| Abstract: | Outbreaks of salmonellosis related to consumption of fresh produce have raised interest in Salmonella-plant interactions leading to plant colonization. Incubation of gfp-tagged Salmonella enterica with iceberg lettuce leaves in the light resulted in aggregation of bacteria near open stomata and invasion into the inner leaf tissue. In contrast, incubation in the dark resulted in a scattered attachment pattern and very poor stomatal internalization. Forcing stomatal opening in the dark by fusicoccin had no significant effect on Salmonella internalization. These results imply that the pathogen is attracted to nutrients produced de novo by photosynthetically active cells. Indeed, mutations affecting Salmonella motility and chemotaxis significantly inhibited bacterial internalization. These findings suggest a mechanistic account for entry of Salmonella into the plant''s apoplast and imply that either Salmonella antigens are not well recognized by the stoma-based innate immunity or that this pathogen has evolved means to evade it. Internalization of leaves may provide a partial explanation for the failure of sanitizers to efficiently eradicate food-borne pathogens in leafy greens.Salmonella enterica is a common cause of food-borne gastroenteritis, with an estimated number of 1 to 3 million human cases per year in the United States (15). Outbreaks related to consumption of fresh produce have been increasingly reported (28) and result in morbidity and high economic losses. For example, the recent produce-associated salmonellosis outbreak (5), the largest yet reported, has resulted in more than 1,400 persons infected with S. enterica serovar Saintpaul in 43 U.S. states and in Canada. Needless to say, such outbreaks are economically destructive to farmers and the fresh produce industry and damage consumer confidence in the safety of the food supply.Plants might become contaminated in the field through the use of contaminated irrigation water, such as raw sewage or partially treated recycled water, as well as through the use of animal manure for fertilization (2, 4, 16). Fresh produce can also become contaminated during harvest and at postharvest stages due to poor worker hygiene and low sanitation in the processing plant (2, 4). Enteropathogens can adapt to the phyllosphere environment, where they might interact with epiphytic bacteria and gain a foothold (3, 4, 14). It was suggested that transient occupants of the leaf, such as enteropathogens, may become incorporated into phylloplane biofilms and consequently gain protection from environmental stress (11). Studies of the interactions between Escherichia coli O157:H7 and cut lettuce leaves demonstrated attachment of bacteria to the surface, trichomes, stomata, and cut edges. Bacteria were also seen entrapped 20 to 100 μm below the surface in stomata and cut edges (27). Potential localization of human pathogens in the phyllosphere at sites inaccessible to sanitizers may lead to contamination of the food supply.The produce industry currently lacks an efficient control method to ensure complete removal or killing of food-borne pathogens in fresh or minimally processed fruits and vegetables. Therefore, understanding the contamination routes and the interplay between food-borne pathogens and plant tissues is essential in order to design new intervention strategies for ensuring the safety of fresh produce. Lettuce was associated with several outbreaks related to contamination with Salmonella (12, 16, 23, 32); therefore, interactions between this pathogen and lettuce leaves were investigated in this study. |
