Protective effects of cold temperature and surface-contact on acid tolerance of Salmonella spp.

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Abstract
Aims: To determine the effect of cold-storage temperature and surface contact on the survival of Salmonella spp. during acid challenge. To determine the contribution of sigmatranscrip- tional factor (encoded by rpoS) in surface contact-mediated acid tolerance. Methods and Results:Salmonella serovar Typhimurium cells in exponential and stationary growth phase were subjected to acid challenge in planktonic and surface-associated states. Low temperatures offered protection against acid challenge to cells which were in stationary growth phase (but not to those in exponential growth phase). The cells from stationary, as well as logarithmic growth phase, acquired increased acid tolerance upon surface contact with various surfaces, such as fresh-cut apples, agar and polyethersulphone membranes. The alternative sigma transcription factor was not required to acquire surface contact-mediated acid tolerance. Conclusions:Salmonella spp. take advantage of low temperature and surface association to overcome acid challenge. Some of the acid tolerance mechanisms are independent of the sigma transcription factor regulon. Significance and Impact of the Study: Effective control measures during produce processing must take into account the different susceptibilities of planktonic vs surface-associated food-borne pathogens.