Serovar-specific lipopolysaccharide modifications induced by phytogenic biocide drive differential bacterial mobility in soil

The presence of biocidal compounds in the rhizosphere complicates the prediction of bacterial fate and transport in soil. This study investigated how phloretin, a plant-derived biocide, alters the mobility of Salmonella typhimurium and Salmonella enteritidis in soil by inducing structural adaptations in bacterial membranes. Phloretin exposure triggered consistent changes in physicochemical properties of both serovars (e.g., increasing zeta potential and decreasing hydrophobicity). Despite these shared trends, mobility responses diverged markedly. Following adaption at Minimal Inhibition Concentration of phloretin, S. enteritidis showed an increasing deposition rate from 0.17 to 1.68, while S. typhimurium showed a decreasing deposition rate from 0.18 to 0.14. These contrasting outcomes suggest that electrostatic and hydrophobic interactions alone could not explain the observed mobility patterns. Alternatively, membrane characterization results suggest that serovar-specific structural remodeling of lipopolysaccharide (LPS) may be responsible for the divergent behaviors. Phloretin induced an elongation of LPS chains by about 30 % in S. enteritidis , promoting adhesion and immobilization, while S. typhimurium retained shorter LPS structures, reducing adhesion and facilitating transport. Since LPS also constitutes a protective layer against harsh environment, the observed inverse link between LPS length and mobility thusly underscores a trade-off between bacterial endurance and dissemination. Furthermore, the finding that lower mobility of S. enteritidis occurring under higher biocidal stress implies higher pathogen accumulation closer to the root surfaces, where the biocidal levels are higher, thereby amplifying localized antimicrobial efficacy. The insights gained are useful in prediction of pathogen risks in the rhizosphere.