Investigating the effect of mutations in lipopolysaccharides (LPS) (O-antigen and Core region) of Salmonella Typhimurium on the colonisation of “microgreens”

In recent years, foodborne illnesses linked to the consumption of raw leafy greens, particularly microgreens, have significantly increased. Salmonella Typhimurium, a common foodborne pathogen, has been implicated in several outbreaks, underscoring the importance of understanding its interactions with fresh produce. This study aimed to investigate the effect of mutations in lipopolysaccharides (LPS) of S. Typhimurium on the colonisation of microgreens. Extracts from nine different microgreens root and leaf samples infected by S. Typhimurium ATCC 14028 were plated on Xylose Lysine Deoxycholate (XLD) agar to quantify bacterial proliferation and attachment. These results demonstrated that S. Typhimurium ATCC 14028 successfully colonised the roots of all nine microgreens, with higher bacterial counts observed in rhubarb chard, Swiss chard, and celery compared to other tested microgreens. LPS mutant strains (SGSC225, SGSC227, SGSC229, SGSC230, and SGSC389) were further studied using the roots of rhubarb chard, Swiss chard, and celery. O-antigen mutant strains SGSC225 and SGSC389 exhibited colonization patterns similar to the wild-type. This suggests that the O-antigen is not essential for bacterial proliferation or suppression of plant immune responses. In contrast, the SGSC230 mutant strain, which has mutations in the LPS core region, showed significantly reduced attachment after multiple washes, emphasising the importance of the LPS core in bacterial adhesion and survival on plant roots. Interestingly, this study indicates that S. Typhimurium induced an increase in root hair production on microgreens, although bacterial attachment did not directly correlate with root hair density. It is hypothesised that S. Typhimurium may synthesise plant hormones, such as auxin, to stimulate root hair production and enhance colonisation. Additionally, the findings indicate that root exudates enhance bacterial colonisation and biofilm formation through chemotactic responses to metabolites. Gene expression analyses showed differential regulation of plant defence-related genes, highlighting the complex dynamics of S. Typhimurium infection. Moreover, structural components of LPS were identified as critical for effective attachment and biofilm formation. These new insights into plant-pathogen interactions underscore the necessity for targeted food safety strategies in microgreen cultivation and handling, informing future agricultural practices aimed at mitigating the risk of Salmonella contamination.