NFT systems was found to be highly farm specific. The strong biofilm formers Corynebacterium tuberculostearicum C2 and Pseudoxanthomonas mexicana C3 were found to facilitate the attachment and colonization of Salmonella on PVC coupons. When forming dual-species biofilms, the presence of C2 and C3 also significantly promoted the growth of Salmonella ( P < 0.05). Compared with hydrogen peroxide (H 2 O 2 ) and sodium percarbonate (SPC), sodium hypochlorite (NaOCl) exhibited superior efficacy in biofilm removal. At 50 ppm, NaOCl reduced the Salmonella Typhimurium, C2, and C3 counts to <1 log CFU/cm 2 within 12 h, whereas neither 3% H 2 O 2 nor 1% SPC achieved this effect. In operational hydroponic systems, the concentration of NaOCl needed to achieve biofilm elimination increased to 500 ppm, likely due to the presence of organic matter accumulated during crop cultivation and the greater persistence of naturally formed multispecies biofilms. Sanitization using 500 ppm NaOCl for 12 h did not impede subsequent plant growth, but chlorination byproduct chlorate was detected at high levels in the hydroponic solution and in plants in the sanitized systems without rinsing., Importance: This study's significance lies first in its elucidation of the necessity of sanitizing hydroponic farming systems. The microbiome in hydroponic systems, although mostly nonpathogenic, might serve as a hotbed for pathogen colonization and thus pose a risk for food safety. We thus explored sanitization solutions with both laboratory-scale and field-scale tests. Of the three tested sanitizers, NaOCl was the most effective and economical option, whereas one must note the vital importance of rinsing the hydroponic systems after sanitization with NaOCl., Competing Interests: The authors declare no conflict of interest.