Investigating the attachment of Salmonella enterica strains to bacterial cellulose-based plant cell wall models

In this study, the role that different plant cell wall (PCW) components (cellulose, pectin and xyloglucan) play in Salmonella (S. Enteritidis ATCC 13076, S. Typhimurium ATCC 14028 and S. enterica M4) attachment to cut PCWs was investigated using bacterial cellulose (BC)-derived PCW models. Pectin significantly increased Salmonella attachment (~0.5 to 0.9 log CFU/cm²) whereas xyloglucan had no effect on Salmonella attachment but significantly increased Salmonella attachment when present with pectin (~0.5 to 1.1 log CFU/cm²). Attachment of Listeria monocytogenes ATCC 7644, which was used as a control strain in the study, to the PCW models was influenced by pectin and xyloglucan in a different way to Salmonella. This suggests that interactions between the PCW components and bacterial species are specific. Attachment studies with monosaccharides making up pectin and xyloglucan polymers indicated that Salmonella attachment to plants was not mediated by receptor-ligand interactions involving carbohydrates and bacterial surface adhesins. The mechanism was instead suggested to be due to the effect of pectin and xyloglucan on the surface morphology of the PCW models. Salmonella attachment was not directly determined by the physicochemical properties of the attachment surface (hydrophobicity) nor the bacterial surface (hydrophobicity and aggregation abilities). This study established that flagella, fimbriae and cellulose all play a role in Salmonella attachment to plant surfaces. The effect of incubation temperature on the role of cell surface components in attachment was investigated. All three cell surface components were produced at 28°C but only flagella were produced at 37°C. Of these structures, flagella appeared to be most important (a reduction of ~1.5 and 0.9 log CFU/cm² when grown at 28°C and 37°C respectively). Using this knowledge, the effectiveness of using ultrasonic waves which can disrupt bacterial surface structures (including flagella) to reduce levels of bacterial attachment to PCWs was investigated. Sonication proved to have potential use as the process resulted in significantly lower numbers of Salmonella cells attach to PCW models and cut plant materials (between 0.5 to 1.0 log CFU/cm²). A linear mathematical model was constructed to predict the numbers of Salmonella cells that would attach to natural PCWs based on cell concentration. This was achievable as the number of Salmonella cells attaching to the PCW models increased linearly with cell concentration. Validation of the model for a range of variables (different natural plant tissues, different subspecies of S. enterica strains and other common human foodborne pathogens) supported its robustness. Development of the mathematical model has the potential to feed into risk-assessment tools for the control of Salmonella associated with the processing of fresh produce.