Salmonella typhimurium cells were subjected to pulsed electric field (PEF) treatment at 25 kV/cm for 0–4 ms to investigate the effect of PEF on the cytoplasmic membrane lipids and oxidative injury of cells. Results indicated that PEF treatment induced a decrease of membrane fluidity of Salmonella typhimurium (S. typhimuriumi), possibly due to the alterations of fatty acid biosynthesis-associated gene expressions (down-regulation of cfa and fabA gene expressions and the up-regulation of fabD gene expression), which, in turn, modified the composition of membrane lipid (decrease in the content ratio of unsaturated fatty acids to saturated fatty acids). In addition, oxidative injury induced by PEF treatment was associated with an increase in the content of malondialdehyde. The up-regulation of cytochrome bo oxidase gene expressions (cyoA, cyoB, and cyoC) indicated that membrane damage was induced by PEF treatment, which was related to the repairing mechanism of alleviating the oxidative injury caused by PEF treatment. Based on these results, we achieved better understanding of microbial injury induced by PEF, suggesting that micro-organisms tend to decrease membrane fluidity in response to PEF treatment and, thus, a greater membrane fluidity might improve the efficiency of PEF treatment to inactivate micro-organisms.
The effect of culture temperature on the conformation of membrane lipid of Salmonella Typhimurium as well as their pulsed electric field (PEF) resistance was investigated. Results revealed that S. Typhimurium cells cultured at relatively lower temperature (10 and 25°C) were more easily inactivated by PEF treatment than those cultured at relatively higher temperature (37 and 45°C). Using a Weibull model, it was determined that the PEF treatment time to inactivate 90% S. Typhimurium cells cultured at 45°C was almost fourfold compared to those cultured at 10°C. Results of micro-Raman spectroscopy indicated that as the culture temperature increased, the order degree of C-C and lateral packing order of membrane lipid chain also increased, resulting in a drop in the membrane fluidity. These results are important in considering the use of heat and PEF to inactivate microbe contaminants in food.
Hierarchical spherical Bi 2 S 3 particles with nanorod were synthesized by hydrothermal method, and then BiCl 3 /Bi 2 S 3 composite powders with different molar ratios were consolidated into bulk samples by spark plasma sintering (SPS) technique. The addition of BiCl 3 with appropriate amount not only increased the electrical conductivity, but also decreased the thermal conductivity of Bi 2 S 3. The Bi 2 S 3 sample doped with 0.5mol% BiCl 3 shows a maximum electrical conductivity of 45.1 S•cm-1 at 762 K, which is much higher than that of pure Bi 2 S 3 at 762 K (12.9 S•cm 1). The minimum thermal conductivity is 0.31 W•m 1 •K 1 for the Bi 2 S 3 sample doped with 0.25mol% BiCl 3 at 762 K, which is lower than that of pure Bi 2 S 3 (0.47 W•m 1 •K 1) at the same temperature. The maximum ZT value of 0.63 at 762 K was achieved by Bi 2 S 3 doped with 0.25mol% BiCl 3 , which is almost two times higher than that of pure Bi 2 S 3 (0.22).
Summary
Effects of various growth temperatures on cell membrane fatty acid composition of Salmonella Typhimurium (S. Typhimurium) and the resistance to Pulsed Electric Fields (PEF) treatments, as well as PEF combining with mild‐thermal (35, 45, 55 °C) treatments were investigated. Results indicated that the PEF resistance of S. Typhimurium at stationary phase was varied markedly at different growth temperatures. S. Typhimurium grown at 45 °C exhibited greater PEF resistance than cells grown at a lower temperature. Alteration of membrane fatty acid composition decreased in the unsaturated to saturated fatty acids ratio (UFA/SFA) and increased in cyclopropane fatty acids (CFA) proportion as growth temperature increased. It was found that the PEF resistance of S. Typhimurium at stationary phase was in a membrane fluidity dependent manner. Thermal combined PEF treatment improved the PEF lethality which indicated that the PEF resistance of S. Typhimurium was greatly affected by the fluidity of cytomembrane.
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