טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKisluk Guy
SubjectSalmonella Enterica in Leafy Greens: Contamination,
Survival and Interactions
DepartmentDepartment of Biotechnology and Food Engineering
Supervisor Professor Sima Yaron


Abstract

The growing number of human infections via fresh produce makes it the subject of increased concerns. However, the factors that govern the occurrence and fitness of enteric pathogens in fresh produce are not well understood. This work presents in planta studies aimed at evaluating the potential of Salmonella enterica to contaminate and persist on leafy greens. Furthermore, bacterial gene expression which allows its survival in planta was studied.

In order to study the potential for contamination of plants via irrigation with contaminated water two model plants were used: basil and parsley. Following overhead irrigation of basil, Salmonella was readily detected on the phyllosphere, however, following drip irrigations Salmonella was not detected, with the exception of plants irrigated with heavily contaminated water. Thus, overhead irrigation is a major contributing factor for contamination of leafy greens. Another major factor is the type of plant subjected for irrigation, since up to 20-fold higher levels of Salmonella were detected in the parsley phyllosphere compared with basil. The extent of contamination of leafy greens was also affected by the quantity of Salmonella in irrigation water, time of irrigation and the time period between exposure and harvest. Prolonged persistence of Salmonella was observed on plants, at levels sufficient to cause illness if consumed. The present research provides applicable knowledge of the potential for contamination of produce and calls for the reassessment of current field methodologies in order to improve its microbial safety.

The ability of Salmonella to persist in plants raises questions about the nature of bacteria-plant interactions. Differential induction of molecular mechanisms during bacterial persistence was discovered. Genes encoding for Type 3 secretion systems (T3SSs), flagella, biofilm and fimbriae were significantly induced in basil- but not in parsley-associated Salmonella. This is indicative of plant-specific survival strategies and adaptation. Furthermore, Salmonella cells not only passively survive but also exercise an active mode of survival in planta, which depends on multiple molecular mechanisms, as shown in the case of basil. A mutant strain impaired in two T3SSs and the flagella exhibited reduced levels of survival compared with the w.t. and with mutants impaired in each of these systems.

Persistence of Salmonella in planta and the plant-specific survival strategies strengthen the arguments that enteric bacteria have adapted to survive in plants and that plants can be considered as secondary hosts or vehicles for enteric pathogens.