Sublethal injury and recovery of Escherichia coli O157:H7 after freezing and thawing

BCP4: A novel antimicrobial peptide with potent efficacy against Bacillus cereus in rice porridge

Bacillus cereus is a common foodborne pathogen that frequently contaminates rice products and produces cereulide toxins, presenting a significant risk to food safety and human health. In contrast, Bacillus subtilis is a promising source of antimicrobial peptides (AMPs). In this research, a novel AMP named BCP4 (KGKTLLQ) was discovered through the fermentation of shrimp waste with B. subtilis, which speculated that BCP4 might be generated through enzymatic hydrolysis catalyzed by endogenous enzymes naturally present in shrimp waste. BCP4 demonstrated potent antibacterial activity against B. cereus with a minimum bactericidal concentration (MBC) of 62.5 μg/mL and bacterial time-kill of 3 h. BCP4 surpassed the bactericidal efficiency of nisin (500 μg/mL), a commonly used AMP of microbial origin. BCP4 operates by causing damage to the bacterial cell wall and membrane, which allows the contents of the cell to flow out. BCP4 also penetrates the cell membrane and binds with DNA, effectively sterilizing the bacteria. Meanwhile, treatment of BCP4 with mammalian red blood cells revealed that it was nonhemolytic. Furthermore, the growth of B. cereus in rice porridge was significantly inhibited by BCP4 at a concentration of 62.5 μg/mL. This study provides a theoretical basis for using BCP4 to control B. cereus contamination.

Inactivation and sublethal injury of Salmonella Typhimurium on beef and in aqueous solution treated with lactic acid

Salmonella Typhimurium, a common foodborne pathogen, is widespread in foods. Lactic acid (LA) has been employed to control bacteria in food, while it can induce the formation of sublethally injured bacteria. The sublethal injury of LA against S. Typhimurium on beef was determined in this study, meanwhile the inactivation and sublethal injury kinetics of S. Typhimurium in aqueous solution treated with LA at different temperatures (4, 25, and 37 °C) and concentrations (1, 2, 4, and 6 mM) were also investigated and fitted. The sublethal injury was also validated by flow cytometry (FCM) and high content imaging system (HCS). LA could induce sublethally injured S. Typhimurium on beef and in aqueous solution, the injury ratios in both the cytoplasmic and outer membrane increased with increasing exposure time (0-180 min), concentration (1-6 mM) and temperature (4-37 °C). Modified Logistic and modified Weibull models fitted the inactivation kinetics well, and the parameters indicated that LA caused accumulated damage to S. Typhimurium. The inactivation and sublethal injury effects of LA on S. Typhimurium were verified by FCM and HCS analysis.

Changes to Pork Bacterial Counts and Composition After Dielectric Barrier Discharge Plasma Treatment and Storage in Modified-Atmosphere Packaging

The aim of this study was to compare the succession of natural microbiota in pork held under refrigerated storage for up to 10 days after dielectric barrier discharge (DBD) plasma treatment. Two methods were used to assess the impact of DBD on microorganisms. Firstly, traditional selective media (SM) were employed to detect the bactericidal effects of DBD on Pseudomonas spp., Enterobacteriaceae, Lactic acid bacteria (LAB), and Brochothrix thermosphacta. Secondly, the thin agar layer (TAL) method was used to further evaluate the bactericidal effects of DBD. In addition, the Baranyi and Roberts model was applied to explore the kinetic parameters of Pseudomonas spp., Enterobacteriaceae, LAB, and B. thermosphacta during storage. Finally, the modified Lotka-Volterra model was used to describe the interactions between each microorganism. The study found that when using traditional selective media (SM), 85 kV DBD had a significant bactericidal effect on Pseudomonas spp., Enterobacteriaceae, LAB, and Brochothrix thermosphacta. However, when using the thin agar layer (TAL) method, the results suggested that DBD had no significant bactericidal effect, suggesting that DBD caused sublethal damage to the natural microorganisms on pork. Analysis with the Baranyi and Roberts model showed that DBD treatment significantly extended the lag phase of these four types of microorganisms and significantly reduced the μmax of all microorganisms except LAB. The analysis results of the modified Lotka-Volterra model showed that LAB had a greater impact on Pseudomonas spp., Enterobacteriaceae, and B. thermosphacta (a21 > a12). In conclusion, DBD treatment was shown to have a significant sublethal bactericidal effect that impacted both the count and composition of natural microorganisms found on pork.

Microbial Growth, Survival, and Cryotolerance in Plant-Based Milk Alternatives: A Study with Listeria monocytogenes

The increasing popularity of plant-based milk products as an alternative to traditional bovine milk has sparked concerns about their safety and nutritional impact. This study focuses on the growth, survival, and cryotolerance behavior of Listeria monocytogenes (strains: ATCC 19115 and RS1) in various plant-based milk substitutes. Samples of almond milk, oat milk, soy milk, and bovine milk, all subjected to ultra-high temperature treatment, were evaluated for their influence on L. monocytogenes growth at 4°C and survival through repeated freezing and thawing cycles. Despite the nutritional differences, the growth rates of two L. monocytogenes strains at 4°C in plant-based milk alternatives and bovine milk displayed similarity (p > 0.05). Both strains of L. monocytogenes demonstrated similar biofilm formation abilities in plant-based milk alternatives and bovine milk. However, L. monocytogenes exhibited different levels of tolerance to repeated freezing and thawing cycles depending on plant-based milk alternatives in which they were grown at 4°C (p < 0.05). In the case of L. monocytogenes ATCC19115, cells cultured in almond milk at 4°C showed a significant reduction in their freezing and thawing tolerance (2.80 log reduction), followed by cells grown in soy milk (2.09 log reduction) when compared with oat and bovine milk (p < 0.05). A parallel trend of tolerance was evident in L. monocytogenes RS1 (2.82 and 3.22 log reduction in almond milk and soy milk, respectively). These findings underscore the need for comprehensive assessments of microbial behavior in emerging food products like plant-based milk alternatives. As these alternatives continue to gain traction, ensuring their safety and stability remains important. With insights into L. monocytogenes growth and survival in milk alternatives, this study will contribute to the evolving understanding of microbial dynamics in response to changing dietary trends.

Sublethal injury and recovery of Escherichia coli O157:H7 after dielectric barrier discharge plasma treatment

Dielectric barrier discharge (DBD) plasma can be used to control food spoilage and food pathogens. However, DBD plasma may induce sublethal injury in microorganisms, constituting a considerable risk to food safety. This research was designed to investigate the sublethal injury and recovery of Escherichia coli O157:H7 after DBD plasma treatment. The results indicated that the sublethal injury ratios of cells rose along with the augmentation of treatment time and input power of DBD plasma under mild treatment conditions, whereas injury accumulation ultimately culminated in cell death. The highest sublethal ratio of 99.3% was obtained after DBD plasma treatment at 18 W for 40 s. When solutions such as phosphate buffered saline (PBS), peptone water, glucose solution, and tryptic soy broth (TSB) were used for cell recovery, TSB was proven to be the most efficacious, facilitating the completion of recovery within 2 h. The repair ratio of injured cells increased as the recovery pH (3.0-7.0) and temperature (4-37 ºC) increased. Moreover, Mg2+ and Zn2+ were demonstrated to be necessary for the recovery process, while Ca2+ presented a weak effect. Understanding the sublethal injury of bacteria resulting from DBD plasma treatment and their repair conditions can provide useful insight into avoiding the occurrence of sublethal injury as well as inhibiting the occurrence of recovery during food processing and storage.

Do Temperature Abuses Along the Frozen Açaí Pulp Value Chain Increase Microbial Hazards?

Does temperature abuse during storage, distribution, marketing, and consumption of unpasteurized frozen açaí pulp increase microbial hazards? This study investigated the behavior of potentially pathogenic (Escherichia coli, Listeria monocytogenes and Salmonella spp.) and spoilage (mesophilic bacteria, yeasts and molds) microorganisms in two simulated thawing conditions: under refrigeration and at room temperature. The effect of repeated cold chain abuse was observed by thawing and refreezing (-20°C) açaí pulp four times over a period of 90 days. Freezing resulted in inhibition of all microorganisms except for mesophilic aerobic bacteria in one single sample. After thawing at 5°C, the kinetic parameters obtained by the Weibull model indicated that mesophilic aerobic bacteria, yeasts and molds and L. monocytogenes showed a longer inactivation time with δ values reaching 35, 126, and 46 days, respectively. The shortest inactivation time for a reduction of 4 log CFU.g-1 was for E. coli. The concentration of Salmonella spp. and L. monocytogenes in control samples was higher (p < 0.01) than in samples exposed to abusive conditions after 90 days of storage. The results indicate that the abusive thawing conditions studied do not increase the potential hazards of pathogens.

Changes in stresses sensitivity of ohmic heating-induced sublethally injured Staphylococcus aureus during repair: Potential mechanisms at the cellular and molecular levels

Ohmic heating (OH), an emerging food processing technology employed in the food processing industry, raises potential food safety concerns due to the recovery of sublethally injured pathogens such as Staphylococcus aureus (S. aureus). In the present study, sensitivity to various stress conditions and the changes in cellular-related factors of OH-injured S. aureus during repair were investigated. The results indicated that liquid media differences (nutrient broth (NB), phosphate-buffered saline (PBS), milk, and cucumber juice) affected the recovery process of injured cells. Nutrient enrichment determines the bacterial repair rate, and the rates of repair for these media were milk > NB > cucumber juice > PBS. The sensitivity of injured cells to various stressors, including different acids, temperature, nisin, simulated gastric fluid, and bile salt, increased during the injury phase and subsequently diminished upon repair. Additionally, the intracellular ATP content, enzyme activities (Na+/K+-ATPase, Ca2+/Mg2+-ATPase, and T-ATPase) and ion concentrations (Mg2+, K+, and Ca2+) gradually increased during repair. After 5 h of repair, the intracellular substances content of cell's was significantly higher than that of the injured bacteria without repair, while some indicators (e.g., Na+/K+-ATPase, K+, and Ca2+) were not restored to the untreated level. The results of this study indicated that OH-injured S. aureus exhibited strengthened resistance post-recovery, potentially due to the restoration of cellular structures. These findings have implications for optimizing food storage conditions and advancing OH processes in the food industry.

Ferrous sulfate combined with ultrasound emulsified cinnamaldehyde nanoemulsion to cause ferroptosis in Escherichia coli O157:H7

The purpose of this study was to investigate ferroptosis in Escherichia coli O157:H7 caused by ferrous sulfate (FeSO4) and to examine the synergistic effectiveness of FeSO4 combined with ultrasound-emulsified cinnamaldehyde nanoemulsion (CALNO) on inactivation of E. coli O157:H7 in vitro and in vivo. The results showed that FeSO4 could cause ferroptosis in E. coli O157:H7 via generating reactive oxygen species (ROS) and exacerbating lipid peroxidation. In addition, the results indicated that FeSO4 combined with CALNO had synergistic bactericidal effect against E. coli O157:H7 and the combined treatment could lead considerable nucleic acids and protein to release by damaging the cell membrane of E. coli O157:H7. Besides, FeSO4 combined with CALNO had a strong antibiofilm ability to inhibit E. coli O157:H7 biofilm formation by reducing the expression of genes related on biofilm formation. Finally, FeSO4 combined with CALNO exhibited the significant antibacterial activity against E. coli O157:H7 in hami melon and cherry tomato.

Sublethally injured microorganisms in food processing and preservation: Quantification, formation, detection, resuscitation and adaption

Sublethally injured state has been recognized as a survival strategy for microorganisms suffering from stressful environments. Injured cells fail to grow on selective media but can normally grow on nonselective media. Numerous microorganism species can form sublethal injury in various food matrices during processing and preservation with different techniques. Injury rate was commonly used to evaluate sublethal injury, but mathematical models for the quantification and interpretation of sublethally injured microbial cells still require further study. Injured cells can repair themselves and regain viability on selective media under favorable conditions when stress is removed. Conventional culture methods might underestimate microbial counts or present a false negative result due to the presence of injured cells. Although the structural and functional components may be affected, the injured cells pose a great threat to food safety. This work comprehensively reviewed the quantification, formation, detection, resuscitation and adaption of sublethally injured microbial cells. Food processing techniques, microbial species, strains and food matrix all significantly affect the formation of sublethally injured cells. Culture-based methods, molecular biological methods, fluorescent staining and infrared spectroscopy have been developed to detect the injured cells. Cell membrane is often repaired first during resuscitation of injured cells, meanwhile, temperature, pH, media and additives remarkably influence the resuscitation. The adaption of injured cells negatively affects the microbial inactivation during food processing.

The effect of manganese and iron on mediating resuscitation of lactic acid-injured Escherichia coli

Lactic acid can induce sublethal injury of E. coli through oxidative stress. In this study, we investigated changes in SOD activity, CAT activity, GSH production and ROS production during sublethal injury and resuscitation of E. coli. Then, the effect of manganese and iron during resuscitation were studied. Both cations (≥1 mmol l^-1 ) significantly promoted the resuscitation of sublethally injured E. coli induced by lactic acid and shortened the repair time (P < 0·05). Conversely, addition of N,N,N',N'-tetrakis (2-pyridylmethyl) which is a metal chelator extended the repair time. Compared with minA, manganese and iron significantly improved SOD activity at 40, 80 and 120 min and decreased ROS production at 40 and 80 min, thereby recovering injured E. coli quickly (P < 0·05). The deletion of sodA encoding Mn-SOD, sodB encoding Fe-SOD or gshA/gshB encoding GSH significantly strengthened sublethal injury and extended the repair time (P < 0·05). It meant these genes-related oxidative stress played important roles in the acid resistance of E. coli and recovery of sublethal injury. Therefore, manganese and iron can promote the recovery of lactic-injured E. coli by the way of increasing SOD activity, scavenging ROS, and relieving oxidative stress.

Development of modified enrichment broth for short enrichment and recovery of filter-injured Salmonella Typhimurium

The filter concentration method facilitates the rapid detection of foodborne pathogens. The filter concentration method lowered the limit of detection (LOD) of artificially inoculated cabbage with Salmonella Typhimurium; however, the procedure injured foodborne pathogens during filtering procedure. Thus, to detect injured pathogens under the detection limit, an enrichment broth promoting pathogen resuscitation and growth is required. To rapidly recover, cultivate and lower the time to result (TTR) of S. Typhimurium detection after filter concentration method, a brain heart infusion (BHI) broth-based modified enrichment broth (MEB) was developed. The MEB was developed by fitting growth curves to a modified Gompertz model; 1.00 g/L of sodium pyruvate, 0.20 g/L proline and 2.0 g/L magnesium sulphate additives were optimized as additional components to rapidly grow filter-injured S. Typhimurium. As a result, the rate of filter-injured S. Typhimurium went from 100% to 0.0% using MEB within 3.5 h. In contrast, BHI required 4 h and buffered peptone water (BPW) required more than 4 h to decrease the injury rate to 0.0%. Using MEB, BHI and BPW, filter-injured S. Typhimurium in cabbages were enriched to 4.056 ± 0.026 Log CFU/25 g, 3.571 ± 0.187 Log CFU/25 g and 3.708 ± 0.156 Log CFU/25 g, respectively. Additionally, 1-9 CFU/mL S. Typhimurium in cabbage was detected within 3.0 h, including 1 h enrichment with MEB, whereas 5.0 h was required for BHI and BPW. Thus, the MEB developed in this study showed great potential as a short enrichment broth for the rapid detection of filter-injured S. Typhimurium.

Recovery and virulence factors of sublethally injured Staphylococcus aureus after ohmic heating

Ohmic heating (OH) is an alternative thermal processing technique, which is widely used to pasteurize or sterilize food. However, sublethally injured Staphylococcus aureus induced by OH is a great concern to food safety. The recovery of injured S. aureus by OH and virulence factor changes during recovery were investigated in this study. The liquid media (phosphate-buffered saline, buffered peptone water and nutrient broth (NB)), temperature (4, 25 and 37 °C) and pH (6.0, 7.2 and 8.0) influenced the recovery rate and the injured cells completely repaired in NB at 37 °C, pH 7.2 with the shortest time of 2 h. The biofilm formation ability, mannitol fermentation, hemolysis, and coagulase activities decreased in injured S. aureus and recovered during repair process. Quantitative real-time PCR showed the expression of sek, clfB and lukH involved in virulence factors increased during recovery. The results indicated that the virulence factors of injured S. aureus recovered after repair.

The Effect of Pulsed Electric Fields (PEF) Combined with Temperature and Natural Preservatives on the Quality and Microbiological Shelf-Life of Cantaloupe Juice

Pulsed electric field (PEF) is an innovative, non-thermal technology for food preservation with many superiorities. However, the sub-lethally injured microorganisms caused by PEF and their recovery provide serious food safety problems. Our study examined the effects of pH, temperature and natural preservatives (tea polyphenols and natamycin) on the recovery of PEF-induced, sub-lethally injured Saccharomyces cerevisiae cells, and further explored the bactericidal effects of the combined treatments of PEF with the pivotal factors in cantaloupe juice. We first found that low pH (pH 4.0), low temperature (4 °C), tea polyphenols and natamycin inhibited the recovery of injured S. cerevisiae cells. Then, the synergistic effects of PEF, combined with cold-temperature storage (4 °C), a mild treatment temperature (50 and 55 °C), tea polyphenols or natamycin, on the inactivation of S. cerevisiae in cantaloupe juice were evaluated. Our results showed that the combination of PEF and heat treatment, tea polyphenols or natamycin enhanced the inactivation of S. cerevisiae and reduced the level of sub-lethally injured cells. Moreover, PEF combined with 55 °C heat treatment or tea polyphenols was applied for cantaloupe juice. In the practical application, the two combined PEF methods displayed a comparable inactivation heat pasteurization ability, prolonged the shelf life of juice compared with PEF treatment alone, and better preserved the physicochemical properties and vitamin C levels of cantaloupe juice. These results provide valuable information to inhibit the recovery of PEF-injured microbial cells and shed light on the combination of PEF with other factors to inactivate microorganisms for better food preservation.