VIRULENCE FACTORS INVOLVED IN PERSISTENCE OF SALMONELLA TYPHIMURIUM ON LEAVES

Salmonella enterica serovars have been isolated from a variety of fresh fruit and vegetables and are among the most commonly identified foodborne pathogens associated with fresh produce. Moreover, transfer and colonization of S. enterica on plants grown in contaminated soil, manure-amended soil or exposed to contaminated irrigation water was reported. The ability of S. enterica to survive in the plant environment in spite of the harsh conditions leads to the hypothesis that this pathogen has adapted to the plant niches by utilizing colonization and virulence mechanisms, and raises questions about the nature of these bacteria-plant interactions. Our research was aimed at studying bacterial gene expression on leaves and identifying genes that allow the survival of S. enterica serovar Typhimurium in planta. Comparing levels of gene expression in basil- vs. parsley-associated S. Typhimurium reveals plant-specific transcriptional response. Furthermore, differential induction of molecular mechanisms during bacterial persistence was discovered. Genes encoding for type-3-secretion-systems 1 and 2 (TTSS-1 and TTSS-2), flagella, fimbriae, biofilm formation and ribonucleotide reductases were significantly induced in basil- but not in parsley-associated S. Typhimurium during a 14 day period. This is indicative of plant-specific survival strategies and adaptation on basil, producers of potent antibacterial substances. While deletion of each of the TTSS did not affect the survival of S. Typhimurium on basil or parsley, a mutant strain impaired in two T3SSs and the flagella exhibited reduced levels of survival compared with the wt or the single null mutants. Persistence of Salmonella in planta and the plant-specific survival strategies, strengthen the arguments that S. Typhimurium has adapted to survive in plants and that plants can be considered as secondary hosts for enteric pathogens.