Influence of growth and treatment temperature on Staphylococcus aureus resistance to pulsed electric fields: Relationship with membrane fluidity

Non-Thermal Treatment Mediated by Curcumin for Enhancing Food Product Quality

Increasing antibiotic resistance is one of the world's greatest health problems, and biocide use in food disinfection, alongside other application fields, could increase antibiotic resistance. Effective and eco-friendly food decontamination treatment with minimal chemical intervention in food production is urgently needed. Synergistic antimicrobial interaction of photoactive compounds and blue-light-emitting diodes have recently been proven effective in agricultural and environmental applications. Curcumin-based non-thermal treatment has been reviewed in this work for the development of a safe and effective decontamination tool that could be adapted to the food industry. The antimicrobial mechanism of the synergistic interaction and the inhibitory efficacy against foodborne pathogens (bacteria in both vegetative form and spore, as well as in biofilms) are discussed. Further studies on curcumin and its derivative, as well as light illumination patterns, were compared for enhanced bactericidal efficacy. Moreover, studies relating to photodynamic inactivation treatment for food sanitation and food quality enhancement (cereal grains and other food products) were summarized, as well as the impact on food organoleptic and nutritional quality.

Inactivation of bacteria using synergistic hydrogen peroxide with split-dose nanosecond pulsed electric field exposures

The use of pulsed electric fields (PEF) as a nonthermal technology for the decontamination of foods is of growing interest. This study aimed to enhance the inactivation of Escherichia coli, Listeria innocua, and Salmonella enterica in Gomori buffer using a combination of nsPEF and hydrogen peroxide (H2O2). Three sub-MIC concentrations (0.1, 0.3, and 0.5%) of H2O2 and various contact times ranging from 5-45 min were tested. PEF exposures as both single (1000 pulse) and split-dose (500+500 pulse) trains were delivered via square-wave, monopolar, 600 ns pulses at 21 kV/cm and 10 Hz. We demonstrate that >5 log CFU/mL reduction can be attained from combination PEF/H2O2 treatments with a 15 min contact time for E. coli (0.1%) and a 30 min contact time for L. innocua and S. enterica (0.5%), despite ineffective results from either individual treatment alone. A 5 log reduction in microbial population is generally the lowest acceptable level in consideration of food safety and represents inactivation of 99.999% of bacteria. Split-dose PEF exposures enhance lethality for several tested conditions, indicating greater susceptibility to PEF after oxidative damage has occurred.

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.

Application of electric field treatment (EFT) for microbial control in water and liquid food

Water disinfection and food pasteurization are critical to reducing waterborne and foodborne diseases, which have been a pressing public health issue globally. Electrified treatment processes are emerging and have become promising alternatives due to the low cost of electricity, independence of chemicals, and low potential to form by-products. Electric field treatment (EFT) is a physical pathogen inactivation approach, which damages cell membrane by irreversible electroporation. EFT has been studied for both water disinfection and food pasteurization. However, no study has systematically connected the two fields with an up-to-date review. In this article, we first provide a comprehensive background of microbial control in water and food, followed by the introduction of EFT. Subsequently, we summarize the recent EFT studies for pathogen inactivation from three aspects, the processing parameters, its efficacy against different pathogens, and the impact of liquid properties on the inactivation performance. We also review the development of novel configurations and materials for EFT devices to address the current challenges of EFT. This review introduces EFT from an engineering perspective and may serve as a bridge to connect the field of environmental engineering and food science.

Pulsed electric field processing as an alternative to sulfites (SO2) for controlling saccharomyces cerevisiae involved in the fermentation of Chardonnay white wine

The use of sulfites (SO₂) for microbial control in the winemaking process is currently being questioned due to its potential toxicity. Pulsed Electric Fields (PEF) are capable of inactivating microorganisms at low temperatures, thus avoiding the negative effects of heat on food properties. In this study, the capacity of PEF technology for the decontamination of yeasts involved in the fermentation process of Chardonnay wine from a winery was evaluated. PEF treatments at 15 kV/cm of low (65 µs, 35 kJ/kg) and higher intensity (177 µs 97 kJ/kg) were selected for evaluating the microbial stability, physicochemical and volatile composition of wine. Even with the least intense PEF-treatment, Chardonnay wine remained yeast-free during 4 months of storage without sulfites. PEF-treatments did not affect the wine's oenological parameters or its aroma during storage. This study, therefore, reveals the potential of PEF technology as an alternative to sulfites for the microbiological stabilization of wine.

Mechanism and potential risk of antibiotic resistant bacteria carrying last resort antibiotic resistance genes under electrochemical treatment

The significant rise in the number of antibiotic resistance genes (ARGs) that resulted from our abuse of antibiotics could do severe harm to public health as well as to the environment. We investigated removal efficiency and removal mechanism of electrochemical (EC) treatment based on 6 different bacteria isolated from hospital wastewater carrying 3 last resort ARGs including NDM-1, mcr-1 and tetX respectively. We found that the removal efficiency of ARGs increased with the increase of both voltage and electrolysis time while the maximum removal efficiency can reach 90%. The optimal treatment voltage and treatment time were 3 V and 120 min, respectively. Temperature, pH and other factors had little influence on the EC treatment process. The mechanism of EC treatment was explored from the macroscopic and microscopic levels by scanning electron microscopy (SEM) and flow cytometry. Our results showed that EC treatment significantly changed the permeability of cell membrane and caused cells successively experience early cell apoptosis, late cell apoptosis and cell necrosis. Moreover, compared with traditional disinfection methods, EC treatment had less potential risks. The conjugative transfer frequencies of cells were significantly reduced after treatment. Less than 1% of bacteria entered the viable but nonculturable (VBNC) state and less than 5% of intracellular ARGs (iARGs) turned into extracellular ARGs (eARGs). Our findings provide new insights into as well as important reference for future electrochemical treatment in removing ARB from hospital wastewater.

Effect of Moderate Intensity Pulsed Electric Field on Shelf-life of Chicken Breast Meat

Moderate intensity pulsed electric field (MIPEF) is a fairly new research topic for microbial inactivation on muscle foods. In this study, indirect application of MIPEF (2.5 kV/cm, 4.67 kV/cm, 7 kV/cm) was evaluated in terms of the growth of some pathogenic bacteria and the quality characteristics of chicken breast meat during cold storage (4°C). Broth cultures of E. coli and C. jejuni showed a remarkable resistance to MIPEF under 4.67 kV/cm and 7 kV/cm, respectively. The limit for total mesophilic aerobic bacteria (TMAB) count was exceeded in 4.67 and 7 kV/cm applied groups 2 days later than control (C). In the end, almost 2 log differences were determined for total coliform bacteria in 4.67 and 7 kV/cm applied groups as compared to C (p<0.05). Inoculated P. aeruginosa count remained the same whereas L. monocytogenes growth was promoted by 4.67 kV/cm (p<0.05). pH value, CIE L*, b*, C* color values of chicken breast fillets were not affected by MIPEF application while ΔE values showed the maximum change in the C group. This study demonstrated that improved shelf-life of chicken breast fillets was provided by moderate intensity DC-pulses in combination with cold storage.

Temperature and presence of ethanol affect accumulation of intracellular trehalose in Lactobacillus plantarum WCFS1 upon pulsed electric field treatment

Pulsed electric field (PEF) treatment can be used to increase intracellular small molecule concentrations in bacteria, which can lead to enhanced robustness of these cells during further processing. In this study we investigated the effects of the PEF treatment temperature and the presence of 8% (v/v) ethanol in the PEF medium on cell survival, membrane fluidity and intracellular trehalose concentrations of Lactobacillus plantarum WCFS1. A moderate PEF treatment temperature of 21 °C resulted in a high cell survival combined with higher intracellular trehalose concentrations compared to a treatment at 10 and 35 °C. Interestingly, highest intracellular trehalose concentrations were observed upon supplementing the PEF medium with 8% ethanol, which resulted in more than a doubling in intracellular trehalose concentrations, while culture survival was retained. Overall, this study shows that treatment temperature and PEF medium optimization are important directions for improving molecule uptake upon PEF processing.

Pseudomonas aeruginosa PA14 Enhances the Efficacy of Norfloxacin against Staphylococcus aureus Newman Biofilms

The thick mucus within the airways of individuals with cystic fibrosis (CF) promotes frequent respiratory infections that are often polymicrobial. Pseudomonas aeruginosa and Staphylococcus aureus are two of the most prevalent pathogens that cause CF pulmonary infections, and both are among the most common etiologic agents of chronic wound infections. Furthermore, the ability of P. aeruginosa and S. aureus to form biofilms promotes the establishment of chronic infections that are often difficult to eradicate using antimicrobial agents. In this study, we found that multiple LasR-regulated exoproducts of P. aeruginosa, including 2-heptyl-4-hydroxyquinoline N-oxide (HQNO), siderophores, phenazines, and rhamnolipids, likely contribute to the ability of P. aeruginosa PA14 to shift S. aureus Newman norfloxacin susceptibility profiles. Here, we observe that exposure to P. aeruginosa exoproducts leads to an increase in intracellular norfloxacin accumulation by S. aureus We previously showed that P. aeruginosa supernatant dissipates the S. aureus membrane potential, and furthermore, depletion of the S. aureus proton motive force recapitulates the effect of the P. aeruginosa PA14 supernatant on shifting norfloxacin sensitivity profiles of biofilm-grown S. aureus Newman. From these results, we hypothesize that exposure to P. aeruginosa PA14 exoproducts leads to increased uptake of the drug and/or an impaired ability of S. aureus Newman to efflux norfloxacin. Surprisingly, the effect observed here of P. aeruginosa PA14 exoproducts on S. aureus Newman susceptibility to norfloxacin seemed to be specific to these strains and this antibiotic. Our results illustrate that microbially derived products can alter the ability of antimicrobial agents to kill bacterial biofilms.IMPORTANCE Pseudomonas aeruginosa and Staphylococcus aureus are frequently coisolated from multiple infection sites, including the lungs of individuals with cystic fibrosis (CF) and nonhealing diabetic foot ulcers. Coinfection with P. aeruginosa and S. aureus has been shown to produce worse outcomes compared to infection with either organism alone. Furthermore, the ability of these pathogens to form biofilms enables them to cause persistent infection and withstand antimicrobial therapy. In this study, we found that P. aeruginosa-secreted products dramatically increase the ability of the antibiotic norfloxacin to kill S. aureus biofilms. Understanding how interspecies interactions alter the antibiotic susceptibility of bacterial biofilms may inform treatment decisions and inspire the development of new therapeutic strategies.

Reversibility of membrane permeabilization upon pulsed electric field treatment in Lactobacillus plantarum WCFS1

Pulsed electric field (PEF) treatment, or electroporation, can be used to load molecules into cells. The permeabilizing effect of the PEF treatment on the cellular membrane can be either reversible or irreversible depending on the severity of the PEF treatment conditions. The influence of PEF on the reversibility of membrane permeabilization in Lactobacillus plantarum WCFS1 by two different fluorescent staining methods was investigated in this study. Whereas staining with propidium iodide (PI) before and after PEF treatment indicated small reversible permeabilized fractions of maximum 14%, the use of a double staining method with PI and SYTOX Green suggested larger reversible permeabilized fractions up to 40% of the population. This difference shows that the choice for a fluorescent staining method affects the conclusions drawn regarding reversibility of membrane permeabilization. Additionally, the effect of PEF treatment conditions on membrane integrity was compared, indicating a relation between critical electric field strength, cell size and membrane permeabilization. Overall this study showed the possibilities and limitations of fluorescent membrane integrity staining methods for PEF studies.

Proteomics-Based Mechanistic Investigation of Escherichia coli Inactivation by Pulsed Electric Field

The pulsed electric field (PEF) technology has been widely applied to inactivate pathogenic bacteria in food products. Though irreversible pore formation and membrane disruption is considered to be the main contributing factor to PEF's sterilizing effects, the exact molecular mechanisms remain poorly understood. In this study, by using mass spectrometry (MS)-based label-free quantitative proteomic analysis, we compared the protein profiles of PEF-treated and untreated Escherichia coli. We identified a total of 175 differentially expressed proteins, including 52 candidates that were only detected in at least two of the three samples in one experiment group but not in the other group. Functional analysis revealed that the differential proteins were primarily involved in the regulation of cell membrane composition and integrity, stress response, as well as various metabolic processes. Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analysis was conducted on the genes of selected differential proteins at varying PEF intensities, which were known to result in different cell killing levels. The qRT-PCR data confirmed that the proteomic results could be reliably used for further data interpretation, and that the changes in the expression levels of the differential candidates were, to a large extent, caused directly by the PEF treatment. The findings of the current study offered valuable insight into PEF-induced cell inactivation.

Rapid determination of the electroporation threshold for bacteria inactivation using a lab-on-a-chip platform

Electroporation based locally enhanced electric field treatment (LEEFT) is an emerging bacteria inactivation technology for drinking water disinfection. Nevertheless, the lethal electroporation threshold (LET) for bacteria has not been studied, partly due to the tedious work required by traditional experimental methods. Here, a lab-on-a-chip device composed of platinum electrodes deposited on a glass substrate is developed for rapid determination of the LET. When voltage pulses are applied, an electric field with a linear strength gradient is generated on a channel between the electrodes. Bacterial cells exposed to the electric field stronger than the LET are inactivated, while others remain intact. After a cell staining process to differentiate dead and live bacterial cells, the LETs are obtained by analyzing the fluorescence microscopy images. Staphylococcus epidermidis has been utilized as a model bacterium in this study. The LETs range from 10 kV/cm to 35 kV/cm under different pulsed electric field conditions, decreasing with the increase of pulse width, effective treatment time, and pulsed electric field frequency. The effects of medium properties on the LET were also investigated. This lab-on-a-chip device and the experimental approach can also be used to determine the LETs for other microorganisms found in drinking water.

Pseudomonas aeruginosa Increases the Sensitivity of Biofilm-Grown Staphylococcus aureus to Membrane-Targeting Antiseptics and Antibiotics

The thick mucus in the airways of cystic fibrosis (CF) patients predisposes them to frequent, polymicrobial respiratory infections. Pseudomonas aeruginosa and Staphylococcus aureus are frequently coisolated from the airways of individuals with CF, as well as from diabetic foot ulcers and other wounds. Both organisms form biofilms, which are notoriously difficult to eradicate and promote chronic infection. In this study, we have shown that P. aeruginosa-secreted factors can increase the efficacy of compounds that alone have little or no bactericidal activity against S. aureus biofilms. In particular, we discovered that P. aeruginosa exoproducts can potentiate the antistaphylococcal activity of phenol-based antiseptics and other membrane-active drugs. Our findings illustrate that polymicrobial interactions can dramatically increase antibacterial efficacy in vitro and suggest that altering membrane physiology promotes the ability of certain drugs to kill bacterial biofilms—knowledge that may provide a path for the discovery of new biofilm-targeting antimicrobial strategies.

Pseudomonas aeruginosa and Staphylococcus aureus often cause chronic, recalcitrant infections in large part due to their ability to form biofilms. The biofilm mode of growth enables these organisms to withstand antibacterial insults that would effectively eliminate their planktonic counterparts. We found that P. aeruginosa supernatant increased the sensitivity of S. aureus biofilms to multiple antimicrobial compounds, including fluoroquinolones and membrane-targeting antibacterial agents, including the antiseptic chloroxylenol. Treatment of S. aureus with the antiseptic chloroxylenol alone did not decrease biofilm cell viability; however, the combination of chloroxylenol and P. aeruginosa supernatant led to a 4-log reduction in S. aureus biofilm viability compared to exposure to chloroxylenol alone. We found that the P. aeruginosa-produced small molecule 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO) is responsible for the observed heightened sensitivity of S. aureus to chloroxylenol. Similarly, HQNO increased the susceptibility of S. aureus biofilms to other compounds, including both traditional and nontraditional antibiotics, which permeabilize bacterial membranes. Genetic and phenotypic studies support a model whereby HQNO causes an increase in S. aureus membrane fluidity, thereby improving the efficacy of membrane-targeting antiseptics and antibiotics. Importantly, our data show that P. aeruginosa exoproducts can enhance the ability of various antimicrobial agents to kill biofilm populations of S. aureus that are typically difficult to eradicate. Finally, our discovery that altering membrane fluidity shifts antimicrobial sensitivity profiles of bacterial biofilms may guide new approaches to target persistent infections, such as those commonly found in respiratory tract infections and in chronic wounds.

Cellular events involved in E. coli cells inactivation by several agents for food preservation: A comparative study

Traditional and novel technologies for food preservation are being investigated to obtain safer products and fulfil consumer demands for less processed foods. These technologies inactivate microorganisms present in foods through their action on different cellular targets, but the final cause of cell loss of viability often remains not well characterized. The main objective of this work was to study and compare cellular events that could play a role on E. coli inactivation upon exposure to treatments with technologies of different nature. E. coli cells were exposed to heat, high hydrostatic pressure (HHP), pulsed electric fields (PEF) and acid treatments, and the occurrence of several alterations, including presence of sublethal injury, membrane permeabilization, increased levels of reactive oxygen species (ROS), DNA damage and protein damage were studied. Results reflected differences among the relevance of the several cellular events depending on the agent applied. Sublethally injured cells appeared after all the treatments. Cells consistently recovered in a higher percentage in non-selective medium, particularly in minimal medium, as compared to selective medium; however this effect was less relevant in PEF-treated cells. Increased levels of ROS were detected inside cells after all the treatments, although their order of appearance and relationship with membrane permeabilization varied depending on the technology. A high degree of membrane permeabilization was observed in PEF treated cells, DNA damage appeared as an important target in acid treatment, and protein damage, in HHP treated cells. Results obtained help to understand the mode of action of food preservation technologies on bacterial cells.

Pulsed electric field effects on inactivation of microorganisms in acid whey

Prospects of pulsed electric field technology application on acid whey concentrate pretreatment were analyzed. Stationary and flow pre-treatment systems were combined with different treatment parameters: electric field strength (E = 39 kV/cm, 95 kV/cm, 92 kV/cm), pulse duration (τ = 60 ns, 90 ns, 1000 ns) and pulse number (pn = up to 100 pulses). Isolates of Saccharomyces sp. and Lactobacillus sp. were predominant in concentrate. Significant non-thermal inactivation effect was achieved after PEF treatment. Exposure to short pulses selectively inactivated yeast cells, as a result PEF technology can be applied for low-energy acid whey processing.

Application of Acid and Cold Stresses to Enhance the Production of Clavulanic Acid by Streptomyces clavuligerus

Clavulanic acid (CA) is frequently prescribed for treatment of bacterial infections. Despite the large number of studies concerning CA production, there is still a need to search for more effective and productive processes because it is mainly produced by biochemical route and is chemically unstable. This paper evaluates the influence of acid and cold stresses on CA production by Streptomyces clavuligerus in bench scale stirred tank bioreactor. Four batch cultures were conducted at constant pH (6.8 or 6.3) and temperature (30, 25, or 20 °C) and five batch cultures were performed with application of acid stress (pH reduction from 6.8 to 6.3), cold stress (reduction from 30 to 20 °C), or both. The highest maximum CA concentration (684.4 mg L^-1) was obtained in the culture conducted at constant temperature of 20 °C. However, the culture under acid stress, in which the pH was reduced from 6.8 to 6.3 at a rate of 0.1 pH unit every 6 h, provided the most promising result, exhibiting a global yield coefficient of CA relative to cell formation (Y_CA/X) of 851.1 mg_CA g_X^-1. High Y_CA/X values indicate that a small number of cells are able to produce a large amount of antibiotic with formation of smaller amounts of side byproducts. This could be especially attractive for decreasing the complexity and cost of the downstream processing, enhancing CA production.