Insights into the food safety implications of a commercial Bacillus thuringiensis product in hydroponic systems

This study examined the food safety implications of a commercial Bacillus thuringiensis (Bt) product by assessing its pathogenicity and impact on the microbiological quality of crops grown in hydroponic systems. B. thuringiensis B3 isolated from the product showed genomic similarity to the foodborne pathogen Bacillus cereus , as it possessed complete sets of enterotoxin-encoding genes ( cytK , nheABC , and hblCDAB ). Caco-2 cytotoxicity assay demonstrated in vitro cytotoxicity as B3 cell-free supernatant reduced cell viability at 20% (vol/vol). The food safety implications of Bt product treatment in hydroponic systems cultivating lettuce were monitored for 5 weeks with polymyxin pyruvate egg yolk mannitol bromothymol blue agar to enumerate B. cereus group populations. Throughout the cultivation period, B. cereus group populations were significantly greater in reservoirs and surfaces of treated systems than controls ( P < 0.05), which demonstrated sustained elevation of B. cereus group populations associated with Bt product application. In contrast, the populations on edible lettuce parts remained at 3.46 ± 0.35 log CFU/g, which was insignificantly different from the control group ( P > 0.05). Changes in the lettuce leaf microbiome composition and functions also appeared unlikely to compromise food safety. Overall, these results indicate that Bt product usage in hydroponics may increase system-wide B. cereus group, but the population levels in the edible parts remain below 5 log CFU/g, the threshold associated with diarrhea syndrome. Moreover, 16S rRNA gene analysis of the Bt product revealed the presence of non- Bacillus genera, emphasizing the importance of quality control measures for microbial plant-beneficial products. IMPORTANCE Safety evaluations of biologically derived fertilizers and control agents are essential to ensure that crops grown in treated systems are safe for consumption. This includes an assessment of the production composition for contaminants, the potential of the intended organism to cause foodborne illnesses, and any effects on the microbiological quality of the crop. These considerations are especially critical in hydroponic systems, where the recirculating system can amplify the spread and persistence of applied products. In this study, we investigated the safety of a commercial Bacillus thuringiensis product due to its widespread use in agriculture and close genomic similarity to the foodborne pathogen Bacillus cereus . Our findings underscore the importance of considering the food safety implications when applying biological products in hydroponics and lay the groundwork for safety evaluations of these inputs in food production systems.