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Preservation technologies subject bacterial cells to different levels of stress, which in the most effective cases lead to their inactivation and death. The term “stress” can refer to any extracellular influence that threatens the ability of microorganisms to perform their living functions. The food preservation technologies designed to rapidly inactivate microbial cells include thermal processes, irradiation, high-pressure processing, and the use of strong oxidant compounds. Other technologies accomplish the preservation of foods by inhibiting growth; the most extensively used are low-temperature storage (refrigeration and freezing), reduction of moisture content (concentration and drying), control of ox-redox potential (use of controlled atmospheres and vacuum packaging), and acidification (fermentation and addition of organic acids). In nature, microorganisms are constantly exposed to similar changes in temperature, oxygen, moisture, light, pH, and chemical composition. Bacteria are able to survive thanks to a wide array of molecular responses that provide cellular protection against stresses. Bacteria are protected from changes in pH, temperature, oxidative conditions, solute concentrations, and pressure by a network of sophisticated global genetic regulatory systems and molecular stress responses specific to individual chemical or physical threats. The most important general regulators and specific genetic systems reported in representative foodborne pathogenic bacteria are highlighted in this chapter.

This chapter describes the most important global regulator systems in representative food-borne pathogens and reviews the individual molecular mechanisms of survival against specific food-related stresses. It talks about representative gram-positive bacteria (Listeria monocytogenes and Staphylococcus aureus) and gram-negative (Escherichia coli, Salmonella, and Campylobacter) food-borne pathogens which have been thoroughly studied. Weber et al. have classified all the RpoS-regulated genes into six major categories based on their functions: metabolism, regulation, transport, adaptation to stress, protein processing, and unknown. Gram-negative and gram-positive bacteria use global strategies in their response to osmotic stress as well as some unique species-specific responses. The chapter reviews the current knowledge on the molecular response to osmotic stress as manifested by a few food-borne pathogens. In most bacteria, glycine betaine is the most effective osmoprotectant, as it increases the volume of available water in the cytoplasm. Low-temperature storage of foods is an extremely successful preservation technology. Advancement of one's knowledge about the molecular basis for bacterial survival under stressful conditions is critical to the assurance of safe and palatable foods, whether using traditional or novel preservation technologies. The advent of genomics-, proteomics-, and metabolomics-based techniques has accelerated one's knowledge of the components involved in novel stress responses. In addition, the availability of an increasing number of fully sequenced bacterial genomes should facilitate further advances in the field of bacterial stress responses.

Enterococcus, Bifidobacterium, and coliphages have been proposed as fecal indicator bacteria for foods, but to this date E. coli continues to be used as the preferred indicator of fecal contamination. Although there is significant controversy whether the term "fecal indicator"; can be applied unequivocally to any of the groups of microorganisms discussed in this chapter, presently, most microbiologists agree on the value of testing some of them as general indicators of the quality and hygiene of foods. The characteristic IMViC profile of E. coli and other Enterobacteriaceae classified as coliforms is provided in the chapter. Investigators often compared the usefulness of coliforms and enterococci as indicators of safety in foods. The term coliphages is used to refer to specific types of viruses or bacteriophages that are capable of infecting enteric bacteria and, specifically, E. coli. The rationale for the use of a fecal indicator microorganism is ultimately to signify the potential presence of pathogenic microorganisms. The distribution of fecal food-borne pathogens in animals is quite diverse. Although some bacteria have the unique ability to colonize the intestine of many different animal species, others are confined to a single or few species. The presence of other food-borne pathogens, such as Staphylococcus aureus, is also tolerated in some foods, but for the most part the use of pathogenic bacteria as indicator microorganisms is not encouraged. With the advent of novel, rapid, sensitive, and specific genomic and metabolomictechniques, better alternatives may be soon be discovered.