|M.Sc Student||Lapidot Anat|
|Subject||The Role of Biofilm Formation in the Attachment, Survival|
and Resistance of Salmonella Typhimurium on
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Sima Yaron|
|Full Thesis text|
Pathogens on fresh produce present a significant potential source for human illness. Salmonella and other enteric bacteria may contaminate fresh produce pre- and post-harvest. Our hypothesis was that the ability of Salmonella to produce biofilm enhances the adhesion and survival on leafy vegetables, and improves its resistance to chlorination.
Therefore, we compared the adhesion and resistance of Salmonella enterica Serovar Typhimurium WT and its biofilm-deficient isogenic mutant to chlorine disinfection. Parsley was contaminated with Salmonella (10^7CFU/ml) and then rinsed with water or chlorine solution. It was found that the WT and the mutant strains had similar survival properties, and up to 3-log reduction were observed, depending on the chlorine concentration. After a week of storage, the biofilm producing strains survived chlorination significantly better than the biofilm deficient mutant. However, the recovery of the mutant was still high, indicating that the biofilm matrix has a limited role in the persistence of Salmonella after the chlorination treatment of parsley. We also identified some S. Typhimurium virulence genes required for lipopolysaccharide or flagella biosynthesis and to mammalian cell-invasion as important for the attachment to plant tissue and resistance to chlorine.
The second part of this research investigated the role of biofilm formation in transmission of Salmonella from irrigation water to parsley and its consequent colonization on the plant. Mature parsley plants were drip-irrigated directly to the soil with contaminated water containing S. Typhimurium or its biofilm-deficient mutant. Superior stems and leaves were harvested and CFU of Salmonella was counted.
Results demonstrated the ability of S. Typhimurium in transfer from irrigation water to soil, and its survival in the soil during the course of the study. The biofilm producing strain was observed in the soil (10^5 CFU/g), on the leaves (10^4 CFU/g) and in the stems (10^3 CFU/g) a day after the irrigation, while CFU counts of the biofilm-deficient mutant were 2-log higher in the soil but 1-log lower on the plant. These evidences indicate that the formation of biofilm indeed confers an advantage to Salmonella in transfer and survival on the plant.
In addition, CLSM images of parsley revealed that S. Typhimurium cells internalized into the leaves and formed aggregates or micro-colonies. Therefore, we concluded that the formation of the biofilm is not the primary solution to prevent attachment of Salmonella to plants, and probably other mechanisms, such as invasion to the internal tissue of the plant, provide the protection and enhance the survival of the pathogen on the plant.