In vitro antimicrobial effects and mechanisms of direct current air-liquid discharge plasma on planktonic Staphylococcus aureus and Escherichia coli in liquids

Stress response of Salmonella Newport with various sequence types toward plasma-activated water: Viable but nonculturable state formation and outer membrane vesicle production

This study aims to investigate the response of Salmonella Newport to plasma-activated water (PAW), a novel disinfectant that attracts attention due to its broad-spectrum antimicrobial efficacy and eco-friendliness. In this work, we demonstrated that S. Newport of different sequence types (STs) could be induced into the viable but nonculturable (VBNC) state by PAW treatment. Notably, a remarkable 99.96% of S. Newport ST45 strain entered the VBNC state after a 12-min PAW treatment, which was the fastest observed among the five S. Newport STs (ST31, ST45, ST46, ST166, ST2364). Secretion of outer membrane vesicles was observed in ST45, suggesting a potential strategy against PAW treatment. Genes related to oxidative stress (sodA, katE, trxA), outer membrane proteins (ompA, ompC, ompD, ompF) and virulence (pagC, sipC, sopE2) were upregulated in the PAW-treated S. Newport, especially in ST45. A reduction of 38-65% in intracellular ATP level after PAW treatment was observed, indicating a contributor to the formation of the VBNC state. In addition, a rapid method for detecting the proportion of VBNC cells in food products based on pagC was established. This study contributes to understanding the formation mechanism of the VBNC state in S. Newport under PAW stress and offers insights for controlling microbial risks in the food industry.

Cold atmospheric plasma for surface disinfection: a promising weapon against deleterious meticillin-resistant Staphylococcus aureus biofilms

BACKGROUND

Bacteria are becoming increasingly resistant to classical antimicrobial agents, so new approaches need to be explored.

AIM

To assess the potential of cold atmospheric plasma for the management of meticillin-resistant Staphylococcus aureus (MRSA).

METHODS

The 24, 48, and 72 h resistant and susceptible S. aureus biofilms were exposed to 60, 120, and 180 s treatment with plasma.

FINDINGS

Increasing the treatment time results in higher cell reduction for both susceptible and resistant strains of S. aureus (P < 0.05). Up to log10 reduction factor of 5.24 cfu/cm2 can be achieved in 180 s of plasma treatment. Furthermore, plasma can substantially alter the cell's metabolisms and impact cell membrane integrity. However, it has not been shown that plasma can reduce biofilm biomass in the case of 24 h and 48 h biofilms, although the 72 h biofilm was more susceptible, and its biomass was decreased (P < 0.05). The accumulation of intrabacterial reactive oxygen species was also observed, which confirms the plasma's induction of oxidative stress. Finally, it was shown that continuous plasma exposure of bacterial cells does not cause resistance to plasma, nor is resistance developed to cefoxitin.

CONCLUSION

Cold atmospheric plasma is a good candidate for S. aureus and MRSA biofilm treatment and may therefore be of value in the bacterial resistance crisis.

Simultaneous removal of antibiotic-resistant Escherichia coli and its resistance genes by dielectric barrier discharge plasma

As emerging contaminants, antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) have been widely detected in various aqueous environments. For antibiotic resistance to be inhibited in the environment, it is essential to control ARB and ARGs. In this study, dielectric barrier discharge (DBD) plasma was used to inactivate antibiotic resistant Escherichia coli (AR E. coli) and remove ARGs simultaneously. Within 15 s of plasma treatment, 108 CFU/mL of AR E. coli were inactivated by 97.9%. The rupture of the bacterial cell membrane and the increase of intracellular ROS are the main reasons for the rapid inactivation of bacteria. Intracellular ARGs (i-qnrB, i-blaCTX-M, i-sul2) and integron gene (i-int1) decreased by 2.01, 1.84, 2.40, and 2.73 log after 15 min of plasma treatment, respectively. In the first 5 min of discharge, extracellular ARGs (e-qnrB, e-blaCTX-M, e-sul2) and integron gene (e-int1) decreased by 1.99, 2.22, 2.66, and 2.80 log, respectively. The results of the ESR and quenching experiments demonstrated that ·OH and 1O2 played important roles in the removal of ARGs. This study shows that DBD plasma is an effective technique to control ARB and ARGs in waters.

Plasma activated Ezhangfeng Cuji as innovative antifungal agent and its inactivation mechanism

Candida albicans is a highly drug-resistant fungus for which new treatments are urgently needed due to the lack of clinically effective options. In this study, we evaluated the antifungal activity and mechanism of plasma-activated Ezhangfeng Cuji (PAEC) against Candida albicans and compared it with physiological saline (PS), plasma-activated physiological saline (PAPS) and Ezhangfeng Cuji (EC). After dielectric barrier discharge (DBD) plasma treatment with EC for 20 min followed by a 10 min immersion of Candida albicans, the fungus was reduced by approximately 3 orders of magnitude. High performance liquid chromatography (HPLC) results showed an increase of 41.18% and 129.88% in the concentration of oxymatrine and rhein, respectively, after plasma-treated EC. The concentrations of reactive species (RS), such as H₂O₂, [Formula: see text], and O₃, were found to be higher and the pH value was getting lower in PS after plasma treatment. Detailed analysis of intracellular material leakage, reactive oxygen species (ROS), apoptosis for Candida albicans and observation by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) demonstrated that PAPS, EC and PAEC disrupt the morphological structure of Candida albicans to varying degrees.Additionally, specific analyses on Candida albicans virulence factors, such as adhesion to tissue surfaces, cell surface hydrophobicity (CSH), the transition of yeast-phase cells to mycelium-phase cells, and the secretion of hydrolytic enzymes for Candida albicans were conducted and found to be inhibited after PAPS/EC/PAEC treatment. In our investigation, the inhibitory effects on Candida albicans were ranked from strong to weak as follows: PAEC, EC, PAPS, and PS.

Role and Mechanism of Cold Plasma in Inactivating Alicyclobacillus acidoterrestris in Apple Juice

A. acidoterrestris has been identified as the target bacterium in fruit juice production due to its high resistance to standard heat treatment. Multiple studies have shown that cold plasma can effectively inactivate pathogenic and spoilage microorganisms in juices. However, we are aware of only a few studies that have used cold plasma to inactivate A. acidoterrestris. In this study, the inactivation efficacy of cold plasma was determined using the plate count method and described using a biphasic model. The effects of the food matrix, input power, gas flow rate, and treatment time on inactivation efficacy were also discovered. Scavenging experiments with reactive oxygen species (•OH, •O2−, and 1O2), scanning electron microscopy (SEM), Raman spectra, as well as an in vitro toxicology assay kit, were used to determine the inactivation mechanism. According to the plate count method, a maximum reduction of 4.14 log CFU/ mL could be achieved within 7 s, and complete inactivation could be achieved within 240 s. The scavenging experiments showed that directly cold plasma-produced singlet oxygen plays the most crucial role in inactivation, which was also confirmed by the fluorescence probe SOSG. The scanning electron microscopy (SEM) and Raman spectra showed that the cold plasma treatment damaged the membrane integrity, DNA, proteins, lipids, and carbohydrates of A. acidoterrestris. The plate count results and the apple juice quality evaluation showed that the cold plasma treatment (1.32 kV) could inactivate 99% of A. acidoterrestris within 60 s, with no significant changes happening in apple juice quality, except for slight changes in the polyphenol content and color value.

Plasma-activated liquids for mitigating biofilms on food and food contact surfaces

Plasma-activated liquids (PALs) are emerging and promising alternatives to traditional decontamination technologies and have evolved as a new technology for applications in food, agriculture, and medicine. Contamination caused by foodborne pathogens and their biofilms has posed challenges and concerns to the food industry in terms of safety and quality. The nature of the food and the food processing environment are major factors that contribute to the growth of various microorganisms, followed by the biofilm characteristics that ensure their survival in severe environmental conditions and against traditional chemical disinfectants. PALs show an efficient impact against microorganisms and their biofilms, with various reactive species (short- and long-lived ones), physiochemical properties, and plasma processing factors playing a crucial role in mitigating biofilms. Moreover, there is potential to improve and optimize disinfection strategies using a combination of PALs with other technologies for the inactivation of biofilms. The overarching aim of this study is to build a better understanding of the parameters that govern the liquid chemistry generated in a liquid exposed to plasma and how these translate into biological effects on biofilms. This review provides a current understanding of PALs-mediated mechanisms of action on biofilms; however, the precise inactivation mechanism is still not clear and is an important part of the research. Implementation of PALs in the food industry could help overcome the disinfection hurdles and can enhance biofilm inactivation efficacy. Future perspectives in this field to expand existing state of the art to seek breakthroughs for scale-up and implementation of PALs technology in the food industry are also discussed.

Effects of Atmospheric Plasma Corona Discharge on Saccharomyces cerevisiae: Viability, Permeability, and Morphology

Food spoilage is a routine challenge in food production. Saccharomyces cerevisiae is a major contaminating microorganism associated with fruit pulps and juices. Our study demonstrated the effect of a plasma corona discharge on S. cerevisiae viability, membrane permeability, and morphology when the cells were prepared in both dry and wet modes. The S. cerevisiae viability was examined as a function of the duration of plasma exposure, the sample's distance from the treating head, initial cell concentration, and yeast suspension volume. The results showed a linear correlation between the exposure duration and the CFU/mL in both dry and wet modes. When the initial yeast concentration was 10⁶ CFU/mL, complete eradication in the dry and wet modes occurred after 45 and 240 s, respectively. Exposure of different initial concentrations of S. cerevisiae to plasma in dry (20 s) or wet (90 s) mode led to 2 to 3 orders of magnitude reduction. In both modes, there was total eradication when the initial cell concentration was about 10³ CFU/mL. The cell-membrane permeability was examined using a flow cytometer and the fluorescent dye propidium iodide (PI). Plasma treatment in the dry mode for 30 and 45 s led to 51% and 76% PI-positive cells. Similar results were obtained in the wet mode but with a longer exposure for 120 and 240 s, respectively. Atmospheric plasma may provide disinfection technology for the food industry in a short process without heating.

Synergistic antibacterial mechanism of different essential oils and their effect on quality attributes of ready-to-eat pakchoi (Brassica campestris L. ssp. chinensis)

The mixture of garlic essential oil (GEO), ginger essential oil (GIEO) and litsea cubeba essential oil (LCEO) was prepared and its effect on the antibacterial activity of E. coli, S. aureus and P. aeruginosa, as well as properties of ready-to-eat pakchoi during storage were assessed. GEO, GIEO or LCEO treatment significantly enhanced the accumulation of reactive oxygen species (ROS) levels, resulting in disruption of the permeability of cell membrane, the leakage of cytoplasmic contents, and the alteration of the secondary structure of bacterial proteins. Meanwhile, GEO, GIEO or LCEO treatment repressed the key enzyme in tricarboxylic acid (TCA) and Hexose monophosphate pathway (HMP) cycle of E. coli, S. aureus and P. aeruginosa. Essential oil treatments (p < 0.05) could significantly prolong the shelf life of pakchoi, total bacterial count (TBC) values and chlorophyll content of GEO/GIEO/LCEO sample were 3.47 log cfu/g and 0.82 mg/g, respectively, after storage for 7 days. E. coli, S. aureus and P. aeruginosa counts in GEO/GIEO/LCEO samples decreased by 56.76 %, 70.10 %, 73.95 % compared to CK (no essential oil) samples. The comprehensive results from the sensory (flavor and color) and microbial analysis (especially TBC) showed that GEO/GIEO/LCEO could extend the shelf life of ready-to-eat pakchoi from 4 d to 7 d. As compared with GEO, GIEO or LCEO individually, the combination of GEO, GIEO and LCEO exhibited synergistic effect and more pronouncedly antibacterial activity to improve quality of ready-to-eat pakchoi.

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.

Upcycle hazard against other hazard: Toxic fluorides from plasma fluoropolymer etching turn novel microbial disinfectants

The release of toxic fluoride byproducts is a seemingly unavoidable artifact of surface engineering, causing severe environmental and human health problems. Here we propose and implement a new "upcycle hazard against other hazard" concept in the case study of cold atmospheric plasma surface modification of fluoropolymers such as polytetrafluorethylene (PTFE). Capitalizing on the excellent controllability, precision and energy efficiency of the plasma surface processing, complemented with the recently discovered ability of plasmas to activate water to produce a potent electrochemical disinfectant, referred to as the plasma-activated water (PAW), we demonstrate a radically new solution to capture the hazardous gaseous fluorides into the PAW and use the as-fluorinated PAW (F-PAW) as a very effective antimicrobial disinfectant. A customized surface discharge reactor is developed to evaluate the effects of fluorides released from the plasma etching of PTFE on the chemistries in gas-phase plasmas and F-PAW, as well as the antibacterial effect of F-PAW. The results show that gaseous fluorides, including COF₂, CF₃COF, and SiF₄ are produced in gas-phase plasmas, and the dissolution of thus-generated fluorides into PAW has a strong effect on inactivating catalase and destroying the oxidation resistance of bacterial cells. As a result, the antibacterial effect of PAW-fluorides against the methicillin-resistant Staphylococcus aureus (MRSA) is enhanced by > 5 log reductions, suggesting that otherwise hazardous fluorides from the plasma processing of PTFE can be used to enhance the microbial disinfection efficiency of PAW. The demonstrated approach opens new avenues for sustainable hazard valorization exemplified by converting toxic fluoride-etching products into potent antimicrobial and potentially anti-viral disinfectants.

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.

codY and pdhA Expression Is Induced in Staphylococcus epidermidis Biofilm and Planktonic Populations With Higher Proportions of Viable but Non-Culturable Cells

Staphylococcus epidermidis biofilm cells can enter a physiological state known as viable but non-culturable (VBNC), where, despite being alive, they do not grow in conventional laboratory media. As such, the presence of VBNC cells impacts the diagnosis of S. epidermidis biofilm-associated infections. Previous transcriptomics analysis of S. epidermidis strain 9142 biofilms with higher proportions of VBNC cells suggested that the genes pdhA, codY and mazEF could be involved in the induction of the VBNC state. However, it was previously demonstrated that VBNC induction is strain-dependent. To properly assess the role of these genes in VBNC induction, the construction of mutant strains is necessary. Thus, herein, we assessed if VBNC cells could be induced in strain 1457, a strain amenable to genetic manipulation, and if the previously identified genes were involved in the modulation of the VBNC state in this strain. Furthermore, we evaluated the formation of VBNC cells on planktonic cultures. Our results showed that despite being commonly associated with biofilms, the proportion of VBNC cells can be modulated in both biofilm and planktonic cultures and that the expression of codY and pdhA was upregulated under VBNC inducing conditions in both phenotypes. Overall, our study revealed that the formation of VBNC cells in S. epidermidis is independent of the mode of growth and that the genes codY and pdhA seem to be relevant for the regulation of this physiological condition.

Cold plasma technology: advanced and sustainable approach for wastewater treatment

Cold plasma has been a potent energy-efficient and eco-friendly advanced oxidation technology which has gained attention in recent decades as a non-thermal approach in diverse forms of applications. This review highlights a comprehensive account of the implementation of this technology in the field of wastewater treatment to resolve certain issues regarding the degradation of numerous aqueous pollutants and water-borne pathogenic microorganisms including viruses up to a significant level. The paper addresses plasma chemistry sources and mechanisms on wastewater treatment and impact on various physical, chemical, and biological characteristics of treated water. Furthermore, studies have revealed that this emerging technology is effective in inactivating SARS-CoV-2 or coronavirus, which serves as a transmission channel for this lethal virus in wastewater. Despite these benefits, the development of cold plasma as a wastewater treatment technique is still hampered by a lack of information like capital investment, proficient application, liveability, and operating cost, thus necessitating additional research for its booming commercialization, as this can be an emerging approach to solving water crises and meeting the demand for fresh or potable water resources.

Plasma induced efficient removal of antibiotic-resistant Escherichia coli and antibiotic resistance genes, and inhibition of gene transfer by conjugation

Antibiotic-resistant bacteria (ARB) and their resistance genes (ARGs) are emerging environmental pollutants that pose great threats to human health. In this study, a novel strategy using plasma was developed to simultaneously remove antibiotic-resistant Escherichia coli (AR bio-56954 E. coli) and its ARGs, aiming to inhibit gene transfer by conjugation. Approximately 6.6 log AR bio-56954 E. coli was inactivated within 10 min plasma treatment, and the antibiotic resistance to tested antibiotics (tetracycline, gentamicin, and amoxicillin) significantly decreased. Reactive oxygen and nitrogen species (RONS) including •OH, ¹O₂, O₂•^-, NO₂^-, and NO₃^- contributed to ARB and ARGs elimination; their attacks led to destruction of cell membrane, accumulation of excessive intracellular reactive oxygen substances, deterioration of conformational structures of proteins, and destroy of nucleotide bases of DNA. As a result, the ARGs (tet(C), tet(W), blaTEM-1, aac(3)-II), and integron gene intI1), and conjugative transfer frequency of ARGs significantly decreased after plasma treatment. The results demonstrated that plasma has great prospective application in removing ARB and ARGs in water, inhibiting gene transfer by conjugation.

In vitro antimicrobial effect and mechanism of action of plasma-activated liquid on planktonic Neisseria gonorrhoeae

Neisseria gonorrhoeae (Ng) is highly resistant to treatment, and there is an urgent need for new treatments to alleviate gonococcal resistance caused by antibiotic monotherapy. The antimicrobial effect and mechanism of plasma-activated liquid (PAL) on Ng were evaluated in this study. Upon PAL treatment, extensively analyses on cell culturability, metabolic capacity, intracellular reactive oxygen species (ROS),membrane integrity and nucleic acids for Ng were carried out and significant antimicrobial effects observed.PAL exerted antibacterial effect on Ng and induced bacterial death (6.71-log) following immersion for 30 min and treatment for 120 s. However, bacterial viability test revealed that after immersion in the same PAL, 10.17% of bacteria retained their metabolic capacity. This indicates that bacteria enter a physiological viable but non-culturable state to protect themselves from environmental stress. Confocal fluorescence microscopy and transmission electron microscopy demonstrated that PAL exerts bactericidal effect on Ng and disrupts its morphological structure. PAL may upregulate inflammatory factors and genes to modulate the resistance of Ng and affect the immune status of the host during infection.

Guidelines on reporting treatment conditions for emerging technologies in food processing

In the last decades, different non-thermal and thermal technologies have been developed for food processing. However, in many cases, it is not clear which experimental parameters must be reported to guarantee the experiments' reproducibility and provide the food industry a straightforward way to scale-up these technologies. Since reproducibility is one of the most important science features, the current work aims to improve the reproducibility of studies on emerging technologies for food processing by providing guidelines on reporting treatment conditions of thermal and non-thermal technologies. Infrared heating, microwave heating, ohmic heating and radiofrequency heating are addressed as advanced thermal technologies and isostatic high pressure, ultra-high-pressure homogenization sterilization, high-pressure homogenization, microfluidization, irradiation, plasma technologies, power ultrasound, pressure change technology, pulsed electric fields, pulsed light and supercritical CO₂ are approached as non-thermal technologies. Finally, growing points and perspectives are highlighted.

Aramid nanofibers reinforced polyvinyl alcohol/tannic acid hydrogel with improved mechanical and antibacterial properties for potential application as wound dressing

The poor mechanical properties and the lack of antibacterial ability of hydrogels limit their applications as wound dressing. In this work, a novel and high strength polyvinyl alcohol (PVA)/tannic acid (TA) hydrogel with aramid nanofibers (ANFs) as the reinforcement was successfully fabricated. The surface composition and microstructure of the hydrogel were characterized by fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties, water content and swelling behaviors, as well as the antibacterial abilities and biocompatibility of the prepared hydrogel were systematically analyzed as well. The results indicated that the prepared hydrogel showed excellent mechanical properties. The tensile strength and elongation of the prepared hydrogel can respectively reach 2.06 MPa and 950% owing to the formation of the multiple H bonds among PVA, ANFs and TA. What's more, PVA/ANFs/TA (PAT) hydrogel possessed shape memory and broad-spectrum antibacterial properties against S. aureus, E. coli and P. aeruginosa (100% antibacterial rate) at the concentration of 12 mg/mL. PAT hydrogels also had low cytotoxicity, affirming its potential application as wound dressing.

Plasma-activated water: Physicochemical properties, microbial inactivation mechanisms, factors influencing antimicrobial effectiveness, and applications in the food industry

Novel nonthermal inactivation technologies have been increasingly popular over the traditional thermal food processing methods due to their capacity in maintaining microbial safety and other quality parameters. Plasma-activated water (PAW) is a cutting-edge technology developed around a decade ago, and it has attracted considerable attention as a potential washing disinfectant. This review aims to offer an overview of the fundamentals and potential applications of PAW in the agri-food sector. A detailed description of the interactions between plasma and water can help to have a better understanding of PAW, hence the physicochemical properties of PAW are discussed. Further, this review elucidates the complex inactivation mechanisms of PAW, including oxidative stress and physical effect. In particular, the influencing factors on inactivation efficacy of PAW, including processing factors, characteristics of microorganisms, and background environment of water are extensively described. Finally, the potential applications of PAW in the food industry, such as surface decontamination for various food products, including fruits and vegetables, meat and seafood, and also the treatment on quality parameters are presented. Apart from decontamination, the applications of PAW for seed germination and plant growth, as well as meat curing are also summarized. In the end, the challenges and limitations of PAW for scale-up implementation, and future research efforts are also discussed. This review demonstrates that PAW has the potential to be successfully used in the food industry.

Synergistic bactericidal effect of clove oil and encapsulated atmospheric pressure plasma against Escherichia coli O157:H7 and Staphylococcus aureus and its mechanism of action

We investigated the bactericidal effect of clove oil and encapsulated atmospheric pressure plasma (EAP), individually or in combination, against Escherichia coli O157:H7 and Staphylococcus aureus. The bactericidal effect of the combined treatment was also investigated in inoculated beef jerky. For both pathogens, clove oil and EAP single treatments resulted in less than 3.0-log reductions, whereas the combined treatment resulted in more than 7.5-log reductions. The disc-diffusion assay and gas chromatography-mass spectrometry showed no changes in both the clear zone diameter and chemical composition of clove oil before and after the EAP treatment. Significant changes in cell membrane permeability and cell morphology resulting from the combined treatment of clove oil and EAP were evidenced by increased in UV absorption of cell supernatants, increased cell staining with propidium iodide, and changes in cell structure revealed by transmission electron microscopy. The synergistic bactericidal effects of clove oil and EAP against both pathogens were also observed in inoculated beef jerky, but the treatments were less effective against S. aureus, presumably due to thicker peptidoglycan layer. Experiments also demonstrated that the synergistic bactericidal effects between clove oil and EAP are due to clove oil increasing the susceptibility of the bacteria to subsequent EAP treatment, and does not involve alteration of the antibacterial activity of clove oil by EAP.

Effects of Atmospheric Plasma Corona Discharges on Soil Bacteria Viability

Crop contamination by soil-borne pathogenic microorganisms often leads to serious infection outbreaks. Plant protection requires disinfection of agricultural lands. The chemical and the physical disinfection procedures have several disadvantages, including an irreversible change in the soil ecosystem. Plasma, the "fourth state of matter" is defined as an ionized gas containing an equal number of negatively and positively charged particles. Cold-plasma technology with air or oxygen as the working gas generates reactive oxygen species, which are found to efficiently eradicate bacteria. In this study, we examined the effect of atmospheric plasma corona discharges on soil bacteria viability. Soil that was exposed to plasma for 60 s resulted in bacterial reduction by two orders of magnitude, from 1.1 × 10⁵ to 2.3 × 10³ cells g^-1 soil. Exposure for a longer period of 5 min did not lead to further significant reduction in bacterial concentration (a final reduction of only 2.5 orders of magnitude). The bacterial viability was evaluated using a colorimetric assay based on the bacterial hydrogenases immediately after exposure and at selected times during 24 h. The result showed no recovery in the bacterial viability. Plasma discharged directly on bacteria that were isolated from the soil resulted in a reduction by four orders of magnitude in the bacterial concentration compared to untreated isolated bacteria: 2.6 × 10^-3 and 1.7 × 10^-7, respectively. The plasma-resistant bacteria were found to be related to the taxonomic phylum Firmicutes (98.5%) and comprised the taxonomic orders Bacillales (95%) and Clostridiales (2%). To our knowledge, this is the first study of soil bacteria eradication using plasma corona discharges.

Prevention of damage caused by corona discharge-generated reactive oxygen species under electrostatic aerosol-to-hydrosol sampling

Human exposure to airborne pathogens is a major cause of health concerns; therefore, it is imperative to monitor, sample, and detect airborne bio-particles. Among various bio-aerosol sampling methods, electrostatic precipitation (EP) is an efficient technique for capturing bio-aerosols as hydrosols due to a lower pressure drop and less damage to sensitive bio-particles. Corona discharge is the main EP mechanism; however, this inevitably generates reactive oxygen species (ROS), which can be transported and dissolved in the sampling liquid. ROS can modify cellular component structures and damage DNA. Additionally, during the sampling process, the liquid flow rate and sampling liquid type can highly affect sampling efficiency. Here, different liquid types and flow rates are examined and ascorbic acid (AA), known as vitamin C, is added to prevent bio-particle damage. However, a high AA concentration can cause oxidative damage. Therefore, the optimal AA concentration should be chosen to obtain the greatest protective effect.

Plant Disease Control by Non-Thermal Atmospheric-Pressure Plasma

Disease stresses caused by pathogenic microorganisms are increasing, probably because of global warming. Conventional technologies for plant disease control have often revealed their limitations in efficiency, environmental safety, and economic costs. There is high demand for improvements in efficiency and safety. Non-thermal atmospheric-pressure plasma has demonstrated its potential as an alternative tool for efficient and environmentally safe control of plant pathogenic microorganisms in many studies, which are overviewed in this review. Efficient inactivation of phytopathogenic bacterial and fungal cells by various plasma sources under laboratory conditions has been frequently reported. In addition, plasma-treated water shows antimicrobial activity. Plasma and plasma-treated water exhibit a broad spectrum of efficiency in the decontamination and disinfection of plants, fruits, and seeds, indicating that the outcomes of plasma treatment can be significantly influenced by the microenvironments between plasma and plant tissues, such as the surface structures and properties, antioxidant systems, and surface chemistry of plants. More intense studies are required on the efficiency of decontamination and disinfection and underlying mechanisms. Recently, the induction of plant tolerance or resistance to pathogens by plasma (so-called "plasma vaccination") is emerging as a new area of study, with active research ongoing in this field.

Protective Effect of the Golden Staphyloxanthin Biosynthesis Pathway on Staphylococcus aureus under Cold Atmospheric Plasma Treatment

Staphylococcus aureus infection poses a serious threat to public health, and antibiotic resistance has complicated the clinical treatment and limited the solutions available to solve this problem. Cold atmospheric plasma (CAP) is a promising strategy for microorganism inactivation. However, the mechanisms of microbial inactivation or resistance remain unclear. In this study, we treated S. aureus strains with a self-assembled CAP device and found that CAP can kill S. aureus in an exposure time-dependent manner. In addition, the liquid environment can influence the survival rate of S. aureus post-CAP treatment. The S. aureus cells can be completely inactivated in normal saline and phosphate-buffered saline but not in tryptic soy broth culture medium. Scanning and transmission electron microscopy revealed that the CAP-treated S. aureus cells maintained integrated morphological structures, similar to the wild-type strain. Importantly, the CAP-treated S. aureus cells exhibited a reduced pigment phenotype. Deletion of the staphyloxanthin biosynthetic genes crtM and crtN deprived the pigmentation ability of S. aureus Newman. Both the Newman-ΔcrtM and Newman-ΔcrtN mutants presented high sensitivity to CAP treatment, whereas Newman-ΔcrtO exhibited a survival rate comparable to wild-type Newman after CAP treatment. Our data demonstrated that the yellow pigment intermediates of the staphyloxanthin biosynthetic pathway are responsible for the protection of S. aureus from CAP inactivation. The key enzymes, such as CrtM and CrtN, of the golden staphyloxanthin biosynthetic pathway could be important targets for the design of novel sterilization strategies against S. aureus infections.IMPORTANCEStaphylococcus aureus is an important pathogen that can be widely distributed in the community and clinical settings. The emergence of S. aureus with multiple-antibiotic resistance has complicated staphylococcal infection control. The development of alternative strategies with powerful bactericidal effects is urgently needed. Cold atmospheric plasma (CAP) is a promising strategy for microorganism inactivation. Nevertheless, the underlying mechanisms of microbial inactivation or resistance are not completely illustrated. In this study, we validated the bactericidal effects of CAP on S. aureus, including antibiotic-resistant strains. We also found that the golden staphyloxanthin, as well as its yellow pigment intermediates, protected S. aureus against CAP, and blocking the staphyloxanthin synthesis pathway at the early steps could strengthen the sensitivity of S. aureus to CAP treatment. These data provide insights into the germicidal mechanism of CAP from the aspect of bacteria and suggest new targets against S. aureus infections.

Direct Treatment of Liquids Using Low-Current Arc in Ambient Air for Biomedical Applications

In this work, we developed a portable device with low production and operation costs for generating an ambient air low-current arc (AALCA) that is transferred to the surface of a treated liquid. It was possible to generate a stable discharge, irrespective of the conductivity of the treated liquid, as a sequence of corona, repeating spark, and low-current arc discharges. The estimated concentration of reactive oxygen and nitrogen species (RONS) in plasma-treated water (PTW) produced using AALCA treatment was two orders of magnitude higher than that of PTW produced using conventional He nonequilibrium atmospheric pressure plasma jets or dielectric barrier discharges. The strong bactericidal effect of the treatment using AALCA and the water treated using AALCA was confirmed by survival tests of Escherichia coli. Further, the possibility of treating a continuous flow of liquid using AALCA was demonstrated.

Antimicrobial mechanism and the effect of atmospheric pressure N2 plasma jet on the regeneration capacity of Staphylococcus aureus biofilm

This study systematically assessed the inactivation mechanism on Staphylococcus aureus biofilms by a N₂ atmospheric-pressure plasma jet and the effect on the biofilm regeneration capacity from the bacteria which survived, and their progenies. The total bacterial populations were 7.18 ± 0.34 log10 CFU ml^-1 in biofilms and these were effectively inactivated (>5.5-log10 CFU ml^-1) within 30 min of exposure. Meanwhile, >80% of the S. aureus biofilm cells lost their metabolic capacity. In comparison, ∼20% of the plasma-treated bacteria entered a viable but non-culturable state. Moreover, the percentage of membrane-intact bacteria declined to ∼30%. Scanning electron microscope images demonstrated cell shrinkage and deformation post-treatment. The total amount of intracellular reactive oxygen species was observed to have significantly increased in membrane-intact bacterial cells with increasing plasma dose. Notably, the N₂ plasma treatment could effectively inhibit the biofilm regeneration ability of the bacteria which survived, leading to a long-term phenotypic response and dose-dependent inactivation effect on S. aureus biofilms, in addition to the direct rapid bactericidal effect.

Roles of membrane protein damage and intracellular protein damage in death of bacteria induced by atmospheric-pressure air discharge plasmas

Although plasma sterilization has attracted much attention, the underlying mechanisms and biochemical pathways are still not fully understood. In this work, we investigate the molecular mechanism pertaining to the inactivation of Escherichia coli (E. coli) by air discharge plasmas. The membrane protein YgaP and intracellular protein swc7 are over-expressed in E. coli by genetic recombination and gene inducible expression techniques and plasma exposure is demonstrated to alter the structures of YgaP and swc7 in E. coli. The plasma-induced damage of YgaP and swc7 involves changes in the secondary and tertiary structures instead of the primary structure and the modification effectiveness depends on the storage time after the plasma treatment. Owing to the unique structure of E. coli, YgaP is more susceptible to the plasma treatment than intracellular swc7. Within 1 h after plasma exposure, YgaP is modified but not swc7, but after 1 h or longer, both YgaP and swc7 proteins are indeed modified. By analyzing the plasma-induced antimicrobial efficacy and modification of YgaP and swc7, plasma-induced modification of the membrane proteins is the major cause of bacterial death but there is no identifiable relationship with modification of the intracellular protein. The new results provide insights into the mechanism of multiple plasma-induced damage to bacteria and cells as well as the disinfection mechanism.

Although plasma sterilization has attracted much attention, the underlying mechanisms and biochemical pathways are still not fully understood.