Efficacy of Lactobacillus rhamnosus and Its Metabolites to Mitigate the Risk of Foodborne Pathogens in Hydroponic Nutrient Solution

Hydroponic nutrient solution (HNS) has been established as an ideal conduit for pathogen contamination and proliferation. This study evaluated the efficacy of lactic acid bacteria and their metabolites in mitigating the risk of foodborne pathogens in HNS when compared to conventional chemical treatments. Hoagland’s HNS were prepared according to the manufacturer’s instructions and inoculated with Salmonella Typhimurium, Escherichia coli 0157:H7, and Listeria innocua at 10⁵ CFU/mL cell concentration. These nutrient solutions were subjected to treatment with various concentrations of Lactobacillus rhamnosus live cells, a cell-free extract (CFE) of L. rhamnosus metabolites, sodium hypochlorite and peroxyacetic acid at 22 ± 1 °C for up to 96 h using appropriate controls. The survived cells were enumerated on respective selective media at regular intervals. Additionally, the impact of these treatments on lettuce growth and the physico-chemical properties of HNS, such as pH, electrical conductivity, salinity, total dissolved solids, and % lactic acid content, were determined over 21 days using standard procedures. Both S. Typhimurium and E. coli O157: H7, when in combination with L. rhamnosus, remained stable in HNS over a 96 h period, while L. innocua showed a 3-log reduction. Whereas CFE treatment of HNS showed a significant reduction in Salmonella and E. coli O157: H7 (both undetectable after 96 h; LOD: <1 log CFU/mL). Interestingly, L. innocua levels remained stable after CFE treatment. PAA treatments at 12 mg/L notably reduced Salmonella and L. innocua growth, but not E. coli O157:H7. Lettuce plants in untreated control were significantly taller and heavier compared to those treated with CFE. These findings highlight the potential of biological interventions while emphasizing their limitations in hydroponic systems for pathogen control.