Mechanism Of Bacterial Inactivation Research Articles

Implementing water recirculation in a novel portable plasma-activated water reactor enhances antimicrobial effect against Escherichia coli

The need for sustainable and reliable decontamination methods is driven by concerns regarding antibiotic resistance, as well as environmental and cost-efficiency challenges associated with traditional methods. Plasma-activated water (PAW) holds significant promise as an innovative and eco-friendly decontamination method. Nevertheless, prior to industrial implementation of PAW-based decontamination devices, a deep understanding of the bacterial inactivation mechanisms and its interplay with PAW chemical composition is required. Advancing in this field requires interdisciplinary and collaborative research using standardized practices with cost-effective and shareable PAW reactors that are still missing today. Here, to this end, a portable PAW reactor, featuring a surface dielectric barrier discharge (SDBD) that operates in air, is presented. The gaseous and aqueous phases were chemically characterized by Fourier transform infrared (FTIR) absorption spectroscopy and vis-spectrophotometry, respectively. To assess PAW antimicrobial efficacy Escherichia coli was employed as a model organism on six different PAW samples for three different treatment times. The significance of water recirculation in controlling the chemical composition of PAW, altering the nitrogen species balance in favor of NO2−, is demonstrated. This chemical modification consequently enhanced the antimicrobial effectiveness of PAW, reaching a log reduction of ∼6. Exposing E. coli to equivalent concentrations of RONS to those found in PAW attained similar log-reductions, indicating that acidified nitrites are key to PAW antimicrobial activity. Through an innovative and portable design, this study illustrates the crucial role of water recirculation in tailoring PAW composition, leading to improved decontamination capabilities and establishing a key parameter for further optimization of PAW production.

Copper and Copper Nanoparticles Applications and Their Role against Infections: A Minireview

The focus of this review article is to present a retrospective analysis of copper applications focusing on ions and nanoparticles as broad-spectrum antimicrobials. Copper nanoparticles are presented as an alternative to rising antibiotic resistance. The basic mechanisms of bacterial, fungal, and viral inactivation, which explain their potential, are presented. The green biosynthesis of copper nanoparticles using biomaterials is also presented and considered a very promising trend for future biotechnology and medical applications.

Towards molecular understanding of surface and interface catalytic engineering in TiO2/TiOF2 nanosheets photocatalytic antibacterial under visible light irradiation

TiO2/TiOF2 Z-scheme nanosheets have been successfully synthesized for photocatalytic antibacterial. The antibacterial efficiency of TiO2/TiOF2 against E. coli and S. aureus were 99.90 % and 81.89 % at low material concentration (110 μg/mL), respectively, which are higher than those of pure TiO2, TiOF2, and Degussa P25. In situ molecular spectroscopy results demonstrate that the microstructure of the synthesized material can be reconstructed and optimized to enhance the exposure of the active sites·H2O and O2 are effectively adsorbed on the catalyst surface and activated to form OH…Ti and O…Ti surface active species. Furthermore, the dense interface formed in TiO2/TiOF2 acts as an efficient transport path for photoexcited electrons from TiO2 to TiOF2, and thus accelerates the formation of reactive oxygen species. Finally, the mechanism of bacterial inactivation is systematically discussed considering the main active substances, cell morphological changes, and activity of antioxidant enzymes.

Cold atmospheric plasma (CAP) in wound healing: harnessing a dual-edged sword

Graphical abstract Abstract Chronic wounds take longer to heal and, if left untreated, can result in severe repercussions such as sepsis, gangrene, and amputation. The current treatment procedures followed are wound cleaning and debridement, specialized dressings, antibiotics and antiseptics, hyperbaric oxygen therapy, and vacuum-assisted wound closure. Some of the limitations of these treatment options are multidrug resistance and tissue toxicity. Cold plasma is an emerging technology that has opened a new frontier in biomedical applications and is found to have great utility in wound healing. Cold plasma comprises reactive oxygen and nitrogen species (RONS) that can be targeted against bacterial inactivation and improve wound healing. The amount of RONS produced can be controlled by several parameters such as gas composition, flow rate, power, frequency, voltage, distance, and exposure time. The reactive species causes damage to the cell membrane as well as the intracellular components which ultimately lead to bacterial cell death. It can also accelerate wound healing by activating neutrophils, macrophages, endothelial cells, keratinocytes, and fibroblasts. These help in maintaining tissue oxygenation, initiating angiogenesis, collagen synthesis which aids in rapid wound closure. In this review, we summarize the various characteristics of cold plasma that can be optimized to produce an effective antimicrobial effect. The different mechanisms of bacterial inactivation and the stimulation of wound healing processes by the reactive species are discussed. Furthermore, numerous pieces of evidence from in vitro and in vivo experiments and clinical trials that prove that cold plasma is an effective approach are presented.

Molecular design of ultrafiltration membranes with antibacterial properties for the inactivation of antibiotic-resistant bacteria

Ultrafiltration membranes have gained significant prominence in water and wastewater treatment, holding potential for application in combating antibiotic resistance. Herein, we successfully fabricated a polyvinylidene fluoride copolymer-based ultrafiltration membrane capable of effectively removing antibiotic-resistant bacteria (ARB) from water and wastewater through the molecular design approach, incorporating polyionic liquid (PIL) via atom transfer radical polymerization. The as-prepared membrane exhibited bactericidal properties against wild-type bacteria, ARB and opportunistic pathogens, showcasing an inactivation efficiency exceeding 70 %. These properties were attributed to the increased intracellular accumulation of reactive oxygen species and enhanced membrane permeability, suggesting the involvement of oxidative stress and disruption of the cell outer membrane in bacterial cell lysis. To investigate the interaction with bacterial cells, we utilized liposome vesicles as a model, revealing that the PIL brush effectively disrupted the integrity of the bacterial cell's phospholipid bilayer through its alkyl chain. Furthermore, zeta potential measurements indicated that the role of electrostatic interactions between the imidazole ring and bacteria in bacterial inactivation mechanisms. During wastewater filtration, the PIL-M membrane demonstrated outstanding efficiency in total bacteria removal, exceeding 99.1 %. Additionally, the PIL-M membrane displayed significant bacterial inactivation capabilities against total bacteria as well as sulfamethoxazole-resistant bacteria, chloramphenicol-resistant bacteria, and tetracycline-resi8stant bacteria, achieving respective inactivation efficiencies of 91 %, 93 %, 87 %, and 85 %. These results highlight the membrane's potential in mitigating membrane fouling and inactivating antibiotic-resistant bacteria. Overall, the findings of this study suggest that the PIL-M membrane can effectively contribute to controlling bacterial resistance in water and wastewater environments.

Magnetic carbon nanotubes/red phosphorus/graphitic carbon nitride heterojunction for highly efficient visible-light photocatalytic water disinfection

The development of recyclable antimicrobial catalysts with high photocatalytic disinfection efficiency is urgently needed. To this end, a ternary heterojunction of magnetic carbon nanotubes/red phosphorus/graphitic carbon nitride (MCNTs/RP/CN) is designed and applied as a recyclable photocatalytic disinfectant for the first time. Significantly, the MCNTs/RP/CN outperforms the most-studied P25 TiO2 under both visible-light and real-sunlight illumination, and its disinfection efficiency ranks as one of the best among various CN-based catalysts developed previously. The bacterial inactivation processes, such as the membrane damage at metabolic and structural levels, the leakage of intracellular components, the entrance of extracellular reactive oxygen species (ROS) to cytoplasm, and the decomposition of genomic DNA, are thoroughly investigated. The characterization and theoretical calculation results corroborate the interfacial interaction between the three moieties in the heterojunction, and thus efficient charge transfer occurs, thereby enhancing the photocatalytic ROS generation amount for efficient water disinfection. The bacterial inactivation mechanism is investigated in a scavenger study, •O2− and H2O2 are identified as the major ROS for bacterial inactivation. In addition, the MCNTs/RP/CN shows applicability in the authentic water matrices, toward a variety of pathogenic microorganisms (Gram-positive and Gram-negative bacteria, antibiotic-resistant bacteria, and oocysts of C. parvum), and in the large-scale demonstration application. The results of this work suggest that MCNTs/RP/CN can serve as a high-performance and recyclable photocatalyst for real-world photocatalytic disinfection operations.

Genotypic and phenotypic characterization of a Salmonella Typhimurium strain resistant to pulsed electric fields

Pulsed Electric Fields (PEF) technology is regarded as one of the most interesting alternatives to current food preservation methods, due to its capability to inactivate vegetative microorganisms while leaving the product's organoleptic and nutritional properties mostly unchanged. However, many aspects regarding the mechanisms of bacterial inactivation by PEF are still not fully understood. The aim of this study was to obtain further insight into the mechanisms responsible for the increased resistance to PEF of a Salmonella Typhimurium SL1344 variant (SL1344-RS, Sagarzazu et al., 2013), and to quantify the impact that the acquisition of PEF resistance has on other aspects of S. enterica physiology, such as growth fitness, biofilm formation ability, virulence and antibiotic resistance. WGS, RNAseq and qRT-PCR assays indicated that the increased PEF resistance of the SL1344-RS variant is due to a higher RpoS activity caused by a mutation in the hnr gene. This increased RpoS activity also results in higher resistance to multiple stresses (acidic, osmotic, oxidative, ethanol and UV-C, but not to heat and HHP), decreased growth rate in M9-Gluconate (but not in TSB-YE or LB-DPY), increased ability to adhere to Caco-2 cells (but no significant change in invasiveness) and enhanced antibiotic resistance (to six out of eight agents). This study significantly contributes to the understanding of the mechanisms of the development of stress resistance in Salmonellae and underscores the crucial role played by RpoS in this process. Further studies are needed to determine whether this PEF-resistant variant would represent a higher, equal or lower associated hazard than the parental strain.

In-Package Atmospheric Cold Plasma Treatment and Storage Effects on Membrane Integrity, Oxidative Stress, and Esterase Activity of Listeria monocytogenes.

Atmospheric cold plasma (ACP) treatment can reduce bacterial pathogens in foods. Additional reduction in bacterial cells during storage after ACP treatment was previously reported. The underlying mechanisms of bacterial inactivation during ACP treatment and post-treatment storage need to be understood. This study investigated the changes in the morpho-physiological status of Listeria monocytogenes on ham surfaces after post-ACP-treatment storage of 1 h, 24 h, and 7 days at 4 °C. The membrane integrity, intracellular oxidative stress, and esterase activity of L. monocytogenes were evaluated by flow cytometry. L. monocytogenes cells were under high oxidative stress conditions with slightly permeabilized membranes after 1 h of post-ACP-treatment storage according to the flow cytometry data. During the extended storage of 24 h, the percentage of cells with a slightly permeabilized membrane increased; subsequently, the percentage of cells with intact membranes decreased. The percentage of L. monocytogenes cells with intact membranes decreased to <5% with a treatment time of 10 min and after 7 days of post-treatment storage. In addition, the percentage of L. monocytogenes cells under oxidation stress decreased to <1%, whereas the percentage of cells with completely permeabilized membranes increased to more than 90% for samples treated with ACP for 10 min and 7 days of post-treatment storage. With increased ACP treatment time, for 1 h stored samples, the percentage of cells with active esterase and slightly permeabilized membranes increased. However, during the extended post-treatment storage of 7 days, the percentage of cells with active esterase and slightly permeabilized membranes decreased to below 1%. At the same time, the percentage of cells with permeabilized membrane increased to more than 92% with an increase in ACP treatment time of 10 min. In conclusion, the higher inactivation after 24 h and 7 days post-ACP-treatment storage compared to 1 h stored samples correlated with the loss of esterase activity and membrane integrity of L. monocytogenes cells.

Evaluating the effects of plasma-activated slightly acidic electrolyzed water on bacterial inactivation and quality attributes of Atlantic salmon fillets

In this study, an intervention non-thermal processing technology plasma-activated slightly acidic electrolyzed water (PASW) was developed to better preserve salmon fillets. Compared to the plasma-activated water (PAW) and slightly acidic electrolyzed water (SAEW), PASW treatment was found to be more effective in inactivating microorganisms. After PAW, SAEW, or PASW treatment for 120 s, the population of Shewanella putrefaciens (S. putrefaciens) was reduced by 2.04, 2.62 and 2.08 Log CFU/mL (P < 0.05) using plate counts, respectively. The test for the leakage of nucleic acids and protein in intracellular contents confirmed that PASW caused serious damage to the microbial cell structural integrity compared to that alone PAW or SAEW. Meanwhile, scanning electron microscopic observations also showed that PASW caused apparent bacterial structural changes. Besides, the PASW treatment did not alter color and textural properties of salmon fillets, and restrained lipid oxidation as compared to the control and SAEW treatments. In all, this study compared the bacterial inactivation mechanisms for PAW, SAEW, and PASW, and suggested that PASW was effective in inactivating S. putrefaciens of salmon fillets. Industrial relevancePlasma-activated slightly acidic electrolyzed water, an emerging technology in food processing, can be a potential green technology for the processing of aquatic food products. PASW treatment had higher disinfection efficacy than that of SAEW or PAW treatment alone, and no adverse effect on the quality of Atlantic salmon fillets. These results boost knowledge in the food preservation field, as well as the application of non-thermal processing in the food industry.

Treatment of wastewater for reuse using advanced oxidation process: a bacterial inactivation mechanism approach

Treatment of wastewater for reuse using advanced oxidation process: a bacterial inactivation mechanism approach

Efficient wastewater disinfection by raised 1O2 yield through enhanced electron transfer and intersystem crossing via photocatalysis of peroxymonosulfate with CuS quantum dots modified MIL-101(Fe)

Peroxymonosulfate (PMS)-based photocatalysis is a promising alternative approach for wastewater disinfection. Singlet oxygen (1O2) is sensitive and efficient for bacterial inactivation. This study developed a 1O2-predominated PMS disinfection technique under visible light with CuS quantum dots (QDs) modified MIL-101(Fe) (CSQDs@MF). CuS QDs modification greatly enhanced the 1O2 quantum yield by 80% than that of MIL-101(Fe). Photoelectricity and photoluminescence tests demonstrated that both the enhanced electron transfer and energy transfer were responsible for improved 1O2 generation in Vis/PMS/CSQDs@MF system. The system took 60 min to inactivate 7.5-log E. coli, and it could be applied in a broad pH and dissolve oxygen range. Bacterial inactivation mechanism suggested that 1O2 attacked cell membrane first, then induced oxidative stress, up-regulated intracellular ROS level, eventually broke DNA strand. The system showed good disinfection performance on Gram-positive B. subtilis and fecal coliforms in practical wastewater, implying it is a promising alternative disinfection technology for wastewater treatment.

Antimicrobial properties dependence on the composition and architecture of copper-alumina coatings prepared by plasma electrolytic oxidation (PEO)

This study presents environmentally friendly and low-cost synthetic routes to produce antimicrobial coatings over 5052 Al alloy based on plasma electrolytic oxidation (PEO) technology. Two methodologies were explored: the decoration with copper and anodic doping with copper ions. The porous oxide layers produced in silicate media presented two porous layers consisting of γ-Al2O3 crystalline phase and amorphous phases of aluminosilicate, silica, and Al(OH)3. Small amounts of copper (<0.3 at.%) were detected in the PEO films. In the Cu-decorated film, copper clusters composed of Cu0 and Cu2+ species were observed visually as small black dots on the surface. In the Cu-doped film, the Cu2+ and Cu+ species were homogeneously distributed on the surface. The copper content affected the corrosion performance in aggressive corrosive media. The PEO coatings showed a remarkable antimicrobial activity after 24 h in standard tests. The antimicrobial effectiveness of the Cu-decorated sample was higher against S. aureus, while the Cu-doped sample was more effective against E. coli. The results demonstrated that differences in the PEO coating architecture can affect the material composition and, consequently, the bacterial inactivation mechanism. These findings can serve as a guide to tailor aluminum alloys for specific antimicrobial surfaces.

Antimicrobial photodynamic inactivation of wastewater microorganisms by halogenated indole derivative capped zinc oxide

Novel 5-bromoindole (5B)-capped zinc oxide (ZnO) nanoparticles (5BZN) were synthesized to improve the antibacterial, antibiofilm, and disinfection processes for the control of microorganisms in wastewater treatment. When exposed to 5BZN, the biofilm density and cell attachment were reduced dramatically, as measured by scanning electron microscopy (SEM). The 5BZN were also investigated for photodynamic treatment of multidrug-resistant (MDR) bacteria and toxicity. The combination of 5B and ZnO exhibited strong antibacterial and antibiofilm activities against MDR bacteria even at low doses (20 μg/mL). After 12.5 mW/cm2 blue LED irradiation, the composite 5BZN showed superior photodynamic inactivation of two wastewater MDR, Enterobacter tabaci E2 and Klebsiella quasipneumoniae SC3, with cell densities reduced by 3.9 log CFU/mL and 4.7 log CFU/mL, respectively, after 120 min. The mechanism of bacterial inactivation was studied using a scavenging investigation, and H2O2 was identified mainly as the reactive species for bacterial inactivation. The 5BZN exhibited higher photodynamic inactivation towards the total coliform bacteria in wastewater effluents under a blue LED light intensity of 12.5 mW/cm2 with almost complete inactivation of the coliform bacteria cells within 40 min. Furthermore, when 5BZN (100 mg/L) was added to the reactor, the level of tetracycline antibiotic degradation was increased by 63.6% after 120 min. The toxicity test, animal model nematode studies and seed germination assays, showed that 5BZN is harmless, highlighting its tremendous potential as a self-healing agent in large-scale photodynamic disinfection processes.

Surface-enhanced Raman spectroscopy enabled evaluation of bacterial inactivation

An improved understanding of bacterial inactivation mechanisms will provide useful insights for infectious disease control and prevention. We evaluated bacterial response to several inactivation methods using surface-enhanced Raman spectroscopy (SERS). The results indicate that changes in the SERS signal are highly related to cellular disruption and that cellular changes arising after cell inactivation cannot be ignored. The membrane integrity of heat and the combination of UV254 and free chlorine (UV254/chlorine) treated Pseudomonas syringae (P. syringae) cells were severely disrupted, leading to significantly increased peak intensities. Conversely, ethanol treated bacteria exhibited intact cell morphologies and the SERS spectra remained virtually unchanged. On the basis of time dependent SERS signals, we extracted dominant SERS patterns. Peaks related to nucleic acids accounted for the main changes observed during heat, UV254, and UV254/chlorine treatment, likely due to their outward diffusion from the cell cytoplasm. For free chlorine treated P. syringae, carbohydrates and proteins on the cell membrane were denatured or lost, resulting in a decrease in related peak intensities. The nucleobases were likely oxidized when treated with UV254 and chlorine, thus leading to shifts in the related peaks. The generality of the method was verified using two additional bacterial strains: Escherichia coli and Bacillus subtilis as well as in different water matrices. The results suggest that SERS spectral analysis is a promising means to examine bacterial stress response at the molecular level and has applicability in diverse environmental implementations.

Inactivation efficacy of combination treatment of blue light-emitting diodes (LEDs) and riboflavin to control E. coli O157:H7 and S. Typhimurium in apple juice

In the present study, we assessed the bactericidal effect of a combination of blue light (BL) and riboflavin (Rb) and elucidated the bacterial inactivation mechanism. Higher Rb concentrations (0.005–50 μM) resulted in greater inactivation of Escherichia coli O157:H7 and Salmonella Typhimurium suspended in phosphate buffer saline (PBS). In the present dose-dependent study, significant inactivation was observed following BL + Rb treatment at 30 J/cm2 (19 min 23 s) in PBS. In apple juice (pH 3.2), BL + Rb treatment resulted in a 3–4-log reduction in the pathogens at 30 J/cm2. As expected, ROS was the key factor for pathogen inactivation. Moreover, cell membrane damage was observed on propidium iodide (PI) uptake analysis and transmission electron microscopy (TEM). These results indicated that BL + Rb treatment can be used as a powerful intervention system to ensure microbial safety in liquid beverages, in addition to enhancing the nutritional value.

Bacterial inactivation mechanism of SC-CD and TEO combinations in watermelon and melon juices

Inactivation mechanism of supercritical carbon dioxide (SC-CD) combination with Thymbra spicata essential oil (TEO) treatments on bacteria in tryptone soy broth (TSB) and juices was investigated. Salmonella Typhimurium was significantly inactivated by 3.45 and 5.55 logs after SC-CD+TEO treatment of TBS for 35 min at 7.6 and 10.13 MPa respectively. Klebsiella pneumoniae showed higher resistance to treatments than other two bacteria and Escherichia coli showed higher resistance to treatments than S. Typhimurium. Approximately 5.27, 4.68 and 3.82 logs inactivation of S. Typhimurium, E. coli and K. pneumoniae were obtained in the treatment of watermelon juice with SC-CD+TEO at 10.13 MPa after 35 min, while 6.89, 4.82 and 4.55 logs inactivated, respectively, were observed in melon juice. Research indicated that the inactivation mechanism involves inducing cell damage, increasing membrane permeability, increasing cell rupture, precipitation of cytoplasmic contents, extracting cellular substances and lysing cells. Different sensitivities of Gram-negative bacteria to the treatments may be attributed to the differences in the cell structure, compositions of membrane and presence of capsule, pressure level and exposure time. pH, total soluble solid and browning degree of juice did not significantly change after treatments and during 7 days of storage.

Electro catalytic generation of reactive species at diamond electrodes and applications in microbial inactivation

Use of robust and safe water disinfection technologies which are inexpensive and energy-efficient are need of the hour to combat the problem of inadequate access of safe and clean drinking water. Energy and chemically intensive water treatment technologies warrant the need for a safe and environmentally sound treatment technology. Electrochemical disinfection or electrodisinfection (ED) is experiencing a great resurgence among the scientific communities owing to its novel use of electrode materials and electric current in an inexpensive and energy-efficient way for achieving the inactivation of microorganisms. Among the various electrodes used in the ED, boron-doped diamonds emerge as a sustainable alternate for their ability to electro generate strong potent oxidants which result in effective pathogen control in drinking water. ED for disinfecting waters occurs via generation of the reactive species which act in the bacterial inactivation mechanisms. In this mini-review, a critical discussion on the fundamentals and applications of promising electrochemical methods using boron-doped diamond anodes (namely electrochemical oxidation), evidencing their advantages for the remediation of drinking water infected with waterborne agents, is given.

The Pathway towards Photoelectrocatalytic Water Disinfection: Review and Prospects of a Powerful Sustainable Tool

Photoelectrocatalysis is a hybrid photon/electron-driven process that benefits from the synergistic effects of both processes to enhance and stabilize the generation of disinfecting oxidants. Photoelectrocatalysis is an easy to operate technology that can be scaled-up or scaled-down for various water treatment applications as low-cost decentralized systems. This review article describes the fundamentals of photoelectrocatalysis, applied to water disinfection to ensure access to clean water for all as a sustainable development goal. Advances in reactor engineering design that integrate light-delivery and electrochemical system requirements are presented, with a description of photo-electrode material advances, including doping, nano-decoration, and nanostructure control. Disinfection and cell inactivation are described using different model microorganisms such as E. coli, Mycobacteria, Legionella, etc., as well the fungus Candida parapsilosis, with relevant figures of merit. The key advances in the elucidation of bacterial inactivation mechanisms by photoelectrocatalytic treatments are presented and knowledge gaps identified. Finally, prospects and further research needs are outlined, to define the pathway towards the future of photoelectrocatalytic disinfection technologies.

H2O2 production and in situ sterilization over a ZnO/g-C3N4 heterojunction photocatalyst

In this work, a novel ZnO/g-C3N4 heterojunction catalyst was prepared by a thermal polycondensation process and has been applied for H2O2 production and in situ sterilization in natural waters irradiated by simulated sunlight. The H2O2 production efficiency of this process was measured, and its in situ sterilization ability in natural waters was evaluated. In addition, the remaining sterilization ability of the process was also assessed. The ZnO/g-C3N4 heterojunction catalyst with 10 wt% ZnO exhibited the best performance in H2O2 production of 5312.45 μmol·L−1 after 8 h, which was nearly 2.65 times greater than that of pure g-C3N4. Moreover, the bactericidal rate reached 97.4% on 10% ZnO/g-C3N4 after 60 min, while 47.3% on pure g-C3N4. H2O2 and sunlight alone had limited bactericidal effects, only 10.3% and 27.6%. It was proven that the H2O2 generated in situ achieved high-efficiency bacterial inactivation through the effect of sunlight excitation. ·OH and ·O2– were verified to play main roles in the inactivation of bacteria, which contributed to about 44.2% and 40.4% respectively. In addition, the mold was completely inactivated after 20 min and had similar mechanism of bacterial inactivation, showing good applicability of this process. This work may provide new ideas for realizing the in situ disinfection using sunlight as driving force and cleaning treatment of natural water bodies.

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

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