Effect of electropermeabilization by ohmic heating for inactivation of Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in buffered peptone water and apple juice

Development and investigation of ultrasound-assisted pulsed ohmic heating for inactivation of foodborne pathogens in milk with different fat content

The central objective of this research was to develop an ultrasound-assisted pulsed ohmic heating (POH) system for inactivation of food-borne pathogens in phosphate buffered saline (PBS) and milk with 0-3.6% fat and investigate its bactericidal effect. Combining ultrasound with POH did not significantly affect the temperature profile of samples. Both POH alone and ultrasound-assisted POH took 120 s to heat PBS 60℃. Milk with 0, 1, and 3.6% fat was heated to 60℃ by POH alone and ultrasound-assisted POH after 335, 475, and 525 s, respectively. This is because the electrical conductivity of the samples was the same for POH alone and ultrasound-assisted POH. Despite identical temperature profiles, ultrasound-assisted POH exerted a synergistic effect on the reduction of Escherichia coli O157:H7, Salmonella Typhimurium, Listeria monocytogenes, and Staphylococcus aureus. In particular, the inactivation level of S. Typhimurium in PBS subjected to ultrasound-assisted POH treatment for 120 s corresponding to a treatment temperature of 60℃ was 3.73 log units higher than the sum of each treatment alone. A propidium iodide assay, intracellular protein measurements, and scanning electron microscopy revealed that ultrasound-assisted POH treatment provoked lethal cell membrane damage and leakage of intracellular proteins. Meanwhile, fat in milk reduced the efficacy of the bacterial inactivation of the ultrasound-assisted POH system due to its low electrical conductivity and sonoprotective effect. After ultrasound-assisted POH treatment at 60℃, there were no significant differences (P > 0.05) in the pH, color, and apparent viscosity of milk between the untreated and treated group.

Inactivation effects and mechanism of ohmic heating on Bacillus cereus

The inactivation effects and mechanism of ohmic heating (OH) on Bacillus cereus ATCC 11778 were investigated in this study, conventional heating (CH) was also carried out and served as control. All OH treatments (10 V/cm 50 Hz, 10 V/cm 500 Hz, 5 V/cm 50 Hz and 5 V/cm 500 Hz) could achieve a comparable inactivation effect with CH, while OH treatments significantly shortened the processing time. OH treated cells exhibited significantly higher leakage of metal ions (Mg²⁺ and K⁺) and biomacromolecules (nucleic acids and proteins) than those treated with CH when bacterial suspensions were heated to the same temperature. Moreover, OH treatment caused more damage on membrane structure, greatly decreased the cell membrane potential and endogenous enzyme activity than that of CH. The results of this study indicated that OH is more efficient in the inactivation of bacteria.

Alkyl ferulic acid esters: Evaluating their structure and antibacterial properties

Ferulic acid (FA) is a natural antibacterial agent rich in plants, FA has excellent antioxidant and antibacterial properties. However, because of its short alkane chain and large polarity, FA is difficult to penetrate the soluble lipid bilayer in the biofilm to enter the cell to play an inhibitory role, limiting its biological activity. To improve the antibacterial activity of FA, with the catalytic condition of Novozym 435, four alkyl ferulic acid esters (FCs) with different alkyl chain lengths were obtained by fatty alcohols (including 1-propanol (C3), 1-hexanol (C6), nonanol (C9), and lauryl alcohol (C12)) modification. The effect of FCs on P. aeruginosa was determined by Minimum inhibitory concentrations (MIC), minimum bactericidal concentrations (MBC), Growth curves, alkaline phosphatase (AKP) activity, crystal violet method, scanning electron microscopy (SEM), membrane potential, PI, cell contents leakage. Results showed that the antibacterial activity of FCs increased after esterification, and the antibacterial activity significantly increased and then decreased with the extension of the alkyl chain of the FCs. Hexyl ferulate (FC6) showed the best antibacterial activities against E. coli and P. aeruginosa (MIC for E. coli was 0.5 mg/ml, MIC for P. aeruginosa was 0.4 mg/ml). And Propyl ferulate (FC3) and FC6 showed the best antibacterial activities S. aureus and B. subtilis (MIC for S. aureus was 0.4 mg/ml, The MIC of B. subtilis was 1.1 mg/ml). In addition, the growth, AKP activity, bacterial biofilm, bacterial cell morphology, membrane potential and cell contents leakage of P. aeruginosa after different FCs were investigated, which found that FCs could damage the cell wall of P. aeruginosa and showed different effects on the P. aeruginosa cell biofilm. FC6 showed the best inhibition on the biofilm formation of P. aeruginosa cells, which caused the surface of P. aeruginosa cells to be rough and wrinkled. Some P. aeruginosa cells showed aggregation and adhesion, even rupture. The membrane hyperpolarization was obvious, which appeared as holes, leading to cell contents leakage (protein and nucleic acid). All these results concluded that the antibacterial activities FCs against foodborne pathogens depended on different fatty alcohol esterification of FA. FC6 showed the best inhibition on P. aeruginosa due to its effect on P. aeruginosa cell walls and biofilms and the leak of the cell contents. This study provides more practical methods and a theoretical basis for giving full play to the bacteriostatic effect of plant FA.

Antibacterial properties of cyclolinopeptides from flaxseed oil and their application on beef

The objective of this study was to investigate the antibacterial activity and potential mechanism of cyclolinopeptides, a type of cyclic hydrophobic peptides present in flaxseed oil. In this study, 1-Mso cyclolinopeptides B and 1-Mso, 3-Mso-cyclolinopeptides F from flaxseed oil exhibited excellent antibacterial activity against Listeria monocytogenes through destroying bacterial cell membrane. Our results indicated that cyclolinopeptides are one of the antibacterial components in flaxseed oil. Also, the application of cyclolinopeptides B and 1-Mso, 3-Mso-cyclolinopeptides F in inhibiting the microbial contamination of beef was investigated as well. Thus, our study highlights the promising potential of cyclolinopeptides to serve as food additives or food preservations due to their strong antibacterial activity against Listeria monocytogenes.

Chitosan-grafted-phenolic acid copolymers against Shewanella putrefaciens by disrupting the permeability of cell membrane

Chitosan (CS) is a kind of high molecular polymer with antibacterial properties. A copolymer with high bacteriostatic activity can be formed by grafting phenolic acid compounds into the chitosan molecular chain, which can inhibit the growth of dominant spoilage bacteria in aquatic products. The study aimed to investigate the antibacterial effect and mechanism of chitosan-grafted-phenolic acid copolymers on Shewanella putrefaciens (S. putrefaciens). CS-grafted-protocatechuic acid (CS-g-PA) and CS-grafted-gallic acid (CS-g-GA) were attained by EDC/NHS coupling reaction. The antibacterial tests indicated that CS-g-PA and CS-g-GA had the same minimum inhibitory concentration (MIC) (1.25 mg/mL) and minimum bactericidal concentration (MBC) (5.0 mg/mL) against S. putrefaciens. According to the change trend of growth curve, the growth of S. putrefaciens was significantly restrained under 2MIC graft copolymers (P < 0.05). Moreover, the increment of alkaline phosphatase (AKPase) activity and electrical conductivity demonstrated that the cell wall and membrane permeability of S. putrefaciens were damaged respectively. In addition, the increase of lactate dehydrogenase (LDHase) activity, protein and nucleic acid absorbance and the decrease of adenosine triphosphatase (ATPase) activity suggested that the cell membrane was incomplete and poor fluidity. The irregular shape of bacteria and the outflow of intercellular contents were also observed from scanning electron microscope (SEM). The above results manifested a great potential of CS-g-PA and CS-g-GA for use as food preservatives to aquatic products.

The application of photodynamic inactivation to microorganisms in food

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Recent progresses in the development of photodynamic inactivation (PDI) of bacteria were summarized.

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Key factors influencing the PDI effects were firstly reviewed.

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Photosensitizers which can be applied in food products for PDI are summarized.

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Application of PDI in various food substrates are also reviewed.

Nowadays, food safety issues have drawn increased attention due to the continual occurrence of infectious diseases caused by foodborne pathogens, which is an important factor causing food safety hazard. Meanwhile, the emergence of an increasing number of antibiotic-resistant pathogens is a worrisome phenomenon. Therefore, it is imperative to find new technologies with low-cost to inactivate pathogenic microorganisms and prevent cross-contamination. Compared with traditional preservatives, photodynamic inactivation (PDI) has emerged as a novel and promising strategy to eliminate foodborne pathogens with advantages such as non-toxic and low microbial resistance, which also meets the demand of current consumers for green treatment. Over the past few years, reports of using this technology for food safety have increased rapidly. This review summarizes recent progresses in the development of photodynamic inactivation of foodborne microorganisms. The mechanisms, factors influencing PDI and the application of different photosensitizers (PSs) in different food substrates are reviewed.

Antimicrobial mechanism of alkyl gallates against Escherichia coli and Staphylococcus aureus and its combined effect with electrospun nanofibers on Chinese Taihu icefish preservation

The objective of this study was to investigate the antibacterial activity and potential mechanism of alkyl gallates against Escherichia coli and Staphylococcus aureus. Results show that the length of the alkyl chain plays a pivotal role in eliciting the activity and octyl gallate (OG) exerted excellent bactericidal activity through a multiple bactericidal mechanism. OG functions against both bacteria through damaging bacterial cell wall integrity, permeating into cells and then interacting with DNA, as well as disturbing the activity of the respiratory electron transport chain to induce a high-level toxic ROS (hydroxyl radicals) generation and up-regulation of the ROS genes. Also, electrospun nanofibers with OG have unique superiorities for maintaining the freshness of the icefish (4 °C). This research not only provides a more in-depth understanding of the interaction between OG and microorganisms but also highlights the great promise of using OG as a safe multi-functionalized food additive for food preservations.

Ohmic heating as a method of obtaining paraprobiotics: Impacts on cell structure and viability by flow cytometry

This study aimed to evaluate the effects of ohmic heating (OH) on probiotic inactivation, cell viability and morphology of the probiotic strains Lactobacillus acidophilus LA 05 (LA), Lacticaseibacillus casei 01 (LC), and Bifidobacterium animalis Bb 12 (BA) to develop paraprobiotics. OH at different electric field magnitudes (4, 8, and 12 V/cm at 60 Hz) and conventional heat treatment (CONV) were performed to determine the most adequate condition for the obtainment of paraprobiotics. Analysis of culturability, flow cytometry (FC), and Scanning electron microscope (SEM) was carried out. The complete inactivation by CONV was achieved only in the following conditions: LA - 95 °C/5 min, LC and BA - 95 °C/7 min. The same temperature profile was used in OH treatments to study the OH electrical effects. The OH treatment (8 V/cm) caused lower damage to the cell membrane integrity compared to the CONV treatment (p < 0.05). The OH showed to be adequate technology for the efficient production of paraprobiotics.

Effect of Ohmic Heating on Sensory, Physicochemical, and Microbiological Properties of "Aguamiel" of Agave salmiana

The use of ohmic heating (OH) processing technologies in beverages might provide a higher quality value to the final product; consumers tended to prefer more natural products with minimum preservative substances. The aim of this work was to evaluate the effect of OH over the presence of microorganisms in "aguamiel" as well as to study the effects on physicochemical analysis like total sugars, soluble solids, electric conductivity pH, and color. The results showed that the conductivity of "aguamiel" was 0.374 s/m, this as temperature increased, conductivity rose as well. During OH a bubbling was observed when reaching 70 °C due to the generation of electrochemical reactions during the OH process. OH had a significant effect in the reduction of E. coli, yeast, and lactobacillus compared to conventional pasteurization, reaching optimal conditions for its total inactivation. Regarding its physicochemical properties, both treatments, conventional pasteurization and OH, did not show negative changes in aguamiel, demonstrating that OH technology can be a feasible option as a pasteurization technique. In conclusion it is important to notice that negative changes were not found in quality, color and sugars of "aguamiel". Therefore, ohmic heating can be an option to replace traditional methods used for pasteurization.

Bactericidal and synergistic effects of X-ray irradiation and gallic acid against foodborne pathogens on lettuce

The objectives of this study were to evaluate the bactericidal effects of X-ray irradiation and gallic acid (GA) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on lettuce leaves and in phosphate-buffered saline (PBS). Inoculated PBS and lettuce were exposed to X-rays (0.05, 0.1, and 0.15; 0.1, 0.2, and 0.3 kGy, respectively), and GA was applied to lettuce leaves as a solution and in PBS at concentrations of 0.5% (w/v). Combined treatment with 0.3 kGy and 0.5% GA reduced E. coli O157:H7, S. Typhimurium, and L. monocytogenes cell counts 5.41, 2.57, and 1.36 log CFU/cm2 on lettuce, respectively. Combined treatment with 0.15 kGy X-ray and 0.5% GA reduced counts for the same species by 6.54, 4.24, and 1.51 log CFU/mL in PBS. The combined treatments exerted a synergistic antibacterial effect against E. coli O157:H7 on lettuce, but not against S. Typhimurium or L. monocytogenes. In PBS, the synergistic effect was confirmed in both E. coli O157:H7 and S. Typhimurium cells. Mechanistic investigations indicated that the synergistic antibacterial effect was associated with intracellular reactive oxygen species (ROS) generation and bacterial cell membrane damage. Additionally, the X-ray and GA combination treatment did not adversely affect the color, total phenol content, and texture of lettuce. These findings demonstrate that treatment with X-ray radiation and GA can enhance the microbiological safety of fresh produce.

Synergistic bactericidal effect of hot water with citric acid against Escherichia coli O157:H7 biofilm formed on stainless steel

This study investigated the antimicrobial effect of hot water with citric acid against Escherichia coli O157:H7 biofilm on stainless steel (SS). Hot water (50, 60, or 70 °C) with 2% citric acid exhibited a synergistic bactericidal effect on the pathogen biofilm. It was revealed that hot water and citric acid combination induced sub-lethally injured cells. Additionally, mechanisms of the synergistic bactericidal effects of hot water with citric acid were identified through several approaches. In terms of biofilm matrix, hot water removes exopolysaccharides, a major component of extracellular polymeric substances (EPS), thereby increasing contact between surface cells and citric acid, resulting in a synergistic bactericidal effect. In terms of the cell itself, increased permeability of citric acid through cell membranes destructed by hot water promotes the inactivation of superoxide dismutase (SOD) in E. coli O157:H7, which induce synergistic generation of reactive oxygen species (ROS) which promote inactivation of cell by activating lipid peroxidation, resulting in destruction of the cell membrane. Therefore, it is interpreted that when hot water with citric acid is applied to E. coli O157:H7 biofilm, synergy effects on the biofilm matrix and cell itself have a complex interaction with each other, thus causing a dramatic synergistic bactericidal effect.

Synergistic effect of 222-nm krypton-chlorine excilamp and mild heating combined treatment on inactivation of Escherichia coli O157:H7 and Salmonella Typhimurium in apple juice

Simultaneous treatment with 222-nm KrCl excilamp and mild heating (EX-MH) at 45, 50 and 55 °C showed synergistic bactericidal effects on non-acid and acid adapted cells of Escherichia coli O157:H7 and Salmonella Typhimurium in apple juice. In particular, acid-adapted pathogens exhibited increased resistance to EX-MH compared to pathogenic bacteria that were not acid-adapted. Also, elucidation of the synergistic bactericidal mechanism of EX-MH was performed through several assays and this mechanism was described as follows: (i) when KrCl excilamp (EX) and mild heating (MH) are applied simultaneously, MH reversibly inactivates the antioxidant enzyme, superoxide dismutase (SOD), thereby increasing accumulation of reactive oxygen species (ROS) generated by EX and thus inducing synergistic ROS generation, (ii) ROS production induces lipid peroxidation occurrence in the cell membrane, (iii) this lipid peroxidation occurrence in the cell membrane induces synergistic destruction of cell membrane, resulting in synergistic cell death. While EX-MH of 45, 50, or 55 °C reduced E. coli O157:H7 (the pathogen most resistant to EX-MH) in apple juice by 5-log, the qualities such as color (L*, a*, and b*), total phenolic compounds (TPC), and DPPH free radical scavenging activity of apple juice did not change significantly (P > 0.05). This study not only suggests the applicability of EX-MH to the apple juice industry, but also can be used as baseline data for future relevant research.

Innovative induction heating of grapefruit juice via induced electric field and its application in Escherichia coli O157:H7 inactivation

The proposed induction heating method was applied in the pasteurization of grapefruit juice. In this processing, an alternating magnetic field acted as the stimulus instead of conventional electrodes to create an induced electric field (IEF) for heat treatment of the continuous-flow juice sample, which excluded the possibility of electrochemical reaction and electrode corrosion that might occur in conventional electric field treatments. As a typical food pathogen, Escherichia coli O157:H7 was selected as a representative to investigate its inactivation by the heating process under different voltages and frequencies, initial temperatures, and flow rates (or retention time). The grapefruit juice was successfully heated up by IEF and the temperature curve was achieved when the juice exposed to IEF. The heating rate and terminal temperature increased with the increasing induced voltage, decreasing frequency and at higher initial temperature. A highest terminal temperature of 93.7 °C for grapefruit juice with an initial temperature of 20 °C was achieved under induced voltage of 2700 V, frequency of 300 Hz and residence time of 400 s. At the same time, E. coli O157:H7 in the grapefruit juice was thoroughly inactivated. There was a trend that the pathogen survival rate was reduced at higher induced voltage, lower frequency and higher initial temperature during the heating treatment. No significant changes in pH and °Brix was observed after this innovative induction heating, but the color of grapefruit juice was brightened. The proposed induction heating can be regarded as a sister technology of ohmic heating, and it provide a reference for the application of this heating method in liquid food pasteurization.

Innovative induction heating method is applied in the pasteurization of grapefruit juice.

Inactivation Kinetics and Membrane Potential of Pathogens in Soybean Curd Subjected to Pulsed Ohmic Heating Depending on Applied Voltage and Duty Ratio

The aim of this research was to investigate the efficacy of the duty ratio and applied voltage in the inactivation of pathogens in soybean curd by pulsed ohmic heating (POH). The heating rate of soybean curd increased rapidly as the applied voltage increased, although the duty ratio did not affect the temperature profile. We supported this result by verifying that electrical conductivity increased with the applied voltage. Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in soybean curd were significantly (P < 0.05) inactivated by more than 1 log unit at 80 Vrms (root mean square voltage). To elucidate the mechanism underlying these results, the membrane potential of the pathogens was examined using DiBAC4(3) [bis-(1,3-dibutylbarbituric acid)trimethine oxonol] on the basis of a previous study showing that the electric field generated by ohmic heating affected the membrane potential of cells. The values of DiBAC4(3) accumulation increased under increasing applied voltage, and they were significantly (P < 0.05) higher at 80 Vrms, while the duty ratio had no effect. In addition, morphological analysis via transmission electron microscopy showed that electroporation and expulsion of intracellular materials were predominant at 80 Vrms Moreover, electrode corrosion was overcome by the POH technique, and the textural and color properties of soybean curd were preserved. These results substantiate the idea that the applied voltage has a profound effect on the microbial inactivation of POH as a consequence of not only the thermal effect, but also the nonthermal effect, of the electric field, whereas the duty ratio does not have such an effect.IMPORTANCE High-water-activity food products, such as soybean curd, are vulnerable to microbial contamination, which causes fatal foodborne diseases and food spoilage. Inactivating microorganisms inside food is difficult because the transfer of thermal energy is slower inside than it is outside the food. POH is an adequate sterilization technique because of its rapid and uniform heating without causing electrode corrosion. To elucidate the electrical factors associated with POH performance in the inactivation of pathogens, the effects of the applied voltage and duty ratio on POH were investigated. In this study, we verified that a high applied voltage (80 Vrms) at a duty ratio of 0.1 caused thermal and nonthermal effects on pathogens that led to an approximately 4-log-unit reduction in a significantly short time. Therefore, the results of this research corroborate database predictions of the inactivation efficiency of POH based on pathogen control strategy modeling.

Comparative analysis of quality and microbial safety of ohmic and water bath cooked pork batter during refrigerated storage

In this study, the microbial safety, lipid and protein oxidation, and water characteristics of ohmic (OH) and water bath (WB) cooked pork batter during storage at 4 °C were investigated. The results indicated that the cooking time was much shorter for samples cooked to 72 °C by OH cooking (2 min) than WB cooking (41 min), but OH and WB cooked samples had no significant difference in total viable colony (TVC) at day 28. No significant differences were observed in thiobarbituric acid reactive substances (TBARS) and total sulfhydryl content between OH and WB cooked samples (P > 0.05), but the OH cooked samples had higher carbonyl content (P < 0.05). Although there were no significant differences for water content and drip loss between OH and WB cooked samples, the relaxation time T ₂₂ of the OH cooked samples were longer than WB cooked ones (P < 0.05). On the whole, the qualities of the OH cooked samples were comparable to the WB cooked ones during the entire storage period, indicating that there was a great prospect for OH cooking to be applied in the meat product industry.

Label free-based proteomic analysis of Escherichia coli O157:H7 subjected to ohmic heating

To investigate the inactivation mechanism of ohmic heating (OH) on Escherichia coli O157:H7 at the same inactivation levels, a label-free quantitative proteomic approach was employed in this study. Quantification of 2633 proteins was obtained with high confidence. Compared to untreated samples (CT), a total of 169, 84, and 26 proteins showed significantly differential abundance after high voltage OH (HVOH, 10 V/cm), low voltage OH (LVOH, 5 V/cm), and water bath heating (WB), respectively. Glyoxylate and dicarboxylate metabolism, ABC transporters, biosynthesis of amino acids, glycerophospholipid metabolism, and ribosome pathway were the main KEGG pathways mediated by OH, but only ribosome pathway was greatly affected by WB. The significant differences in proteome changes of E. coli O157:H7 among HVOH, LVOH, and WB treatments, especially the greater number of differential proteins in HVOH, indicated that OH might exert additional effects on proteome of E. coli O157:H7 due to the electric current, particularly in HVOH with higher electric field. This result enriched our understanding of molecular changes of E. coli O157:H7 induced by OH and provided data reference for further research into the inactivation mechanism of OH.

Inactivating foodborne pathogens in apple juice by combined treatment with fumaric acid and ultraviolet-A light, and mechanisms of their synergistic bactericidal action

We evaluated the bactericidal efficacy of the simultaneous application of ultraviolet-A (UV-A) irradiation and fumaric acid (FA) against Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in apple juice and as well as investigated the effects of this treatment on product quality. Further, we elucidated the mechanisms underlying their synergistic bactericidal action. Simultaneous UV-A light irradiation and 0.1% FA treatment for 30 min resulted in 6.65-, 6.27-, and 6.49-log CFU/ml reductions in E. coli O157:H7, S. Typhimurium, and L. monocytogenes, respectively, which involved 3.15, 2.21, and 3.43 log CFU reductions, respectively, and these were attributed to the synergistic action of the combined treatments. Mechanistic investigations suggested that the combined UVA-FA treatment resulted in significantly greater bacterial cell membrane damage and intracellular reactive oxygen species (ROS) generation. UVA-FA treatment for 30 min did not cause significant changes to the color, nonenzymatic browning index, pH, and total phenolic content of apple juice. These results suggest that combined UVA-FA treatment can be effectively used to control foodborne pathogens in apple juice without affecting its quality.

Mechanisms of enhanced bacterial endospore inactivation during sterilization by ohmic heating

During ohmic heating, the electric field may additionally inactivate bacterial endospores. However, the exact mechanism of action is unclear. Thus, a mechanistic study was carried out, investigating the possible target of electric fields inside the spore. Bacillus subtilis spores were heated by conventional and ohmic heating in a capillary system under almost identical thermal conditions. Wild-type (PS533) spores were used, as well as isogenic mutants lacking certain components known for their contribution to spores' heat resistance: small-acid soluble proteins (SASP) protecting DNA (PS578); the coat covering the spore (PS3328); and the spore germination enzyme SleB (FB122(+)). Treatment-dependent release of the spore core's depot of dipicolinic acid (DPA) was further evaluated. Up to 2.4 log₁₀ additional inactivation of PS533 could be achieved by ohmic heating, compared to conventional heating. The difference varied for the mutants, with a decreasing difference indicating a decreased effect of the electric field and vice versa. In particular, mutant spores lacking SASPs showed a behavior more similar to thermal inactivation alone. The combination of heat and electric field was shown to be necessary for enhanced spore inactivation. Thus, it is hypothesized that either the heat treatment makes the spore susceptible to the electric field, or vice versa.