Cross-protective effects of temperature, pH, and osmotic and starvation stresses in Escherichia coli O157:H7 subjected to pulsed electric fields in milk

Antibacterial mechanism of vanillin against Escherichia coli O157: H7

Vanillin, a plant-derived antimicrobial volatile substance, has potential microbial control applications in the food industry. However, the effect of vanillin on the food-borne pathogen Escherichia coli (E. coli) O157:H7 has not been well studied. This study aims to explore the antibacterial mechanism of vanillin against E. coli O157:H7. The minimum inhibitory concentration (MIC) and antibacterial effect of vanillin were determined by microdilution. Scanning electron microscopy (SEM) was used to observe the damage of vanillin to the cell membrane, while cell membrane potential and the leakage of nucleic acid protein were measured to explore the effect of vanillin on the membrane system. Confocal laser scanning and intracellular adenosine triphosphate (ATP) concentration determination were utilized to investigate the effects of vanillin on the energy, life, and death of E. coli. Finally, transcriptome sequencing was conducted to investigate the gene expression differences induced by vanillin treatment. The results showed that vanillin treatment effectively controlled E. coli O157:H7 with an MIC of 2 mg/mL. After treatment, damage to the membrane system, depolarization of the membrane, and leakage of nucleic acid and protein were observed. Meanwhile, vanillin treatment caused decreased ATP content and cell death. Transcriptome analysis showed that vanillin treatment significantly affected the expression of genes involved in cell membrane formation, tricarboxylic acid (TCA) cycling pathway, and oxidative phosphorylation pathway in E. coli O157:H7. In conclusion, membrane damage and energy metabolism disruption are important mechanisms of vanillin's inhibitory effect on E. coli O157:H7. This study provides new insights into the molecular reaction mechanism of vanillin against E. coli O157:H7, highlighting its potential as an antibacterial substance for preventing E. coli contamination in the food industry.

Advances in pulsed electric stimuli as a physical method for treating liquid foods

There is a need for alternative technologies that can deliver safe and nutritious foods at lower costs as compared to conventional processes. Pulsed electric field (PEF) technology has been utilised for a plethora of different applications in the life and physical sciences, such as gene/drug delivery in medicine and extraction of bioactive compounds in food science and technology. PEF technology for treating liquid foods involves engineering principles to develop the equipment, and quantitative biochemistry and microbiology techniques to validate the process. There are numerous challenges to address for its application in liquid foods such as the 5-log pathogen reduction target in food safety, maintaining the food quality, and scale up of this physical approach for industrial integration. Here, we present the engineering principles associated with pulsed electric fields, related inactivation models of microorganisms, electroporation and electropermeabilization theory, to increase the quality and safety of liquid foods; including water, milk, beer, wine, fruit juices, cider, and liquid eggs. Ultimately, we discuss the outlook of the field and emphasise research gaps.

Validation of a High-Throughput Sausage Casing Model for the Assessment of Bacterial Inactivation Affected by Salt Concentration, pH, and Temperature

Previous studies have shown the efficacy of high concentrations of salt as the main preservative against vegetative bacteria present on natural sausage casings. These studies were limited in the number of variables and the interactions between these variables that were assessed. To remedy this situation, a MicroCasing high-throughput model was developed and validated to study the inactivation kinetics of various combinations of parameters (salt concentration, pH, and temperature) on eight bacterial isolates of Salmonella enterica, Staphylococcus aureus, Escherichia coli, and Listeria monocytogenes over a prolonged period. A Weibullian power model was the best fit to show the trends in sensitivity of each bacterial isolate to salt, pH, and temperature over time. The inactivation kinetics generated with this novel approach could serve as a predictive model for the required salting period for casings. The actual bacterial contamination of the product can vary with the respective production step during processing from animal intestine into sausage casings (initial level, ∼10⁵ CFU/g; level after salting, <10² CFU/g). Subsequent selection and grading of these casings will require complete removal of all salt, and upon completion of this production step, the casings will be resalted. By determining the actual contamination level before the salting process, the minimum storage period in salt can be calculated and potentially optimized by adjusting the pH and temperature. As a result, a standard holding period of at least 30 days may no longer be necessary to produce salted natural casings in accordance with validated quality and food safety criteria.

Current and future prospects for the use of pulsed electric field in the meat industry

Pulsed electric field (PEF) is a novel non-thermal technology that has recently attracted the attention of meat scientists and technologists due to its ability to modify membrane structure and enhance mass transfer. Several studies have confirmed the potential of pulsed electric field for improving meat tenderness in both pre-rigor and post-rigor muscles during aging. However, there is a high degree of variability between studies and the underlying mechanisms are not clearly understood. While some studies have suggested physical disruption as the main cause of PEF induced tenderness, enzymatic nature of the tenderization seems to be the most plausible mechanism. Several studies have suggested the potential of PEF to mediate the tenderization process due to its membrane altering properties causing early release of calcium ions and early activation of the calpain proteases. However, experimental research is yet to confirm this postulation. Recent studies have also reported increased post-mortem proteolysis in PEF treated muscles during aging. PEF has also been reported to accelerate curing, enhance drying and reduce the numbers of both pathogens and spoilage organisms in meat, although that demands intense processing conditions. While tenderization, meat safety and accelerated curing appears to be the areas where PEF could provide attractive options in meat processing, further research is required before the application of PEF becomes a commercial reality in the meat industry. It needs to deal with carcasses which vary biochemically and in composition (muscle, fat, and bones). This review critically evaluates the published reports on the topic with the aim of reaching a clear understanding of the possible applications of PEF in the meat sector in addition to providing some insight on critical issues that need to be addressed for the technology to be a practical option for the meat industry.

Effect of heat-assisted pulsed electric fields and bacteriophage on enterohemorrhagic Escherichia coli O157:H7

Pulsed electric fields (PEF), heat-assisted PEF (H-PEF), and virulent bacteriophage (VP) are non-thermal techniques for pathogen inactivation in liquids that were investigated individually, and in combination (PEF/VP, H-PEF/VP) to control enterohemorrhagic Escherichia coli (EHEC) O157:H7 in Luria-Bertani broth (LBB) and Ringer's solution (RS). Treated cells were subsequently incubated at refrigeration (4°C) and temperature-abuse conditions (12°C) for 5 days. When EHEC cells grown in LBB were subjected to non-thermal processing and subsequently stored at 12°C for 5 days, reductions in count of between 0.1 and 0.6 log cycles were observed and following storage at 4°C the decrease in counts varied between 0.2 and 1.1 log10 . For bacteria cells suspended in RS values ranged from 0.1 to ≥3.9 log cycles at both storage temperatures. The most effective treatments were H-PEF and H-PEF/VP, both producing a >3.4 log cycle reduction of cells suspended in non-nutrient RS. Analysis of EHEC recovery on selective and non-selective media indicated no occurrence of sub-lethal damage for VP, PEF/VP, and H-PEF/VP-treated cells. The findings indicate that combining PEF and lytic phage may represent a suitable alternative to conventional fluid decontamination following further process optimization.