This work investigated the role of rpoS in the development of increased cell envelope resilience and enhanced pressure resistance in stationary-phase cells of Escherichia coli. Loss of both colony-forming ability and membrane integrity, measured as uptake of propidium iodide (PI), occurred at lower pressures in E. coli BW3709 (rpoS) than in the parental strain (BW2952). The rpoS mutant also released much higher concentrations of protein under pressure than the parent. We propose that RpoS-regulated functions are responsible for the increase in membrane resilience as cells enter stationary phase and that this plays a major role in the development of pressure resistance. Strains from the Keio collection with mutations in two RpoS-regulated genes, cfa (cyclopropane fatty acyl phospholipid synthase) and osmB (outer membrane lipoprotein), were significantly more pressure sensitive and took up more PI than the parent strain, with cfa having the greatest effect. Mutations in the bolA morphogene and other RpoS-regulated lipoprotein genes (osmC, osmE, osmY, and ybaY) had no effect on pressure resistance. The cytoplasmic membranes of the rpoS mutant failed to reseal after pressure treatment, and strains with mutations in osmB and nlpI (new lipoprotein) were also somewhat impaired in the ability to reseal their membranes. The cfa mutant, though pressure sensitive, was unaffected in membrane resealing, implying that the initial transient permeabilization event is critical for loss of viability rather than the failure to reseal. The enhanced pressure sensitivity of polA, recA, and xthA mutants suggested that DNA may be a target of oxidative stress in pressure-treated cells.Consumer demand for minimally processed, additive-free foods that have a long shelf life has stimulated research into new nonthermal methods of food preservation. High hydrostatic pressure (HHP) can inactivate nonsporing food-borne pathogens and spoilage organisms with minimal effects on food quality and is thus regarded as one of the more promising of the new technologies. The operation of HHP machinery requires relatively low energy consumption, produces little waste, and therefore, has a low impact on the environment (13,37,46). The use of HHP in the food industry is gradually increasing and is now being applied to a wide range of food products, including oysters, crabs and other crustaceans, cured hams, guacamole, salsas, fruit juices, desserts, and prepared meals (50).In order to ensure the safety of pressure-treated foods, it is important not only to have quantitative information on the intrinsic pressure resistance of different microorganisms but also to understand the basis of cellular pressure resistance and how this is affected by conditions during growth, pressure treatment, and recovery. Three main processes are currently believed to contribute to the inactivation of pressure-treated cells. These are protein denaturation, oxidative damage, and loss of membrane integrity (34).In all bacteria examined so far, pressure resistance is highly depende...