Viability of Escherichia coli after combined osmotic and thermal treatment: a plasma membrane implication

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The combined effects of subzero temperature and high pressure on the inactivation of Escherichia coli K12TG1 were investigated. Cells of this bacterial strain were exposed to high pressure (50 to 450 MPa, 10-min holding time) at two temperatures (؊20°C without freezing and 25°C) and three water activity levels (a w ) (0.850, 0.992, and ca. 1.000) achieved with the addition of glycerol. There was a synergistic interaction between subzero temperature and high pressure in their effects on microbial inactivation. Indeed, to achieve the same inactivation rate, the pressures required at ؊20°C (in the liquid state) were more than 100 MPa less than those required at 25°C, at pressures in the range of 100 to 300 MPa with an a w of 0.992. However, at pressures greater than 300 MPa, this trend was reversed, and subzero temperature counteracted the inactivation effect of pressure. When the amount of water in the bacterial suspension was increased, the synergistic effect was enhanced. Conversely, when the a w was decreased by the addition of solute to the bacterial suspension, the baroprotective effect of subzero temperature increased sharply. These results support the argument that water compression is involved in the antimicrobial effect of high pressure. From a thermodynamic point of view, the mechanical energy transferred to the cell during the pressure treatment can be characterized by the change in volume of the system. The amount of mechanical energy transferred to the cell system is strongly related to cell compressibility, which depends on the water quantity in the cytoplasm.Food processing under high hydrostatic pressure is an emerging technology that has stimulated considerable interest within the food industry over the past 15 years. There are currently some interesting commercial opportunities and research challenges in the high-pressure processing of foods (42). This processing technique allows the manufacture of innovative food products while preserving the texture, color, raw flavoring agents, and nutritional value of the food, which are all aspects valued by consumers (22,36). Furthermore, high-pressure treatments cause the denaturation of several enzymes that are responsible for quality deterioration, as well as the inactivation of pathogenic and spoilage microorganisms (15). However, to achieve high or complete microbial inactivation, high pressures and/or long treatment times are required, so that the cost of the process seriously limits its industrial applications. For this reason, it would be beneficial to optimize high-hydrostatic-pressure processes.Numerous studies have demonstrated the temperature dependence of the antimicrobial effects of high pressure (19,25,39). Moreover, the efficiency of high-pressure treatments is controlled by other process parameters such as the applied pressure and the kinetics of pressurization (31), as well as by the physicochemical properties of the medium being treated, such as pH (1, 18) and water activity (10,26,43). Precise control of these parameters is necessary to ...

Section: Resultssupporting

confidence: 90%

Synergistic and Antagonistic Effects of Combined Subzero Temperature and High Pressure on Inactivation ofEscherichia coli

Moussa

Perrier-Cornet

Gervais

2006

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“…Putrescine, the polyamine with the highest intracellular concentration in E. coli, is exported upon osmotic upshock to balance the increase in the internal positive charge resulting from K ϩ uptake (31,40). The response to hyperosmotic conditions increases the tolerance of E. coli and other bacteria to other stresses, including thermal inactivation (27,29). The exceptional expression of NmpC and transport proteins likely contributes to the heat resistance of E. coli AW1.7 through an altered pool of cytoplasmic osmolytes.…”

Section: Discussionmentioning

confidence: 99%

Solute Transport Proteins and the Outer Membrane Protein NmpC Contribute to Heat Resistance of Escherichia coli AW1.7

Ruan

Pleitner

Gänzle

et al. 2011

This study aimed to elucidate determinants of heat resistance in Escherichia coli by comparing the composition of membrane lipids, as well as gene expression, in heat-resistant E. coli AW1.7 and heat-sensitive E. coli GGG10 with or without heat shock. The survival of E. coli AW1.7 at late exponential phase was 100-fold higher than that of E. coli GGG10 after incubation at 60°C for 15 min. The cytoplasmic membrane of E. coli AW1.7 contained a higher proportion of saturated and cyclopropane fatty acids than that of E. coli GGG10. Microarray hybridization of cDNA libraries obtained from exponentially growing or heat-shocked cultures was performed to compare gene expression in these two strains. Expression of selected genes from different functional groups was quantified by quantitative PCR. DnaK and 30S and 50S ribosomal subunits were overexpressed in E. coli GGG10 relative to E. coli AW1.7 upon heat shock at 50°C, indicating improved ribosome stability. The outer membrane porin NmpC and several transport proteins were overexpressed in exponentially growing E. coli AW1.7. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of membrane properties confirmed that NmpC is present in the outer membrane of E. coli AW1.7 but not in that of E. coli GGG10. Expression of NmpC in E. coli GGG10 increased survival at 60°C 50-to 1,000-fold. In conclusion, the outer membrane porin NmpC contributes to heat resistance in E. coli AW1.7, but the heat resistance of this strain is dependent on additional factors, which likely include the composition of membrane lipids, as well as solute transport proteins.Escherichia coli is a common contaminant of the food supply. The majority of the strains of this species are not pathogenic; however, their relatively high resistance to environmental insults and the occurrence of virotypes with a low infectious dose, particularly enterohemorrhagic E. coli, make E. coli an organism of major concern in the production of minimally processed foods, particularly produce and fresh beef (11).Most strains of E. coli have a D 60 value (the duration of heat treatment at 60°C required to reduce the number of microorganisms to 1/10 of the initial value) of less than 1 min. However, the heat resistance of E. coli is highly variable among different strains and individual strains exhibit D 60 values of up to 6.5 min (8,21,25,37). The heat resistance of individual strains of E. coli relates to their ability adapt to heat stress by the homoviscous adaptation of the plasma membrane, as well as the synthesis of heat shock proteins (20, 43). The 32 -induced expression of heat shock proteins after sublethal thermal stress increases resistance to lethal heat treatment (43; for a review, see reference 6). Increased basal expression of the heat shock proteins DnaK, Lon, and ClpX was linked to the increased heat resistance of E. coli mutants LMM1010, LMM1020, and LMM 1030 (1, 21). The S -mediated general stress response additionally contributes to acid, heat, pressure, and salt resistance in E. coli (2,14,22,3...

“…The sensitivity of the permeability coefficient to membrane chain ordering has been found to be particularly marked for acetic acid, with a significant increase in sensitivity observed between formic and acetic acids in dipalmitoylphospatidylcholine bilayer membranes (17). At physiological (isotonic) levels of hydration, the phospholipid bilayer of E. coli is in a fluid, lamellar, liquid-crystalline phase, but the phospholipids undergo phase transition to a gel state with increasing osmotic pressure (6), which may result from alterations in fatty acid structure or changes in packing geometry (7). It has been determined that for E. coli, membrane fluidity is greatly decreased with increasing osmotic pressures up to 40 MPa (2).…”

Section: Discussionmentioning

confidence: 99%

Escherichia coli and Salmonella enterica Are Protected against Acetic Acid, but Not Hydrochloric Acid, by Hypertonicity

Chapman

Ross

2009

) demonstrated that an increased NaCl concentration prolongs survival of Escherichia coli O157 SERL 2 in a broth model simulating the aqueous phase of a food dressing or sauce containing acetic acid. We examined the responses of five other E. coli strains and four Salmonella enterica strains to increasing concentrations of NaCl under conditions of lethal acidity and observed that the average "lag" time prior to inactivation decreases in the presence of hydrochloric acid but not in the presence of acetic acid. For E. coli in the presence of acetic acid, the lag time increased with increasing NaCl concentrations up to 2 to 4% at pH 4.0, up to 4 to 6% at pH 3.8, and up to 4 to 7% (wt/wt of water) NaCl at pH 3.6. Salmonella was inactivated more rapidly by combined acetic acid and NaCl stresses than E. coli, but increasing NaCl concentrations still decreased the lag time prior to inactivation in the presence of acetic acid; at pH 4.0 up to 1 to 4% NaCl was protective, and at pH 3.8 up to 1 to 2% NaCl delayed the onset of inactivation. Sublethal injury kinetics suggest that this complex response is a balance between the lethal effects of acetic acid, against which NaCl is apparently protective, and the lethal effects of the NaCl itself. Compared against 3% NaCl, 10% (wt/wt of water) sucrose with 0.5% NaCl (which has similar osmotic potential) was found to be equally protective against adverse acetic acid conditions. We propose that hypertonicity may directly affect the rate of diffusion of acetic acid into cells and hence cell survival.We previously observed that inactivation of Escherichia coli O157 SERL 2 by acetic acid at adverse pH in a broth model simulating the aqueous phase of acidic sauces and dressings was reduced by the presence of NaCl (4). Specifically, the time to a 3-log 10 -unit reduction (t 3D ) of E. coli SERL 2 as function of NaCl concentration was significantly nonmonotonic; that is, the t 3D initially increased when NaCl was increased (from 1 to 3% [wt/wt] of solution), but the t 3D decreased upon a further increase in NaCl concentration (to 8% [wt/wt] of solution) (4). The statistical significance of this "nonmonotonic" response increased with increasing exposure time from 24 to 72 h (at 23°C), primarily due to a proportionally greater increase in inactivation at 1% (wt/wt) NaCl with increasing treatment time than that which was observed at higher NaCl concentrations (4).The combination of acid and NaCl is a common example of the food industry's "hurdle" approach, which is used to preserve a large and diverse range of foods, including acidic dressings and sauces, fermented meats, cheeses, and preserved vegetables. Given the widespread use of this hurdle combination in food manufacturing, the first aim of this study was to determine whether the observed protection of E. coli SERL 2 from acid inactivation by NaCl is common among E. coli and Salmonella enterica and at what NaCl concentration maximum protection is achieved. A second aim was to determine whether NaCl protection is specific against acid...