Influence of processing parameters and stress adaptation on the inactivation of Listeria monocytogenes by Non-Thermal Atmospheric Plasma (NTAP)

Bactericidal efficacy difference between air and nitrogen cold atmospheric plasma on Bacillus cereus: Inactivation mechanism of Gram-positive bacteria at the cellular and molecular level

As an emerging food processing technology, cold atmospheric plasma (CAP) has attracted great attention in the field of microbial inactivation. Although CAP has been proven to effectively inactivate a variety of foodborne pathogens, there is less research on the inactivation of Bacillus cereus, and the exact inactivation mechanism is still unclear. Elucidating the inactivation mechanism will help to develop and optimize this sterilization method, with the prospective application in industrialized food production. This study aims to explore the bactericidal efficacy difference between air and nitrogen CAP on B. cereus, a typical Gram-positive bacterium, and reveals the inactivation mechanism of CAP at the cellular and molecular level, by observing the change of the cell membrane, cell morphological damage, intracellular antioxidant enzyme activity and cellular biomacromolecules changes. The results showed that both air CAP and nitrogen CAP could effectively inactivate B. cereus, which was due to the reactive oxygen and nitrogen species (RONS) generated by the plasma causing bacterial death. The damage pathways of CAP on Gram-positive bacteria could be explained by disrupting the bacterial cell membrane and cell morphology, disturbing the intracellular redox homeostasis, and destroying biomacromolecules in the cells. The differences in active species generated by the plasma were the main reason for the different bactericidal efficiencies of air CAP and nitrogen CAP, where air CAP producing RONS with stronger oxidative capacity in a shorter time. This study indicates that air CAP is an effective, inexpensive and green technology for B. cereus inactivation, providing a basis for industrial application in food processing.

Technologies to decontaminate bacterial biofilm on hospital surfaces: a potential new role for cold plasma?

Healthcare-associated infections (HCAIs) are a major challenge and the near patient surface is important in harbouring causes such as methicillin-resistant Staphylococcus aureus (MRSA) and Clostridioides difficile. Current approaches to decontamination are sub-optimal and many studies have demonstrated that microbial causes of HCAIs may persist with onward transmission. This may be due to the capacity of these microbes to survive in biofilms on surfaces. New technologies to enhance hospital decontamination may have a role in addressing this challenge. We have reviewed current technologies such as UV light and hydrogen peroxide and also assessed the potential use of cold atmospheric pressure plasma (CAPP) in surface decontamination. The antimicrobial mechanisms of CAPP are not fully understood but the production of reactive oxygen and other species is believed to be important. CAPP systems have been shown to partially or completely remove a variety of biofilms including those caused by Candida albicans, and multi-drug-resistant bacteria such as MRSA. There are some studies that suggest promise for CAPP in the challenge of surface decontamination in the healthcare setting. However, further work is required to define better the mechanism of action. We need to know what surfaces are most amenable to treatment, how microbial components and the maturity of biofilms may affect successful treatment, and how would CAPP be used in the clinical setting.

In Vitro Antibacterial Mechanism of High-Voltage Electrostatic Field against Acinetobacter johnsonii

This study aimed to investigate the antibacterial properties and mechanisms of a high-voltage static electric field (HVEF) in Acinetobacter johnsonii, which were assessed from the perspective of biochemical properties and stress-related genes. The time/voltage-kill assays and growth curves showed that an HVEF decreased the number of bacteria and OD600 values. In addition, HVEF treatment caused the leakage of cell contents (nucleic acids and proteins), increased the electrical conductivity and amounts of reactive oxygen substances (ROS) (16.88 fold), and decreased the activity of Na+ K+-ATPase in A. johnsonii. Moreover, the changes in the expression levels of genes involved in oxidative stress and DNA damage in the treated A. johnsonii cells suggested that HVEF treatment could induce oxidative stress and DNA sub-damage. This study will provide useful information for the development and application of an HVEF in food safety.

Effects of Atmospheric Plasma Corona Discharge on Agrobacterium tumefaciens Survival

Soil-borne pathogenic microorganisms are known to cause extensive crop losses. Agrobacterium tumefaciens, a member of the Proteobacteria, causes the neoplastic crown gall disease in plants. Plant protection is mainly based on toxic chemicals that are harmful to the environment. The use of cold atmospheric-pressure plasma is an attractive method for microbial eradication. Its antimicrobial mechanism includes the formation of large quantities of reactive oxygen species (ROS). The advantages of eradicating bacteria using cold plasma are not needed for chemicals, short treatment, and environmental temperatures. This study examined the impact of plasma corona discharge exposure on A. tumefaciens viability, membrane permeability, relative cell size, and ROS formation. The results showed that 90 s of plasma exposure led to a reduction by four orders of magnitude when the initial concentration was 1 × 107 CFU/mL and in a dry environment. When the initial concentration was 1 × 106 CFU/mL, 45 s of exposure resulted in total bacterial eradication. In a liquid environment, in an initial concentration of 2.02 × 106 CFU/mL, there was no complete bacterial eradication even at the most prolonged examined exposure (90 s). The influence of plasma treatment on the membrane permeability of A. tumefaciens, and their possible recovery, were analyzed using flow cytometer analysis using propidium iodide (PI). When the plasma-treated bacteria were suspended in Luria–Bertani (LB) (rich medium), the PI-positive count of the plasma-treated bacteria after two hours was 12 ± 3.9%. At the 24th hour, this percentage was only 1.74 ± 0.6%, as the control (0.7 ± 0.1%). These results may indicate the repair of the plasma-treated bacteria that were suspended in LB. At the 24th hour, the relative cell size of the treated bacteria shifted to the right, to ~3 × 104 forward side scatter (FSC), about 0.5-fold higher than the untreated cells. Measurement of the ROS showed that the intracellular fluorescence of the 90-s plasma-treated cells led to significant fluorescence formation of 32 relative fluorescence units (RFU)/cell (9 × 104 fold, compared to the nontreated cells). This study showed that cold plasma is a useful method for A. tumefaciens eradication. The eradication mechanism involves ROS generation, membrane permeability, and changes in cell size.

Potential of cold plasma to control Callosobruchus chinensis (Chrysomelidae: Bruchinae) in chickpea cultivars during four year storage

Cold plasma has proven itself as a promising method of food preservation by controlling food spoilage bacteria at very low temperatures. It is showing potential for insect control. Synthetic pesticides are mostly used to control Callosobruchus chinensis L. (Chrysomelidae: Coleoptera) to which it has developed resistance. The prospective potential of cold plasma treatment to control pulse beetle infestation of chickpea in the storage for about four years of plasma treatment was studied. The four chickpea cultivars were treated with cold plasma at different power 40, 50, and 60 W each for 10, 15, 20 min. Plasma treated and untreated chickpeas were stored in an airtight ziplock pouch. At regular intervals, the grains were observed for infestation. It was found most effective in controlling the pulse beetle infestation of treated chickpea samples. While plasma untreated chickpeas were attacked and damaged mostly by pulse beetle within the first quarter of the storage study. To avoid the problems created by the use of pesticides cold plasma treatment is found to be the best alternative in the protection of chickpea invasion by pulse beetle during a longer storage period. The findings in the present research may be used for the preparation of legumes which may also soak and cook faster like quick-cooking legumes and preserved for years without invasion of pulse beetle.

The antimicrobial efficacy of remote cold atmospheric plasma effluent against single and mixed bacterial biofilms of varying age

Cold atmospheric plasma (CAP) is a minimal food processing technology of increasing interest in the food industry, as it is mild in nature compared to traditional methods (e.g. pasteurisation) and thus can maintain the food's desirable qualities. However, due to this mild nature, the potential exists for post-treatment microbial survival and/or stress adaptation. Furthermore, biofilm inactivation by CAP is underexplored and mostly studied on specific foods or on plastic/polymer surfaces. Co-culture effects, biofilm age, and innate biofilm-associated resistance could all impact CAP efficacy, while studies on real foods are limited to the food product investigated without accounting for structural complexity. The effect of a Remote and Enclosed CAP device (Fourth State Medicine Ltd) was investigated on Escherichia coli and Listeria innocua grown as planktonic cells and as single or mixed bacterial biofilms of variable age, on a biphasic viscoelastic food model of controlled rheological and structural complexity. Post-CAP viability was assessed by plate counts, cell sublethal injury was quantified using flow cytometry, and biofilms were characterised and assessed using total protein content and microscopy techniques. A greater impact of CAP on planktonic cells was observed at higher air flow rates, where the ReCAP device operates in a mode more favourable to reactive oxygen species than reactive nitrogen species. Although planktonic E. coli was more susceptible to CAP than planktonic L. innocua, the opposite was observed in biofilm form. The efficacy of CAP was reduced with increasing biofilm age. Furthermore, E. coli produced much higher protein content in both single and mixed biofilms than L. innocua. Consequently, greater survival of L. innocua in mixed biofilms was attributed to a protective effect from E. coli. These results show that biofilm susceptibility to CAP is age and bacteria dependent, and that in mixed biofilms bacteria may become less susceptible to CAP. These findings are of significance to the food industry for the development of effective food decontamination methods using CAP.

Effect of Non-Thermal Atmospheric Plasma on Food-Borne Bacterial Pathogens on Ready-to Eat Foods: Morphological and Physico-Chemical Changes Occurring on the Cellular Envelopes

Currently, there is a need for new technological interventions to guarantee the microbiological safety of ready-to-eat (RTE) foods. Non-thermal atmospheric plasma (NTAP) has emerged as a promising strategy for inactivating microorganisms on thermo-sensitive foods, and the elucidation of its mechanisms of action will aid the rational optimization and industrial implementation of this technology for potential applications in the food industry. In this study, the effectiveness of NTAP for inactivating strains of Salmonella Enteritidis, Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes contaminating the surface of different sliced RTE foods (“chorizo”, salami, bacon, smoked salmon, tofu and apple) was investigated. In addition, to further assess the bacterial inactivation mechanisms of NTAP, the morphological and physico-chemical damages in bacterial cells were analyzed. NTAP was effective for the surface decontamination of all products tested and, especially, of cut apple, where the microbial populations were reduced between 1.3 and 1.8 log units for the two Salmonella strains and E. coli O157: H7, respectively, after 15 min of exposure. In the rest of foods, no significant differences in the lethality obtained for the E. coli O157:H7 strain were observed, with inactivation rates of between 0.6 and 0.9 log cycles after a 15-min treatment. On the other hand, the strains from the rest of pathogenic microorganisms studied were extremely resistant on tofu, where barely 0.2–0.5 log units of inactivation were achieved after 15 min of plasma exposure. S. Enteritidis cells treated for 10 min exhibited noticeable morphological and structural changes, as observed by transmission electron microscopy, which were accompanied by a loss in membrane integrity, with an increased leakage of intracellular components and uptake of propidium iodide and marked changes in regions of their FTIR spectra indicating major alterations of the cell wall components. Overall, this indicates that loss of viability was likely caused for this microorganism by a significant damage in the cellular envelopes. However, the plasma-treated cells of L. monocytogenes did not show such obvious changes in morphology, and exhibited less marked effects on the integrity of their cytoplasmic membrane, what suggests that the death of this pathogenic microorganism upon NTAP exposure is more likely to occur as a consequence of damages in other cellular targets.

Evaluation of Cold Atmospheric Pressure Plasma (CAPP) and plasma-activated water (PAW) as alternative non-thermal decontamination technologies for tofu: Impact on microbiological, sensorial and functional quality attributes

The efficacy of Cold Atmospheric Pressure Plasma (CAPP) for the inactivation of Salmonella enterica serovar Enteritidis, Salmonella enterica serovar Typhimurium, Listeria monocytogenes and Escherichia coli O157:H7 on tofu was evaluated. The potential of using Plasma Activated Water (PAW) as an immersion solution for controlling microbial growth in tofu throughout its shelf-life was also investigated. The effects of these strategies on the physical and functional properties of treated tofu were also studied. CAPP treatment of tofu caused a limited inactivation of microbial populations, with log₁₀ reductions attained ranging from 0.2 to 0.6 log₁₀ for S. Enteritidis and E. coli O157:H7, respectively, after a 15 min treatment. CAPP did not affect tofu's water holding capacity, but it dried it and gave rise to changes in color and texture, which were reverted by immersing the treated product in distilled water. Refrigerated storage of tofu using PAW as an immersion solution was effective in controlling microbial growth. Thus, total counts obtained after 28 days of storage were around 3 log₁₀ units lower than those observed for tofu stored immersed in non-treated deionized water. In addition, this strategy led to a product with a higher functional value than thermally-treated commercial tofu, retaining up to 80% of the initial content of total polyphenols, with better texture properties, less hardness and springiness (approximately 20-30% lower) and with minor changes in its characteristic color. Overall, these results evidence that PAW is a promising non-thermal technology which can facilitate the control of pathogenic microorganisms on tofu while retaining its physical and functional properties.

Potential Applications of Non-thermal Plasma in Animal Husbandry to Improve Infrastructure

Infrastructure in animal husbandry refers to fundamental facilities and services necessary for better living conditions of animals and its economy to function through better productivity. Mainly, infrastructure can be divided into two categories: hard infrastructure and soft infrastructure. Physical infrastructure, such as buildings, roads, and water supplying systems, belongs to hard infrastructure. Soft infrastructure includes services which are required to maintain economic, health, cultural and social standards of animal husbandry. Therefore, the proper management of infrastructure in animal husbandry is necessary for animal welfare and its economy. Among various technologies to improve the quality of infrastructure, non-thermal plasma (NTP) technology is an effectively applicable technology in different stages of animal husbandry. NTP is mainly helpful in maintaining better health conditions of animals in several ways via decontamination from microorganisms present in air, water, food, instruments and surfaces of animal farming systems. Furthermore, NTP is used in the treatment of waste water, vaccine production, wound healing in animals, odor-free ventilation, and packaging of animal food or animal products. This review summarizes the recent studies of NTP which can be related to the infrastructure in animal husbandry.

A Review on Non-thermal Atmospheric Plasma for Food Preservation: Mode of Action, Determinants of Effectiveness, and Applications

Non-thermal Atmospheric Plasma (NTAP) is a cutting-edge technology which has gained much attention during the last decade in the food-processing sector as a promising technology for food preservation and maintenance of food safety, with minimal impact on the quality attributes of foods, thanks to its effectiveness in microbial inactivation, including of pathogens, spoilage fungi and bacterial spores, simple design, ease of use, cost-effective operation, short treatment times, lack of toxic effects, and significant reduction of water consumption. This review article provides a general overview of the principles of operation and applications of NTAP in the agri-food sector. In particular, the numerous studies carried out in the last decade aimed at deciphering the influence of different environmental factors and processing parameters on the microbial inactivation attained are discussed. In addition, this review also considers some important studies aimed at elucidating the complex mechanism of microbial inactivation by NTAP. Finally, other potential applications of NTAP in the agri-food sector, apart from food decontamination, are briefly described, and some limitations for the immediate industrial implementation of NTAP are discussed (e.g., impact on the nutritional and sensory quality of treated foods; knowledge on the plasma components and reactive species responsible for the antimicrobial activity; possible toxicity of some of the chemical species generated; scale-up by designing fit-for-purpose equipment).

The inhibition effect of cold atmospheric plasma-activated media in cutaneous squamous carcinoma cells

AIM

This study investigated the effect and mechanism of cold atmospheric plasma (CAP)-activated media on A431 and HaCaT cells.

MATERIALS & METHODS

Phosphate-buffered saline (PBS) and Dulbecco's Modified Eagle's Medium (DMEM) were treated by CAP to get CAP-activated media. A431 and HaCaT were incubated by CAP-activated media for 2 h. MTT, Annexin-V and propidium iodide (PI), Western blot and reactive oxygen species (ROS) detection assay were utilized to demonstrate the effect.

RESULTS

The CAP-activated media decreased the proliferation of A431 cells in a dose/time-dependent manner. And the CAP-activated media could induce apoptosis in A431 cells. CAP-activated phosphate-buffered saline could increase intracellular ROS level but not CAP-activated DMEM.

CONCLUSION

CAP-activated media could induce apoptosis in A431 cells by production of ROS.

Inactivation Kinetics of Salmonella typhimurium and Staphylococcus aureus in Different Media by Dielectric Barrier Discharge Non-Thermal Plasma

A study was conducted to determine the effect of dielectric barrier discharge non-thermal plasma (DBD-NTP) on Salmonella typhimurium and Staphylococcus aureus populations on solid surfaces and in liquid suspensions. Our results showed that inactivation kinetics of S. typhimurium and S. aureus by DBD-NTP treatments can be well predicted with mathematical models. The survival curves of both S. typhimurium and S. aureus showed a log-linear phase followed by tailing behaviors on solid surfaces, and shoulder behaviors followed by a log-linear phase in liquid suspensions. The D values (decimal reduction time) for S. typhimurium and S. aureus in suspension were higher than those on solid surfaces (p < 0.05). Additionally, the maxima of sublethal injury values under low NaCl concentration and neutral pH condition were higher than those under high NaCl and low pH condition. In addition, mathematical modeling was evaluated to predict the final inactivation result for potential industrial applications. This study indicates that different microbial supporting matrices significantly influence the inactivation effect of DBD-NTP; it also provides useful information for future applications of NTP in enhancing food shelf life and safety.

Effect of preliminary stresses on the resistance of Escherichia coli and Staphylococcus aureus toward non-thermal plasma (NTP) challenge

As the development of hurdle technology, cross-protection of various stresses for pathogens posed the potential risk to food safety and public health. This study tried to explore various preliminary stresses including acidity, osmosis, oxidation, heat and cold on the resistance of microbial cells toward the non-thermal plasma (NTP) exposure. The results indicated that short-term (4h) exposure of Staphylococcus aureus and Escherichia coli to acidity, osmosis, oxidation, heat and cold stresses did not lead to the resistance to the subsequent NTP treatment. On the contrary, acidity, osmosis and heat preadaptation increased the vulnerability of E. coli cells to NTP treatment. After exposing S. aureus to osmosis, oxidation, heat and cold stress for longer period (24h), the reduction level showed significantly (P<0.05) higher. Interestingly, long-term (24h) preliminary exposure of acidic stress exhibited protective effect for S. aureus against the following NTP exposure with less damage in cell membrane integrity, membrane potential and intracellular enzyme activity. It might be due to the protein production for oxidative stress response during preliminary acidic adaptation. In general, the obtained result helped to grasp better understanding of the microbial stress response to NTP treatment and provided insight for the future research in order to accelerate the development of NTP technology in food industry.