In recent years, with consumer trends towards fresher and safer foods, there has been increasing interest in alternative methods for food preservation. These include innovative techniques such as high electric field pulses or ultrahigh pressures, and also combined processes of several agents such as mild heat, natural antimicrobials or low pH. The main problem encountered for the implementation of these new approaches is the lack of knowledge on the effect of the different agents on microorganisms. A better understanding of bacterial inactivation and resistance mechanisms would help in defining safer processes. The resistance of foodborne pathogens such as Salmonella and Escherichia coli to environmental stresses is an increasingly important area of microbio logy. These microorganisms have developed adaptive networks which can be triggered in response to many inimical processes used in the food industry, inducing the expression of genes involved in cellular defence mechanisms. The modification of sigma factors (?) bound to core RNA polymerase, conferring promoter specificity, is possibly the most important regulatory mechanism in gramnegative cells. However, apart from the common regulatory networks, also specific stress resistance mechanisms are present in bac terial cells. In this project we will study the role of general and specific resistance mechanisms in Salmonella against several agents used in food preservation, through the isolation of naturally occurring mutants of either increased or reduced resistanc e. Mutant strains on the general regulation networks (genes encoding for the RNA polymerases sigma factors ?5 and ?32) and strains specifically more resistant to one single agent (either heat, acid, high electric field pulses or oxidative stress) will be s elected and characterized with the final objective of identifying those proteins involved on a higher or lower resistance.
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