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Applied and Environmental Microbiology, January 2002, p. 86-92, Vol. 68, No. 1
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.1.86-92.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Inoculation onto Solid Surfaces Protects Salmonella spp. during Acid Challenge: a Model Study Using Polyethersulfone Membranes

Produce Quality and Safety Laboratory, Henry A. Wallace Beltsville Agricultural Research Center, Agricultural Research Service, USDA, Beltsville, Maryland 20705-2350

Received 20 June 2000/ Accepted 11 October 2001

Salmonellae are the most frequently reported cause of outbreaks of food-borne gastroenteritis in the United States. In clinical trials, the oral infective dose (ID) for healthy volunteers was estimated to be approximately 1 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 surfaces not only provided suitable solid support for pathogen attachment but also played a critical role in increasing the 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°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 occurs when Salmonella strains are 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.