Eradication of Pseudomonas aeruginosa biofilms by atmospheric pressure non-thermal plasma

Cold Atmospheric Pressure Plasma Solutions for Sustainable Food Packaging

Increasing the number of resistant bacteria resistant to treatment is one of the leading causes of death worldwide. These bacteria are created in wounds and injuries and can be transferred through hospital equipment. Various attempts have been made to treat these bacteria in recent years, such as using different drugs and new sterilization methods. However, some bacteria resist drugs, and other traditional methods cannot destroy them. In the meantime, various studies have shown that cold atmospheric plasma can kill these bacteria through different mechanisms, making cold plasma a promising tool to deactivate bacteria. This new technology can be effectively used in the food industry because it has the potential to inactivate microorganisms such as spores and microbial toxins and increase the wettability and printability of polymers to pack fresh and dried food. It can also increase the shelf life of food without leaving any residue or chemical effluent. This paper investigates cold plasma's potential, advantages, and disadvantages in the food industry and sterilization.

Cold Atmospheric Plasma Is a Promising Alternative Treatment Option in Case of Split Skin Graft Failure

Cold atmospheric plasma (CAP) has shown promising potential in promoting wound healing. This case report presents the successful application of CAP in a 42-year-old female patient with extensive wound healing disorders and superinfections following the excision of an abscess in the left thoracic region. After several failed split skin graft attempts, the implementation of CAP led to significant improvements in wound healing. This report highlights the wound healing-promoting effects of CAP and discusses its potential mechanisms of action.

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.

Plasma activated water as a pre-treatment strategy in the context of biofilm-infected chronic wounds

Healing and treatment of chronic wounds are often complicated due to biofilm formation by pathogens. Here, the efficacy of plasma activated water (PAW) as a pre-treatment strategy has been investigated prior to the application of topical antiseptics polyhexamethylene biguanide, povidone iodine, and MediHoney, which are routinely used to treat chronic wounds. The efficacy of this treatment strategy was determined against biofilms of Escherichia coli formed on a plastic substratum and on a human keratinocyte monolayer substratum used as an in vitro biofilm-skin epithelial cell model. PAW pre-treatment greatly increased the killing efficacy of all the three antiseptics to eradicate the E. coli biofilms formed on the plastic and keratinocyte substrates. However, the efficacy of the combined PAW-antiseptic treatment and single treatments using PAW or antiseptic alone was lower for biofilms formed in the in vitro biofilm-skin epithelial cell model compared to the plastic substratum. Scavenging assays demonstrated that reactive species present within the PAW were largely responsible for its anti-biofilm activity. PAW treatment resulted in significant intracellular reactive oxygen and nitrogen species accumulation within the E. coli biofilms, while also rapidly acting on the microbial membrane leading to outer membrane permeabilisation and depolarisation. Together, these factors contribute to significant cell death, potentiating the antibacterial effect of the assessed antiseptics.

Cold atmospheric pressure plasma-antibiotic synergy in Pseudomonas aeruginosa biofilms is mediated via oxidative stress response

Cold atmospheric-pressure plasma (CAP) has emerged as a potential alternative or adjuvant to conventional antibiotics for the treatment of bacterial infections, including those caused by antibiotic-resistant pathogens. The potential of sub-lethal CAP exposures to synergise conventional antimicrobials for the eradication of Pseudomonas aeruginosa biofilms is investigated in this study. The efficacy of antimicrobials following or in the absence of sub-lethal CAP pre-treatment in P. aeruginosa biofilms was assessed. CAP pre-treatment resulted in an increase in both planktonic and biofilm antimicrobial sensitivity for all three strains tested (PAO1, PA14, and PA10548), with both minimum inhibitory concentrations (MICs) and minimum biofilm eradication concentrations (MBECs) of individual antimicrobials, being significantly reduced following CAP pre-treatment of the biofilm (512-fold reduction with ciprofloxacin/gentamicin; and a 256-fold reduction with tobramycin). At all concentrations of antimicrobial used, the combination of sub-lethal CAP exposure and antimicrobials was effective at increasing time-to-peak metabolism, as measured by isothermal microcalorimetry, again indicating enhanced susceptibility. CAP is known to damage bacterial cell membranes and DNA by causing oxidative stress through the in situ generation of reactive oxygen and nitrogen species (RONS). While the exact mechanism is not clear, oxidative stress on outer membrane proteins is thought to damage/perturb cell membranes, confirmed by ATP and LDH leakage, allowing antimicrobials to penetrate the bacterial cell more effectively, thus increasing bacterial susceptibility. Transcriptomic analysis, reveals that cold-plasma mediated oxidative stress caused upregulation of P. aeruginosa superoxide dismutase, cbb3 oxidases, catalases, and peroxidases, and upregulation in denitrification genes, suggesting that P. aeruginosa uses these enzymes to degrade RONS and mitigate the effects of cold plasma mediated oxidative stress. CAP treatment also led to an increased production of the signalling molecule ppGpp in P. aeruginosa, indicative of a stringent response being established. Although we did not directly measure persister cell formation, this stringent response may potentially be associated with the formation of persister cells in biofilm cultures. The production of ppGpp and polyphosphate may be associated with protein synthesis inhibition and increase efflux pump activity, factors which can result in antimicrobial tolerance. The transcriptomic analysis also showed that by 6 h post-treatment, there was downregulation in ribosome modulation factor, which is involved in the formation of persister cells, suggesting that the cells had begun to resuscitate/recover. In addition, CAP treatment at 4 h post-exposure caused downregulation of the virulence factors pyoverdine and pyocyanin; by 6 h post-exposure, virulence factor production was increasing. Transcriptomic analysis provides valuable insights into the mechanisms by which P. aeruginosa biofilms exhibits enhanced susceptibility to antimicrobials. Overall, these findings suggest, for the first time, that short CAP sub-lethal pre-treatment can be an effective strategy for enhancing the susceptibility of P. aeruginosa biofilms to antimicrobials and provides important mechanistic insights into cold plasma-antimicrobial synergy. Transcriptomic analysis of the response to, and recovery from, sub-lethal cold plasma exposures in P. aeruginosa biofilms improves our current understanding of cold plasma biofilm interactions.

Staphylococcus aureus strains exhibit heterogenous tolerance to direct cold atmospheric plasma therapy

The global clinical and socioeconomic impact of chronic wounds is substantial. The main difficulty that clinicians face during the treatment of chronic wounds is the risk of infection at the wound site. Infected wounds arise from an accumulation of microbial aggregates in the wound bed, leading to the formation of polymicrobial biofilms that can be largely resistant to antibiotic therapy. Therefore, it is essential for studies to identify novel therapeutics to alleviate biofilm infections. One innovative technique is the use of cold atmospheric plasma (CAP) which has been shown to possess promising antimicrobial and immunomodulatory properties. Here, different clinically relevant biofilm models will be treated with cold atmospheric plasma to assess its efficacy and killing effects. Biofilm viability was assessed using live dead qPCR, and morphological changes associated with CAP evaluated using scanning electron microscopy (SEM). Results indicated that CAP was effective against Candida albicans and Pseudomonas aeruginosa, both as mono-species biofilms and when grown in a triadic model system. CAP also significantly reduced viability in the nosocomial pathogen, Candida auris. Staphylococcus aureus Newman exhibited a level of tolerance to CAP therapy, both when grown alone or in the triadic model when grown alongside C. albicans and P. aeruginosa. However, this degree of tolerance exhibited by S. aureus was strain dependent. At a microscopic level, biofilm treatment led to subtle changes in morphology in the susceptible biofilms, with evidence of cellular deflation and shrinkage. Taken together, these results indicate a promising application of direct CAP therapy in combatting wound and skin-related biofilm infections, although biofilm composition may affect the treatment efficacy.

Exploring the potential of cold plasma therapy in treating bacterial infections in veterinary medicine: opportunities and challenges

Cold plasma therapy is a novel approach that has shown significant promise in treating bacterial infections in veterinary medicine. Cold plasma possesses the potential to eliminate various bacteria, including those that are resistant to antibiotics, which renders it a desirable substitute for traditional antibiotics. Furthermore, it can enhance the immune system and facilitate the process of wound healing. However, there are some challenges associated with the use of cold plasma in veterinary medicine, such as achieving consistent and uniform exposure to the affected area, determining optimal treatment conditions, and evaluating the long-term impact on animal health. This paper explores the potential of cold plasma therapy in veterinary medicine for managing bacterial diseases, including respiratory infections, skin infections, and wound infections such as Clostridium botulinum, Clostridium perfringens, Bacillus cereus, and Bacillus subtilis. It also shows the opportunities and challenges associated with its use. In conclusion, the paper highlights the promising potential of utilizing cold plasma in veterinary medicine. However, to gain a comprehensive understanding of its benefits and limitations, further research is required. Future studies should concentrate on refining treatment protocols and assessing the long-term effects of cold plasma therapy on bacterial infections and the overall health of animals.

Basic research and clinical exploration of cold atmospheric plasma for skin wounds

Skin wounds, such as burns, diabetic foot ulcers, pressure sores, and wounds formed after laser or surgical treatment, comprise a very high proportion of dermatological disorders. Wounds are treated in a variety of ways; however, some wounds are greatly resistant, resulting in delayed healing and an urgent need to introduce new alternatives. Our previous studies have shown that cold atmospheric plasma (CAP) has antibacterial activity and promotes cell proliferation, differentiation, and migration in vitro. To further advance the role of CAP in wound healing, we evaluated the safety and efficacy of CAP in vitro by irradiation of common refractory bacteria on the skin, irradiation of normal skin of rats and observing reactions, treatment of scald wounds in rats, and treating clinically common acute wounds. Our findings revealed that CAP can eliminate refractory skin bacteria in vitro; CAP positively affected wound healing in a rat scalding wound model; and direct CAP irradiation of low intensity and short duration did not lead to skin erythema or edema. CAP promises to be a new, economical, and safe means of wound treatment.

Foreign Body Reaction (Immune Response) for Artificial Implants Can Be Avoided: An Example of Polyurethane in Mice for 1 Week

Despite great success with artificial implants for the human body, modern implants cannot solve major health problems. The reason is an immune reaction of organisms to artificial implants, known as the foreign body reaction. We have found a way to avoid or decrease the foreign body reaction. The surface of an artificial implant is modified with condensed aromatic structures containing free radicals, which provide a covalent attachment of host proteins in a native conformation. The total protein coverage prevents the direct contact of immune cells with the implant surface, and the immune cells are not activated. As a result, the immune response of the organism is not generated, and the artificial implant is not isolated from the tissue; there is no collagen capsule, low activity of macrophages, low cell proliferation, and low inflammatory activity.

Simulation-based architecture of a stable large-area [Formula: see text] atmospheric plasma source

Unified jet-DBD design, [Formula: see text], proposed in this work presents large-scale plasma in an unbounded region of atmospheric air, without any need for the flow of gas, offering efficient exposure to sizable and complex objects. This is a simulation-based architecture for stable non-thermal plasma source with notable experimental results. [Formula: see text] geometry optimizes the electric field and charge distribution for a diffuse discharge in the steady air by a key design parameter of [Formula: see text]. Teflon insulator with a thickness [Formula: see text] imposes an intense and uniform electric field shaped up at the open area in front of the device and generates radially/axially expanded plasma jet. In the [Formula: see text], phase shift increases by [Formula: see text] and the plasma generates more power than the classical plasma jet. Two distinct states of [Formula: see text] operation indicate the mode-swap at [Formula: see text] and power dissipation. In the reactive [Formula: see text] scheme even small changes in the phase angle effectively improves the electric power.

Reactive species variation in cold atmospheric pressure plasma jet discharge under the influence of intrinsic parameters and its effect on E. coli inactivation

Cold atmospheric pressure plasma jet (CAPJ) has piqued the interest of researchers for various antimicrobial applications such as disinfection, wound decontamination, etc. In the current context, a deeper understanding of the correlation between CAPJ's intrinsic parameters, discharge characteristics, species composition, and antimicrobial activity is required for any successful application. This research evaluated the effect of intrinsic operational parameters such as voltage, frequency, gas flow rate, and operating gas on the reactive species composition of an in-house-developed CAPJ discharge along with the antimicrobial activity. It was observed that the identified excited atoms (Ar I, He I, N2, and O I), ions (Ar+, N2+, N+, H2O+, H3O+, etc.), radical reactive oxygen and nitrogen species (RONS) (OH•), and nonradical RONS (O I, O+, OH+, NO+, O2+, O2-, NO2-, N2O2-, NO3-, N2O3-, etc.) might play a synergistic role in bacterial inactivation via oxidative and electrostatic stress. The variation in voltage, frequency, gas flow rate, and operating gas influenced the discharge chemistry, leading to variation in bacterial inactivation. The reactive species in the discharge responsible for such variation was evaluated extensively. This investigation into various operational parameters would aid in determining the most effective settings for a developed CAPJ to achieve high productivity.

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.

Non-thermal plasma induces changes in aflatoxin production, devitalization, and surface chemistry of Aspergillus parasiticus

Non-thermal plasma (NTP) represents the fourth state of matter composed of neutral molecules, atoms, ions, radicals, and electrons. It has been used by various industries for several decades, but only recently NTPs have emerged in fields such as medicine, agriculture, and the food industry. In this work, we studied the effect of NTP exposure on aflatoxin production, conidial germination and mycelial vitality, morphological and surface changes of conidia and mycelium. When compared with colonies grown from untreated conidia, the colonies from NTP-treated conidia produced significantly higher levels of aflatoxins much earlier during development than colonies from untreated conidia. However, at the end of cultivation, both types of cultures yielded similar aflatoxin concentrations. The increase in the accumulation of aflatoxins was supported by high transcription levels of aflatoxin biosynthetic genes, which indicated a possibility that NTP treatment of conidia was having a longer-lasting effect on colony development and aflatoxins accumulation. NTP generated in the air at atmospheric pressure effectively devitalized Aspergillus parasiticus in conidia and hyphae within a few minutes of treatment. To describe devitalization kinetics, we applied Weibull and Hill models on sets of data collected at different exposure times during NTP treatment. The damage caused by NTP to hyphal cell wall structures was displayed by raptures visualized by scanning electron microscopy. Fourier transform infrared spectroscopy demonstrated that changes in cell envelope correlated with shifts in characteristic chemical bonds indicating dehydration, oxidation of lipids, proteins, and polysaccharides. Key points • Non-thermal plasma increases aflatoxin production shortly after treatment. • Non-thermal plasma rapidly devitalizes Aspergillus parasiticus. • Non-thermal plasma disrupts the cell surface and oxidizes biological components.

Progress in Alternative Strategies to Combat Antimicrobial Resistance: Focus on Antibiotics

Antibiotic resistance, and, in a broader perspective, antimicrobial resistance (AMR), continues to evolve and spread beyond all boundaries. As a result, infectious diseases have become more challenging or even impossible to treat, leading to an increase in morbidity and mortality. Despite the failure of conventional, traditional antimicrobial therapy, in the past two decades, no novel class of antibiotics has been introduced. Consequently, several novel alternative strategies to combat these (multi-) drug-resistant infectious microorganisms have been identified. The purpose of this review is to gather and consider the strategies that are being applied or proposed as potential alternatives to traditional antibiotics. These strategies include combination therapy, techniques that target the enzymes or proteins responsible for antimicrobial resistance, resistant bacteria, drug delivery systems, physicochemical methods, and unconventional techniques, including the CRISPR-Cas system. These alternative strategies may have the potential to change the treatment of multi-drug-resistant pathogens in human clinical settings.

In vitro antibacterial and antibiofilm effects of cold atmospheric microwave plasma against Pseudomonas aeruginosa causing canine skin and ear infections

BACKGROUND

Pseudomonas aeruginosa is an opportunist pathogen that causes purulent inflammation in the skin and in the ears of dogs. Among the various virulence factors of P. aeruginosa, biofilms have been reported to result in antibiotic resistance, leading to therapeutic limitations. Cold atmospheric microwave plasma (CAMP) is known to have a high antimicrobial effect, which causes physical cell wall rupture and DNA damage.

HYPOTHESIS/OBJECTIVES

The objective of this study was to evaluate the antibacterial and antibiofilm effects of CAMP against planktonic bacteria and the biofilm of P. aeruginosa.

METHODS AND MATERIALS

The antibacterial effect of CAMP against P. aeruginosa ATCC10145 and clinical isolates (n = 30) was evaluated using the colony count method. We also assessed the effect of CAMP on biofilm of P. aeruginosa ATCC strain by the colony count method, water-soluble tetrazolium salt (WST) assay and confocal laser scanning microscopy (CLSM).

RESULTS

The complete eradication of P. aeruginosa (ATCC strain and clinical isolates) was achieved within 120 s at 50 W, and clinical isolates required 60 s shorter than the ATCC strain for complete eradication at 50 W. We also confirmed the time-dependent bactericidal effect of CAMP at 50 W against ATCC strain biofilm.

CONCLUSIONS AND CLINICAL IMPORTANCE

CAMP was effective against both planktonic bacteria and biofilm formation of P. aeruginosa. However, further studies on in vivo efficacy and safety in canine skin and ears are necessary to fully validate its clinical application.

Non-thermal Plasma Treatment of ESKAPE Pathogens: A Review

The acronym ESKAPE refers to a group of bacteria consisting of Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. They are important in human medicine as pathogens that show increasing resistance to commonly used antibiotics; thus, the search for new effective bactericidal agents is still topical. One of the possible alternatives is the use of non-thermal plasma (NTP), a partially ionized gas with the energy stored particularly in the free electrons, which has antimicrobial and anti-biofilm effects. Its mechanism of action includes the formation of pores in the bacterial membranes; therefore, resistance toward it is not developed. This paper focuses on the current overview of literature describing the use of NTP as a new promising tool against ESKAPE bacteria, both in planktonic and biofilm forms. Thus, it points to the fact that NTP treatment can be used for the decontamination of different types of liquids, medical materials, and devices or even surfaces used in various industries. In summary, the use of diverse experimental setups leads to very different efficiencies in inactivation. However, Gram-positive bacteria appear less susceptible compared to Gram-negative ones, in general.

Multi-Modal Biological Destruction by Cold Atmospheric Plasma: Capability and Mechanism

Cold atmospheric plasma (CAP) is a near-room-temperature, partially ionized gas composed of reactive neutral and charged species. CAP also generates physical factors, including ultraviolet (UV) radiation and thermal and electromagnetic (EM) effects. Studies over the past decade demonstrated that CAP could effectively induce death in a wide range of cell types, from mammalian to bacterial cells. Viruses can also be inactivated by a CAP treatment. The CAP-triggered cell-death types mainly include apoptosis, necrosis, and autophagy-associated cell death. Cell death and virus inactivation triggered by CAP are the foundation of the emerging medical applications of CAP, including cancer therapy, sterilization, and wound healing. Here, we systematically analyze the entire picture of multi-modal biological destruction by CAP treatment and their underlying mechanisms based on the latest discoveries particularly the physical effects on cancer cells.

Cold atmospheric pressure plasma (CAPP) as a new alternative treatment method for onychomycosis caused by Trichophyton verrucosum: in vitro studies

PURPOSE

Anthropophilic dermatophytes as etiological factors of onychomycoses are more common than zoophilic fungi. In the case of the latter, reverse zoonoses are possible, which poses a threat to the persistence of dermatophytes in the environment. Nevertheless, without treatment, both types of tinea unguium may lead to complete nail plate destruction and secondary mixed infections with fungi and bacteria. One of the zoophilic dermatophytes that cause onychomycosis is Trichophyton verrucosum, whose prevalence has been increasing in recent years. Such infections are usually treated with allylamines and/or azoles, but such a conventional treatment of infections caused by T. verrucosum often fails or is discontinued by patients.

METHODS

Herein, we reveal the results of our in vitro studies related to direct application of cold atmospheric pressure plasma (CAPP) on Trichophyton verrucosum growth, germination and adherence to nail as a new alternative treatment method of such types of dermatomycoses.

RESULTS

Our in vitro studies showed that, while exposure to CAPP for 10 min delays germination of conidia and clearly impairs the fitness of the fungal structures, 15 min is enough to kill all fungal elements exposed to plasma. Moreover, the SEM images revealed that T. verrucosum cultures exposed to CAPP for 10 and 15 min were not able to invade the nail fragments.

CONCLUSION

The results revealed that single exposure to CAPP was able to inhibit T. verrucosum growth and infection capacity. Hence, cold atmospheric pressure plasma should be considered as a promising alternative treatment of onychomycoses.

Inhibitory Effect of Cold Atmospheric Plasma on Chronic Wound-Related Multispecies Biofilms

The presence of microbial biofilms in the wounds affects negatively the healing process and can contribute to therapeutic failures. This study aimed to establish the effective parameters of cold atmospheric plasma (CAP) against wound-related multispecies and monospecies biofilms, and to evaluate the cytotoxicity and genotoxicity of the protocol. Monospecies and multispecies biofilms were formed by methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa and Enterococcus faecalis. The monospecies biofilms were grown in 96 wells plates and multispecies biofilm were formed on collagen membranes. The biofilms were exposed to helium CAP for 1, 3, 5 and 7 min. In monospecies biofilms, the inhibitory effect was detected after 1 min of exposure for E. faecalis and after 3 min for MRSA. A reduction in P. aeruginosa biofilm’s viability was detected after 7 min of exposure. For the multispecies biofilms, the reduction in the overall viability was detected after 5 min of exposure to CAP. Additionally, cytotoxicity and genotoxicity were evaluated by MTT assay and static cytometry, respectively. CAP showed low cytotoxicity and no genotoxicity to mouse fibroblastic cell line (3T3). It could be concluded that He-CAP showed inhibitory effect on wound-related multispecies biofilms, with low cytotoxicity and genotoxicity to mammalian cells. These findings point out the potential application of CAP in wound care.

In vitro and in vivo research of atmosphere pressure nonequilibrium plasmas on root canal disinfection: implication for alternative strategy for irrigation

OBJECTIVE

To investigate an intracanal disinfection methodology of APNPs (atmosphere pressure nonequilibrium plasmas) or modified APNPs in root canal treatment and evaluate the antimicrobial efficiency against in vitro infected dentinal tubules and in vivo experimental apical periodontitis.

MATERIALS AND METHODS

Dentine specimens were centrifugated with Enterococcus faecalis to generate 1-day-old and 3-week-old biofilms, and were treated with 2% chlorhexidine (Chx), APNP or modified APNP for 3 and 10 min (n=4). LIVE/DEAD staining was employed to analyze the ratio of deactivated bacteria. Experimental apical periodontitis in beagles was induced. Root canal therapy with APNPs or modified APNPs was performed and the antimicrobial effect was evaluated by histological and radiographical analyses.

RESULTS

APNP deactivated 1-day-old and 3-week-old E. feacalis in dentinal tubules as much as 2% Chx irrigating. Modified APNP significantly deactivated more E. faecalis biofilms in dentinal tubules for 3-min and 10-min treatments, without thermal damage or dentinal destruction being observed. In beagles' apical periodontitis, significantly increased BV/TV and decreased lesion volume of apical bone were found in modified APNP group than 2% Chx irrigation group according to μCT. Fewer inflammatory cells and bacterial residual in dentine were observed in modified APNP-treated apical tissue by histology staining compared with those in the 2% Chx irrigation group.

CONCLUSION

The antimicrobial effect of APNP jet irradiation was comparable to that of 2% Chx irrigation. No structural damage in dentine or tissue necrosis at the periapical region was induced upon treatment. The modified APNP demonstrated an increased antimicrobial efficacy compared with 2% Chx irrigation both in vitro and in vivo.

CLINICAL RELEVANCE

The modified APNPs can be used as an alternative intracanal disinfection strategy.

Going virtual: a report from the sixth Young Microbiologists Symposium on 'Microbe Signalling, Organisation and Pathogenesis'

The sixth Young Microbiologists Symposium on 'Microbe Signalling, Organisation and Pathogenesis' was scheduled to be held at the University of Southampton, UK, in late August 2020. However, due to the health and safety guidelines and travel restrictions as a response to the COVID-19 pandemic, the symposium was transitioned to a virtual format, a change embraced enthusiastically as the meeting attracted over 200 microbiologists from 40 countries. The event allowed junior scientists to present their work to a broad audience and was supported by the European Molecular Biology Organization, the Federation of European Microbiological Societies, the Society of Applied Microbiology, the Biochemical Society, the Microbiology Society and the National Biofilms Innovation Centre. Sessions covered recent advances in all areas of microbiology including: Secretion and transport across membranes, Gene regulation and signalling, Host-microbe interactions, and Microbial communities and biofilm formation. This report focuses on several of the highlights and exciting developments communicated during the talks and poster presentations.

Investigation of decontamination effect of argon cold plasma on physicochemical and sensory properties of almond slices

Cold plasma is a novel technology for surface decontamination. Almond slices can be contaminated with different microorganisms during their production. In the current research, the atmospheric argon plasma treatment (5, 10, 15, and 20 min) was used to decontamination of almond slices surface. Microbial load, Color, peroxide value, hardness, and sensory properties of slices was comprised with untreated samples (control). Descriptive sensory evaluation about color, flavor, crispiness, crunchiness, and hardness of almond slices were performed. According to the results of the microbial tests, Total count, molds and yeasts, and Staphylococcus aureus of almond surface decreased about 2.95 log cfu/g, 1.81 log cfu/g, and 2.72 log cfu/g after 20 min of plasma treatment, respectively, provided that microbial reduction increased more by increasing the treatment time. Coupled with the color evaluation, peroxide value and sensory attributes didn't change during plasma treatment. Having said that, the hardness of slices was changed by increasing treatment time. Furthermore, Principal Component Analysis and cluster analysis were performed for sensory evaluation. In light of the consumer's point of view, firstly 10 min and secondly 15 min plasma treatment can be more desirable.

Reproducibility of 'COST reference microplasma jets'

Atmospheric pressure plasmas have been ground-breaking for plasma science and technologies, due to their significant application potential in many fields, including medicinal, biological, and environmental applications. This is predominantly due to their efficient production and delivery of chemically reactive species under ambient conditions. One of the challenges in progressing the field is comparing plasma sources and results across the community and the literature. To address this a reference plasma source was established during the 'biomedical applications of atmospheric pressure plasmas' EU COST Action MP1101. It is crucial that reference sources are reproducible. Here, we present the reproducibility and variance across multiple sources through examining various characteristics, including: absolute atomic oxygen densities, absolute ozone densities, electrical characteristics, optical emission spectroscopy, temperature measurements, and bactericidal activity. The measurements demonstrate that the tested COST jets are mainly reproducible within the intrinsic uncertainty of each measurement technique.

The anti-glioblastoma effect of cold atmospheric plasma treatment: physical pathway v.s. chemical pathway

Cold atmospheric plasma (CAP), a near room temperature ionized gas, has shown potential application in many branches of medicine, particularly in cancer treatment. In previous studies, the biological effect of CAP on cancer cells and other mammalian cells has been based solely on the chemical factors in CAP, particularly the reactive species. Therefore, plasma medicine has been regarded as a reactive species-based medicine, and the physical factors in CAP such as the thermal effect, ultraviolet irradiation, and electromagnetic effect have been regarded as ignorable factors. In this study, we investigated the effect of a physical CAP treatment on glioblastoma cells. For the first time, we demonstrated that the physical factors in CAP could reinstate the positive selectivity on CAP-treated astrocytes. The positive selectivity was a result of necrosis, a new cell death in glioblastoma cells characterized by the leak of bulk water from the cell membrane. The physically-based CAP treatment overcomed a large limitation of the traditional chemically based CAP treatment, which had complete dependence on the sensitivity of cells to reactive species. The physically-based CAP treatment is a potential non-invasive anti-tumor tool, which may have wide application for tumors located in deeper tissues.

Use of non-thermal plasma pre-treatment to enhance antibiotic action against mature Pseudomonas aeruginosa biofilms

Non-thermal plasma (NTP), generated at atmospheric pressure by DC cometary discharge with a metallic grid, and antibiotics (gentamicin-GTM, ceftazidime-CFZ and polymyxin B-PMB), either alone or in combination, were used to eradicate the mature biofilm of Pseudomonas aeruginosa formed on Ti-6Al-4V alloy. Our aim was to find the conditions for NTP pre-treatment capable of enhancing the action of the antibiotics and thus reducing their effective concentrations. The NTP treatment increased the efficacy of relatively low concentrations of antibiotics. Generally, the highest effect was achieved with GTM, which was able to suppress the metabolic activity of pre-formed P. aeruginosa biofilms in the concentration range of 4-9 mg/L by up to 99%. In addition, an apparent decrease of biofilm-covered area was confirmed after combined NTP treatment and GTM action by SYTO®13 staining using fluorescence microscopy. Scanning electron microscopy confirmed a complete eradication of P. aeruginosa ATCC 15442 mature biofilm from Ti-6Al-4V alloy when using 0.25 h NTP treatment and subsequent treatment by 8.5 mg/L GTM. Therefore, NTP may be used as a suitable antibiofilm agent in combination with antibiotics for the treatment of biofilm-associated infections caused by this pathogen.

The Combination of Plasma-Processed Air (PPA) and Plasma-Treated Water (PTW) Causes Synergistic Inactivation of Candida albicans SC5314

Microwave-induced plasma was used for the generation of plasma-processed air (PPA) and plasma-treated water (PTW). By this way, the plasma was able to functionalize the compressed air and the used water to antimicrobial effective agents. Their fungicidal effects by single and combined application were investigated on Candida albicans strain SC5314. The monoculture of C. albicans was cultivated on specimens with polymeric surface structures (PE-stripes). The additive as well as the synergistic fungicidal potential of PPA and PTW was investigated by different process windows of plasma exposure time (5–50 s) and sample treatment time with PPA/PTW (1–5 min). For a single PTW or PPA treatment, an increase in the reduction factor with the indicated treatment time was observed (maximum reduction factor of 1.1 and 1.6, respectively). In comparison, the combined application of PTW and then PPA resulted in antagonistic, additive and synergistic effects, depending on the combination. An application of the synergistically acting processes of PTW for cleaning and PPA for drying can be an innovative alternative to the sanitary processes currently used in production plants.

Sensitive detection of HO radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a well-established, highly sensitive absorption technique whose sensitivity and selectivity for trace radical sensing can be further enhanced by measuring the polarization rotation of the intracavity light by the paramagnetic samples in the presence of a magnetic field. In this paper, we highlight the use of this Faraday rotation cavity ring-down spectroscopy (FR-CRDS) for the detection of HO₂ radicals. In particular, we use a cold atmospheric pressure plasma jet as a highly efficient source of HO₂ radicals and show that FR-CRDS in the near-infrared spectral region (1506 nm) has the potential to be a useful tool for studying radical chemistry. By simultaneously measuring ring-down times of orthogonal linearly polarized light, measurements of Faraday effect-induced rotation angles (θ) and absorption coefficients (α) are retrieved from the same data set. The Faraday rotation measurement exhibits better long-term stability and enhanced sensitivity due to its differential nature, whereby highly correlated noise between the two channels and slow drifts cancel out. The bandwidth-normalized sensitivities are α_min=2.2×10^-11 cm^-1 Hz^-1/2 and θ_min=0.62 nrad Hz^-1/2. The latter corresponds to a minimum detectable (circular) birefringence of Δn_min=5×10^-16 Hz^-1/2. Using the overlapping ^qQ₃(N = 4-9) transitions of HO₂, we estimate limits of detection of 3.1 × 10⁸ cm^-3 based on traditional (absorption) CRDS methods and 6.7 × 10⁷ cm^-3 using FR-CRDS detection, where each point of the spectrum was acquired during 2 s. In addition, Verdet constants for pertinent carrier (He, Ar) and bulk (N₂, O₂) gases were recorded in this spectral region for the first time. These show good agreement with recent measurements of air and values extrapolated from reported Verdet constants at shorter wavelengths, demonstrating the potential of FR-CRDS for measurements of very weak Faraday effects and providing a quantitative validation to the computed rotation angles.

Antimicrobial effects of microwave-induced plasma torch (MiniMIP) treatment on Candida albicans biofilms

The susceptibility of Candida albicans biofilms to a non-thermal plasma treatment has been investigated in terms of growth, survival and cell viability by a series of in vitro experiments. For different time periods, the C. albicans strain SC5314 was treated with a microwave-induced plasma torch (MiniMIP). The MiniMIP treatment had a strong effect (reduction factor (RF) = 2.97 after 50 s treatment) at a distance of 3 cm between the nozzle and the superior regions of the biofilms. In addition, a viability reduction of 77% after a 20 s plasma treatment and a metabolism reduction of 90% after a 40 s plasma treatment time were observed for C. albicans. After such a treatment, the biofilms revealed an altered morphology of their cells by atomic force microscopy (AFM). Additionally, fluorescence microscopy and confocal laser scanning microscopy (CLSM) analyses of plasma-treated biofilms showed that an inactivation of cells mainly appeared on the bottom side of the biofilms. Thus, the plasma inactivation of the overgrown surface reveals a new possibility to combat biofilms.

Application of Non-Thermal Plasma on Biofilm: A Review

The formation of bacterial biofilm on implanted devices or damaged tissues leads to biomaterial-associated infections often resulting in life-threatening diseases and implant failure. It is a challenging process to eradicate biofilms as they are resistant to antimicrobial treatments. Conventional techniques, such as high heat and chemicals exposure, may not be suitable for biofilm removal in nosocomial settings. These techniques create surface degradation on the treated materials and lead to environmental pollution due to the use of toxic chemicals. A novel technique known as non-thermal plasma has a great potential to decontaminate or sterilize those nosocomial biofilms. This article aims to provide readers with an extensive review of non-thermal plasma and biofilms to facilitate further investigations. A brief introduction summarizes the problem caused by biofilms in hospital settings with current techniques used for biofilm inactivation followed by the literature review strategy. The remainder of the review discusses plasma and its generation, the role played by plasma reactive species, various factors affecting the antimicrobial efficacy of non-thermal plasma and summarizes many studies published in the field.

Biofilm and methods of its eradication

Microorganisms occur in the natural environment in the form of planktonic or create biofilms, i.e. communities of cells surrounded by the extracellular matrix. This is possible due to the phenomenon of quorum sensing, i.e. the ability of microorganisms to estimate their own density and change the expression of genes in response to them. Within such a structure, microorganisms are protected against harmful environmental conditions, their metabolic profile and the level of expression of individual genes are also changed, which leads to an increase in the pathogenicity of organisms associated in the form of biofilms. They pose a huge threat to hospital patients because they are capable of residing abiotic surfaces, such as catheters and endoprostheses, and can cause infection. The current methods of combating microbes with antibiotics and fungicides lose their effectiveness, both due to the increasing drug resistance of clinically relevant strains, but also to the very properties of biofilms. This determines the need to search for new and effective methods (physical, chemical and biological) to eradicate biofilms

Atmospheric pressure non-thermal plasma exposure reduces Pseudomonas aeruginosa lipopolysaccharide toxicity in vitro and in vivo

Lipopolysaccharide (LPS) is an endotoxin composed of a polysaccharide and lipid component. It is intrinsically responsible for the pathogenicity of Gram-negative bacteria and is involved in the development of bacterial sepsis. Atmospheric pressure non-thermal plasma is proposed as a potential new approach for the treatment of infected tissue such as chronic wounds, with both antibacterial and wound-healing activities extensively described. Using both the RAW264.7 murine macrophage cell line in vitro assays and the Galleria mellonella insect in vivo toxicity model, the effect non-thermal plasma exposure on LPS-mediated toxicity has been characterised. Short (60 s) non-thermal plasma exposures of Pseudomonas aeruginosa conditioned growth media, membrane lysates and purified P. aeruginosa LPS, resulted in a substantial detoxification and reduction of LPS-induced cytotoxicity in RAW264.7 murine macrophages. Non-thermal plasma exposure (60 s) of purified P. aeruginosa LPS led to a significant (p < 0.05) improvement in the G. mellonella health index (GHI) score, a measure of in vivo toxicity. These findings demonstrate the ability of short plasma exposures to significantly reduce LPS-induced cytotoxicity both in vitro and in vivo; attenuating the toxicity of this important virulence factor intrinsic to the pathogenicity of Gram-negative bacteria.

In vivo detection of endotracheal tube biofilms in intubated critical care patients using catheter-based optical coherence tomography

The formation of biofilms in the endotracheal tubes (ETTs) of intubated patients on mechanical ventilation is associated with a greater risk of ventilator-associated pneumonia and death. New technologies are needed to detect and monitor ETTs in vivo for the presence of these biofilms. Longitudinal OCT imaging was performed in mechanically ventilated subjects at 24-hour intervals until extubation to detect the formation and temporal changes of in vivo ETT biofilms. OCT-derived attenuation coefficient images were used to differentiate between mucus and biofilm. Extubated ETTs were examined with optical and electron microscopy, and all imaging results were correlated with standard-of-care clinical test reports. OCT and attenuation coefficient images from four subjects were positive for ETT biofilms and were negative for two subjects. The processed and stained extubated ETTs and clinical reports confirmed the presence/absence of biofilms in all subjects. Our findings confirm that OCT can detect and differentiate between biofilm-positive and biofilm-negative groups (P < 10-5 ). OCT image-based features may serve as biomarkers for direct in vivo detection of ETT biofilms and help drive investigation of new management strategies to reduce the incidence of VAP.

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).

Acinetobacter baumannii biofilm biomass mediates tolerance to cold plasma

Acinetobacter baumannii is an intrinsically multidrug‐resistant pathogen that, when existing as a biofilm, confers increased environmental tolerance to desiccation, nutrient starvation as well as increased tolerance to antimicrobials. Outbreaks of A. baumannii infections within the clinical setting are often associated with the biofilm phenotype. This study investigates the role of biofilm biomass in A. baumannii susceptibility to exposure to a kilohertz‐driven, in‐house–designed, cold plasma jet, through the examination of cold plasma treatment efficacy in A. baumannii biofilms grown over various times for up to 72 h. For biofilms grown for 24, 48 and 72 h, D values were 19·32 ± 2·71, 29·18 ± 3·15 and 24·70 ± 3·07 s respectively. Monitoring A. baumannii biofilm biomass over these time periods revealed that the greatest biomass was observed at 48 h with the lowest biofilm biomass at 24 h growth. Enumeration of viable biofilm colony counts at each time point was comparable. Scanning electron microscopy images of plasma‐treated biofilms revealed extensive surface damage of A. baumannii cells. These results describe the role of biomass in mediating A. baumannii biofilm susceptibility to cold plasma treatment, implicating the biofilm matrix as a protective barrier to the antimicrobial effects of cold plasma.

Significance and Impact of the Study

Acinetobacter baumannii biofilm formation results in increased environmental and antimicrobial tolerance and resistance compared to the planktonic phenotype. Cold plasma technology is increasingly investigated as a new tool for decontamination of biofilm‐contaminated surfaces, especially those found in the clinical setting. This new technology presents a promising approach to the remediation of surfaces contaminated by biofilms. This study identifies the role played by A. baumannii biofilm biomass in mediating tolerance and susceptibility to cold plasma treatment. This work demonstrates that increased biofilm biomass reduces the efficacy of antimicrobial species generated by cold plasma, resulting in greater tolerance to plasma exposure.

The antibacterial effect of non-thermal atmospheric pressure plasma treatment of titanium surfaces according to the bacterial wall structure

Titanium is commonly used as a biomaterial for dental implants. In this study, we investigated the antibacterial properties of titanium samples following treatment with a non-thermal atmospheric pressure plasma jet (NTAPPJ) on bacteria with two different cell wall structures, including gram-positive and gram-negative bacteria. The hydrophilicity and surface energy of titanium surfaces were significantly increased after NTAPPJ treatment without altering topographical features. Changes in the chemical composition and reductive potential were observed on the NTAPPJ-treated titanium surfaces. The adhesion and biofilm formation rate of bacteria were significantly reduced on the NTAPPJ-treated titanium surfaces compared with the untreated samples, which was confirmed by fluorescent imaging. Regarding the comparison between gram-positive and gram-negative bacteria, both adhesion and the biofilm formation rate were significantly lower for gram-negative bacteria than gram-positive bacteria on samples treated for longer durations with the NTAPPJ. Transmission electron microscopy imaging showed a comparably more disruptive membrane structure of gram-negative bacteria than gram-positive bacteria on the NTAPPJ-treated surfaces. Our results indicated that the NTAPPJ treatment could be useful for preventing bacterial adhesion and biofilm formation on titanium dental implant surfaces, while the reductive potential on surfaces treated by the NTAPPJ could cause oxidation of bacteria, which could be more sensitive to gram-negative bacteria due to differences in the cell wall structure.

Treatment of Infected Wounds in the Age of Antimicrobial Resistance: Contemporary Alternative Therapeutic Options

As antibiotic resistance increases and antimicrobial options diminish, there is a pressing need to identify and develop new and/or alternative (non-antimicrobial-based) wound therapies. The authors describe the implications of antibiotic resistance on their current wound treatment paradigms and review the most promising non-antibiotic-based antimicrobial agents currently in research and development, with a focus on preclinical and human studies of therapeutic bacteriophages, antimicrobial peptides, cold plasma treatment, photodynamic therapy, honey, silver, and bioelectric dressings.

Cold plasma effect on the proteome of Pseudomonas aeruginosa - Role for bacterioferritin

Cold atmospheric-pressure plasma (CAP) is a relatively new method used for bacterial inactivation. CAP is ionized gas that can be generated by applying an electric current to air or a feeding gas. It contains reactive species and emits UV radiation, which have antibacterial activity. Previous data suggests that CAP is effective in microbial inactivation and can decontaminate and sterilize surfaces, but its exact mode of action is still under debate. This study demonstrates the effect of CAP on the whole proteome of Pseudomonas aeruginosa PAO1 biofilms, which is a dominant pathogen in cystic fibrosis and medical device-related infections. Liquid chromatography-mass spectrometry (LC-MS) was used to identify differentially regulated proteins of whole cell P. aeruginosa extracts. A total of 16 proteins were identified to be affected by plasma treatment compared to the control. Eight of the identified proteins have functions in transcription and translation and their expression changes are likely to be part of a general physiological response instead of a CAP-specific adaptation. However, CAP also affected bacterioferritin (Bfr), Isocitrate dehydrogenase (Idh), Trigger factor (Tig) and a chemotaxis protein, which may be involved in P. aeruginosa’s specific response to CAP. We confirm that bacterioferritin B plays a role in the bacterial response to CAP because ΔbfrB mutants of both PAO1 and PA14 are more susceptible to plasma-induced cell-death than their corresponding wild-type strains. To our knowledge, this is the first study showing the effect of plasma on the whole proteome of a pathogenic microorganism. It will help our understanding of the mode of action of CAP-mediated bacterial inactivation and thus support a safe and effective routine use of CAP in clinical and industrial settings.

Antibacterial activity of Staphylococcus aureus biofilm under combined exposure of glutaraldehyde, near-infrared light, and 405-nm laser

Healthcare-associated infections have increasingly become problematic in the endoscopic procedures resulting in several severe diseases such as carbapenem-resistant Enterobacteriaceae (CRE)-related infections, pneumonia, and bacteremia. Especially, some bacterial strains are resistant to traditional antimicrobials. Therefore, the necessity of developing new antibiotics or management to deal with bacterial infections has been increasing. The current study combined a low concentration of glutaraldehyde (GTA) with near-infrared (NIR) light and 405-nm laser to entail antibacterial activity on Staphylococcus aureus biofilm. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and colony forming unit (CFU) counting were used to quantify the viable cells while fluorescent and scanning electron microscopic images were used to qualitatively evaluate the cell membrane integrity and structural deformation, respectively. Practically, S. aureus biofilm was highly susceptible (7% cell viability and 6.8-log CFU/cm2 bacterial reduction for MTT assay and CFU analysis, respectively) to the combination of GTA (0.1%), NIR light (270 J/cm2), and 405-nm laser (288 J/cm2) exposure. GTA could form either DNA-protein or protein-protein crosslinks to inhibit DNA and protein synthesis. The NIR light induced the thermal damage on protein/enzymes while 405-nm laser could induce reactive oxygen species (ROS) to damage the bacterial membrane. Thus, the proposed technique may be a feasible modality for endoscope cleaning to prevent any secondary infection in the healthcare industry.

Long-lived and short-lived reactive species produced by a cold atmospheric pressure plasma jet for the inactivation of Pseudomonas aeruginosa and Staphylococcus aureus

Different chemical pathways leading to the inactivation of Pseudomonas aeruginosa and Staphylococcus aureus by a cold atmospheric pressure plasma jet (APPJ) in buffered and non-buffered solutions are reported. As APPJs produce a complex mixture of reactive species in solution, a comprehensive set of diagnostics were used to assess the liquid phase chemistry. This includes absorption and electron paramagnetic resonance spectroscopy in addition to a scavenger study to assess the relative importance of the various plasma produced species involved in the inactivation of bacteria. Different modes of inactivation of bacteria were found for the same plasma source depending on the solution and the plasma feed gas. The inactivation of bacteria in saline is due to the production of short-lived species in the case of argon plasma when the plasma touches the liquid. Long-lived species (ClO^-) formed by the abundant amount of O^. radicals produced by the plasmas played a dominant role in the case of Ar + 1% O₂ and Ar + 1% air plasmas when the plasma is not in direct contact with the liquid. Inactivation of bacteria in distilled water was found to be due to the generation of short-lived species: O^. &O₂^.- for Ar + 1% O₂ plasma and O₂^.- (and ^.OH in absence of saline) for Ar plasma.