Analysis of the Induction of Heat Tolerance by Osmotic Stress in Salmonella

Salts or sugars are often added to foods at high concentrations to retard the growth of contaminating microorganisms. However, in a number of bacteria, osmotic stress that is generated by high concentrations of solutes confers increased tolerance of high temperature. This phenomenon suggests that high concentrations of food additives antagonize the efficacy of heat treatment for the control of bacterial contaminants. We use Salmonella trains, which are the most prevalent causative agents of food-borne infections in the United States as well as world- wide, to study the mechanism of induction of heat tolerance by osmotic stress. We shall carry-out a detailed characterization of the induction of heat tolerance by osmotic stress in S. typhimurium, which is a non-virulent laboratory strain, and in S. enteritidis strains, which are serious food pathogens. In order to identify the gene products that are involved in the osmotic stress-dependent induction of thermotolerance, we isolated a mutation that specifically blocks this process. We shall clone the wild type alleles of the disrupted gene, and determine its DNA sequence. Because high concentrations of salts and sugars induce increased thermotolerance in many other bacteria besides Salmonella, our studies with this organism will provide a basis for the analysis of this response in other food pathogens.