Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2001
Publication Date: N/A
Citation: Interpretive Summary: Salmonellae are the most frequently reported cause of food-borne outbreaks of gastroenteritis in the United States. In clinical trials the minimum number of salmonellae required to cause a disease in healthy volunteers is estimated to be approximately one million cells. However, recent reports from various outbreaks associated with tomatoes and melons involving Salmonella species, showed that much smaller number of cells (approx. 100 cells) were sufficient to cause the disease. An acidic condition in the stomach is our first line of defense against invading pathogens. Therefore, we examined if the acidic nature of some fresh-cut produce (for example, tomatoes and apples) could have a role in making salmonellae resistant to the stomach acidity. Our data indicated that acidity of produce played no role in increasing acid tolerance of salmonellae. However, it provided a suitable surface for the pathogen to attach. Mere attachment to surfaces of fresh-cut produce, as well as to inert surfaces such as nylon and tissue paper, helped salmonellae endure acidity much better. The prevention of contamination caused by human pathogens during the harvesting, washing, cutting, slicing, packaging and transporting is of considerable importance to public health, and consequently is a major issue for the fresh-cut produce industry. Understanding various environmental factors that influence survival of human pathogens will benefit the fresh produce industry. The results from this research will help the produce industry design effective control measures to eliminate surface-attached pathogens during processing of fresh-cut produce.
Technical Abstract: Salmonellae are the most frequently reported cause of food-borne outbreaks of gastroenteritis in the United States. In clinical trials, the oral infective dose (ID) for healthy volunteers is estimated to be approximately one million cells. However, in reports from various outbreaks, the ID of Salmonella species associated with solid foods was estimated to be as few as 100 cells. We found that fresh-cut produce surface provided suitable solid support for pathogen attachment, which played a critical role in increasing acid tolerance of the pathogen. However the acidic nature of certain produce played no role in making salmonellae resistant to stomach acidity. Inoculation onto fresh-cut produce surfaces as well as onto inert surfaces such as polyethersulfone membranes and tissue paper, increased the survival of salmonellae during acid-challenge (50 mM Na-citrate, pH 3.0, 37 degrees C, 2 h) by 4- to 5-log units. Acid-challenge experiments using cells inoculated onto polyethersulfone membranes provided a model system suitable for studying the underlying fundamentals of the protection that occur when S. typhimurium is associated with solid foods. The surface-associated acid protection, which was observed in several Salmonella strains, required de novo protein synthesis, and was independent of stationary phase sigma-transcription factor.