The importance of RpoS in the survival of bacteria through food processing.

The resistance of bacteria to environmental stresses is recognised as an increasingly important area of microbiology. In particular, the alternative sigma factor RpoS has been shown to produce greater stress resistance in stationary phase cells of Salmonella and Escherichia coli compared with those in exponential phase. Our work has shown that RpoS can be induced in exponential phase in response to a number of inimical processes used in the food industry, including changes in water activity produced using a range of humectants and preservatives. The presence of high levels of competitor cells will also lead to early induction of RpoS in Salmonella by an as yet unknown mechanism. High levels of competitor cells also provide Salmonella with an increased resistance to heat and freeze-thaw injury; the mechanism for this, however, is rpoS independent and has lead to the theory of a holistic mechanism for sub-lethal injury in respiring bacteria--the bacterial suicide response. This hypothesis predicts that sub-lethal injury occurs through the production of free radical species and not by the action of the applied inimical process per se. The demonstration of the production of a free radical burst when cells are subjected to differing types of stresses has been shown by a number of methods.