To determine acid tolerance and growth of human pathogens on fresh-cut fruits and vegetables

Part I

Survival of pathogens such as Salmonella typhimurium for extended periods of time on fresh-cut produce is a serous problem. In our study, we demonstrate that acid resistance of S. typhimurium increases by 6 log at low temperatures or if the cells are inoculated onto fresh-cut apples. When acid-challenged in planktonic (free suspension) state, the low temperature-mediated protection was observed only for stationary growth phase, acquired increased acid tolerance upon surface-contact with inert surface such as polyethersulphone membrane. The alternative sigma factor (rpoS) was required to acquire adherence-mediated acid tolerance.

Part II

Salmonella are the most frequently reported cause of food-borne outbreaks of gastroenteritis in the United States. In clinical trials, the oral infection 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 out that fresh-cut produce surfaces not only provided suitable solid support for pathogen attachment, but also played a critical role in increasing acid tolerance. Inoculation on to fresh-cut produce surfaces as well as onto inert surfaces such as polyethersulphone membranes and tissue paper increased the survival of salmonellae during acid challenge (50 mM Na-citrate buffer pH 3, 37°C for 2 h) by 4- to 5- log units. It has been suggested that Salmonella outbreaks with a low ID are often associated with a high fat content. Boiled egg white (but not bolied rice), which is low in fat but rich in proteins, is also reported to protect Salmonella from acid challenge. Based on the data, which we have, it appears that the primary factor in the protection observed by solid foods could be mere availability of a solid support for pathogen attachment. Acid challenge experiments using cells inoculated onto polyethersulphone membranes provided a model system suitable for studying the underlying fundamentals of the protection that occur 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-transcriptional factor. Attempts were also made to study the molecular and genetic analysis of attachment mediated acid shock by means of gene arrays.