Ultrasound-involved emerging strategies for controlling foodborne microbial biofilms

Optimization of calcium chloride and ultrasonication pre-treatment to mitigate the microbial load on fresh carrots using response surface methodology

The quality and safety of fruits and vegetables are the major concerns of the food industry. The quality of these foods depends on their safety during storage and processing. The current research was designed to improve the quality by optimizing the safety parameters to minimize the microbial load of carrots during processing and storage. Three process variables including ultrasonication time, ultrasonication temperature, and calcium chloride concentration were optimized, each at five levels, to find the minimum optimal response of microbial load in terms of total plate count (TPC), total coliform count (TCC) and yeast mold count (YMC) using response surface central composite design. The selected factors showed statistically significant (p < 0.05) linear negative effect on TPC and linear positive effect on YMC. The total plate count, total coliform count, and yeast mold count were reduced to 6.117, 5.689, and 6.021 log CFU/mL respectively, under the optimum conditions i.e., ultrasonication time (17.97 min), temperature (43.885 °C), and CaCl2 concentration (181.211 mg/L). The non-significant interaction or quadratic effect of selected factors was observed on any of the studied responses. Concludingly, the reduction in studied parameters can improve the safety measurements of foods.

Role of advanced cleaning and sanitation techniques in biofilm prevention on dairy equipment

Biofilm formation on dairy equipment is a persistent challenge in the dairy industry, contributing to product contamination, equipment inefficiency, and economic losses. Traditional methods such as manual cleaning and basic chemical sanitation are discussed as foundational approaches, followed by an in-depth investigation of cutting-edge technologies, including clean-in-place systems, high-pressure cleaning, foam cleaning, ultrasonic and electrochemical cleaning, dry ice blasting, robotics, nanotechnology-based agents, enzymatic cleaners, and oxidizing agents. Enhanced sanitation techniques, such as dry steam, pulsed light, acidic and alkaline electrolyzed water, hydrogen peroxide vapor, microbubble technology, and biodegradable biocides, are highlighted for their potential to achieve superior sanitation while promoting sustainability. The effectiveness, feasibility, and limitations of these methods are evaluated, emphasizing their role in maintaining dairy equipment hygiene and reducing biofilm-associated risks. Additionally, challenges, such as equipment compatibility, cost, and regulatory compliance, are addressed, along with insights into future directions and innovations, including automation, smart cleaning systems, and green cleaning solutions. This review provides a comprehensive resource for researchers, industry professionals, and policymakers aiming to tackle biofilm formation in dairy production systems and enhance food safety, operational efficiency, and sustainability.

Recent Advances in Biofilm Control Technologies for the Food Industry

Biofilms remain a major challenge in the food industry due to the increased resistance of foodborne pathogens to antimicrobial agents and food processing stresses, leading to food contamination and significant health risks. Their resistance to preservation techniques, antimicrobial treatments, and processing conditions increases concerns regarding food safety. This review discusses recent developments in physical, chemical, and surface modification strategies to control and remove biofilms in food processing environments. Physical methods, such as thermal treatments, electric fields, and ultrasonic systems, have demonstrated their efficacy in disrupting biofilm structure and improving disinfection processes. Chemical treatments, including the use of sanitizers, disinfectants, acidulants, and enzymes, provide targeted approaches to degrade biofilm matrices and inhibit bacterial adhesion. Furthermore, surface modifications of food contact materials provide innovative solutions for preventing biofilm formation and enhancing food safety. These cutting-edge strategies not only improve food safety but also reduce contamination risk in food processing facilities. The review highlights the mechanisms, efficacy, and applicability of these techniques, emphasizing their potential to mitigate biofilm-associated risks and ensure food quality and safety.

Rapid Detection of Salmonella Typhimurium During Cell Attachment on Three Food-Contact Surfaces Using Long-Read Sequencing

Salmonella spp. are pathogenic microorganisms linked to foodborne outbreaks associated with eggs and egg products. Salmonella can resist sanitation of egg processing equipment and form biofilms on food-contact surfaces. A major challenge for controlling Salmonella is the ability to detect the cells during the early stages of attachment to indicate that interventions are needed to sanitize the surface. This research investigated the use of long-read sequencing to identify Salmonella during the early stages (0-5 h) of cell attachment to three common food-contact surfaces-stainless steel, silicone, and nylon-and compared it with traditional microbiological methods. Results of the conventional plate counts showed that the detection of Salmonella began after three hours of incubation, with less than 1 log CFU/cm2 of growth. Silicone had the highest number of Salmonella attached (0.87 log CFU/cm2), followed by stainless steel (0.70 log CFU/cm2). Long-read whole genome sequencing identified attached Salmonella on stainless steel, silicone, and nylon after only one hour of incubation. The results of this study suggest that long-read sequencing could be a very useful method for detecting Salmonella at low concentrations in the processing environment during the early stages of cell attachment to food-contact surfaces, allowing for correct sanitation intervention.

Improve anti-biofilm efficacy of ultrasound by modulating the phase transition of exopolysaccharides

This study focused on the adverse sonochemical effect of ultrasound on biofilm extracellular polysaccharide and the adaptive biofilm responses for ultrasound resistance. Results showed ultrasound triggered phase transition of polysaccharides within biofilm from solation to gelation, which induced following biofilm viscoelasticity enhancement, consequential failure of biofilm removal and bacteria killing. Introducing additional cationic polysaccharide, 1.25 % chitosan, inhibited the ultrasound responsive polysaccharides gelation and biofilm viscoelasticity enhancement, exerted synergistic antibacterial (97.40 %) and antibiofilm (96.38 %) effects with 120 W ultrasound combined on S. aureus biofilm, prolonged the preservation time of milk 2.45 times longer compared with ultrasound alone. These findings indicated the possible mechanism and solution to improve ultrasound efficacy on biofilm control and bacteria suppression, exhibit the promising prospect of ultrasound combined strategy in hygiene issues of food and medical industry.

Biofilm battleground: Unveiling the hidden challenges, current approaches and future perspectives in combating biofilm associated bacterial infections

A biofilm is a complex aggregation of microorganisms, either of the same or different species, that adhere to a surface and are encased in an extracellular polymeric substances (EPS) matrix. Quorum sensing (QS) and biofilm formation are closely linked, as QS genes regulate the development, maturation, and breakdown of biofilms. Inhibiting QS can be utilized as an effective approach to combat the impacts of biofilm infection. The impact of biofilms includes chronic infections, industrial biofouling, infrastructure corrosion, and environmental contamination as well. Therefore, a deep understanding of biofilms is crucial for enhancing public health, advancing industrial processes, safeguarding the environment, and deepening our knowledge of microbial life as well. This review aims to offer a comprehensive examination of challenges posed by bacterial biofilms, contemporary approaches and strategies for effectively eliminating biofilms, including the inhibition of quorum sensing pathways, while also focusing on emerging technologies and techniques for biofilm treatment. In addition, future research is projected to target the challenges associated with the bacterial biofilms, striving to develop new approaches and improve existing strategies for their effective control and eradication.

Hidden Places for Foodborne Bacterial Pathogens and Novel Approaches to Control Biofilms in the Meat Industry

Biofilms are of great concern for the meat industry because, despite the implementation of control plans, they remain important hotspots of contamination by foodborne pathogens, highlighting the need to better understand the ecology of these microecosystems. The objective of this paper was to critically survey the recent scientific literature on microbial biofilms of importance for meat safety and quality, also pointing out the most promising methods to combat them. For this, the databases PubMed, Scopus, Science Direct, Web of Science, and Google Scholar were surveyed in a 10-year time frame (but preferably papers less than 5 years old) using selected keywords relevant for the microbiology of meats, especially considering bacteria that are tolerant to cleaning and sanitization processes. The literature findings showed that massive DNA sequencing has deeply impacted the knowledge on the species that co-habit biofilms with important foodborne pathogens (Listeria monocytogenes, Salmonella, pathogenic Escherichia coli, and Staphylococcus aureus). It is likely that recalcitrant commensal and/or spoilage microbiota somehow protect the more fastidious organisms from harsh conditions, in addition to harboring antimicrobial resistance genes. Among the members of background microbiota, Pseudomonas, Acinetobacter, and Enterobacteriales have been commonly found on food contact and non-food contact surfaces in meat processing plants, in addition to less common genera, such as Psychrobacter, Enhydrobacter, Brevundimonas, and Rothia, among others. It has been hypothesized that these rare taxa may represent a primary layer in microbial biofilms, offering better conditions for the adhesion of otherwise poor biofilm formers, especially considering their tolerance to cold conditions and sanitizers. Taking into consideration these findings, it is not only important to target the foodborne pathogens per se in cleaning and disinfection plans but the use of multiple hurdles is also recommended to dismantle the recalcitrant structures of biofilms. In this sense, the last part of this manuscript presents an updated overview of the antibiofilm methods available, with an emphasis on eco-friendly approaches.

Predictive Modeling for Inactivation of Escherichia coli Biofilm with Combined Treatment of Thermosonication and Organic Acid on Polystyrene Surface

The present study aimed to evaluate the antibiofilm effect of combined sonication treatment with organic acids on polystyrene surfaces and to develop a predictive model for the inactivation of Escherichia coli biofilms. Polystyrene plates containing E. coli biofilms were subjected to sonication using different inactivation solutions (PBS, lactic acid, and acetic acid) at varying temperatures (20 °C, 40 °C, and 50 °C) and durations (2 and 5 min). The effects of temperature, treatment duration, and inactivation solution on E. coli biofilm removal were statistically significant (p < 0.05). The use of organic acids, along with increased treatment time and temperature, led to a significant reduction in viable cell counts (0.43-6.21 log CFU/mL) and optical density (0.13-0.72 at OD600) of E. coli biofilms (p < 0.05). The highest E. coli biofilm inactivation, with a reduction of 6.21 CFU/mL and 0.72 OD, was achieved by combining organic acid and thermosonication at 50 °C for 5 min. A significant positive correlation was observed between test methods based on viable cell count and optical density (OD) measurements. According to multiple linear regression analysis results, the R2 values of the predictive models for biofilm inactivation, based on viable cell count and OD measurements, were 0.84 and 0.80, respectively. Due to its higher accuracy, the predictive model developed using viable cell count data is recommended for applications in the food industry and processing sectors.

Stimuli-Responsive NO Delivery Platforms for Bacterial Infection Treatment

The prevalence of drug-resistant bacterial infections has emerged as a grave threat to clinical treatment and global human health, presenting one of the foremost challenges in medical care. Thus, there is an urgent imperative to develop safe and efficacious novel antimicrobial strategies. Nitric oxide (NO) is a recognized endogenous signaling molecule, which plays a pivotal role in numerous pathological processes. Currently, NO has garnered significant interest as an antibacterial agent due to its capability to eradicate bacteria, disrupt biofilms, and facilitate wound healing, all while circumventing the emergence of drug resistance. However, the inherently unstable characteristic of NO therapeutic gas renders the controlled administration of NO gases exceedingly challenging. Hence, in this review, the current challenge of bacterial infection is discussed; then it is briefly elucidated the antibacterial mechanism of NO and comprehensively delineate the recent advancements in stimulus-responsive NO delivery platforms, along with their merits, obstacles, and prospective avenues for clinical application. This review offers guidance for future advancements in NO-medicated anti-infection therapy is hoped.

Effects of low frequency ultrasound treatment on dissolved organic nitrogen removal by biological activated carbon: Critical insights into molecular characteristics, microbial traits, and metabolism

Drinking water treatment plants (DWTPs) in China that pioneered the biological activated carbon (BAC) process have reached 10 years of operation. There has been a renewed focus on biofiltration and the performance of old BAC filters for dissolved organic nitrogen (DON) has been poor, requiring replacement and regeneration of the BAC. Therefore, it is necessary to explore a cost-effective way to improve the water quality of the old BAC filters. To address this, low frequency ultrasound is proposed to enhance DON removal efficiency by BAC. In this study, bench and pilot tests were conducted to investigate the effect of low frequency ultrasound on DON removal by 10-year BAC. The results indicated that low frequency ultrasound significantly improved the DON removal rate increased from 15.83 % to 85.87 % and considerably inhibited the nitrogenous disinfection by-products (N-DBPs) formation potential, which was attributed to a decrease in the production of lipid-like, carbohydrate-like, and protein/amino sugar-like DON. The biomass on the BAC was significantly reduced after ultrasound treatment, and it decreased from 349.56∼388.98 nmol P/gBAC to 310.12∼377.63 nmol P/gBAC, enabling the biofilm thickness to decrease and the surface to become sparse and porous, which was conducive to oxygen and nutrients transfer. The Rhizobials associated with microbe-derived DON were stripped away during ultrasound treatment, which reduced microbe-derived DON associated with amino acids. Additionally, ultrasound regulated metabolic pathways, including amino acids, tricarboxylic acid (TCA) cycle, and nucleotide metabolism, to improve the osmotic pressure of the biofilm. In short, low frequency ultrasound treatment can enhance BAC biological properties and effectively remove DON and N-DBPs formation potentials, which provides a viable and promising strategy for improving the safety of drinking water in practice.

Effect of ultrasonic pretreatment on fermentation performance and quality of fermented hawthorn pulp by lactic acid bacteria

In order to explore the effects of ultrasonic pretreatment on the fermentation performance and quality characteristics of fermented hawthorn pulp. Five types of fermented hawthorn pulp were obtained using 0 W for 5 min, 300 W for 5 min, 360 W for 5 min, 420 W for 5 min, 540 W for 5 min. The fermentation performance and quality of fermented hawthorn pulp were characterized. The results indicated Low power ultrasound (360 W) could improve the fermentation performance and quality of FHP, and high power ultrasound (540 W) could reduce the fermentation performance and quality. Under the ultrasonic condition of 360 W for 5 min; the cell membrane of lactic acid bacteria produced repairable damage and the morphology did not change significantly, the consumption of reducing sugar, total acid, soluble solids, amino nitrogen, conductivity, and sensory quality of the fermented hawthorn pulp reached the highest. The fermentation performance and quality of fermented hawthorn pulp were improved by the optimum ultrasonic treatment, which could be used as an effective and alternative method for producing FHP with good flavor, high bioactivity and good quality.

Controlling of foodborne pathogen biofilms on stainless steel by bacteriophages: A systematic review and meta-analysis

This study investigates the potential of using bacteriophages to control foodborne pathogen biofilms on stainless steel surfaces in the food industry. Biofilm-forming bacteria can attach to stainless steel surfaces, rendering them difficult to eradicate even after a thorough cleaning and sanitizing procedures. Bacteriophages have been proposed as a possible solution, as they can penetrate biofilms and destroy bacterial cells within, reducing the number of viable bacteria and preventing the growth and spread of biofilms. This systematic review and meta-analysis evaluates the potential of bacteriophages against different biofilm-forming foodborne bacteria, including Cronobacter sakazakii, Escherichia coli, Staphylococcus aureus, Pseudomonas fluorescens, Pseudomonas aeruginosa and Listeria monocytogenes. Bacteriophage treatment generally causes a significant average reduction of 38 % in biofilm formation of foodborne pathogens on stainless steel. Subgroup analyses revealed that phages are more efficient in long-duration treatment. Also, applying a cocktail of phages is 1.26-fold more effective than applying individual phages. Phages at concentrations exceeding 107 PFU/ml are significantly more efficacious in eradicating bacteria within a biofilm. The antibacterial phage activity decreases substantially by 3.54-fold when applied at 4 °C compared to temperatures above 25 °C. This analysis suggests that bacteriophages can be a promising solution for controlling biofilms in the food industry.

Examining Sound, Light, and Vibrations as Tools to Manage Microbes and Support Holobionts, Ecosystems, and Technologies

The vast array of interconnected microorganisms across Earth's ecosystems and within holobionts has been called the "Internet of Microbes." Bacteria and archaea are masters of energy and information collection, storage, transformation, and dissemination using both "wired" and wireless (at a distance) functions. Specific tools affecting microbial energy and information functions offer effective strategies for managing microbial populations within, between, and beyond holobionts. This narrative review focuses on microbial management using a subset of physical modifiers of microbes: sound and light (as well as related vibrations). These are examined as follows: (1) as tools for managing microbial populations, (2) as tools to support new technologies, (3) as tools for healing humans and other holobionts, and (4) as potential safety dangers for microbial populations and their holobionts. Given microbial sensitivity to sound, light, and vibrations, it is critical that we assign a higher priority to the effects of these physical factors on microbial populations and microbe-laden holobionts. We conclude that specific sound, light, and/or vibrational conditions are significant therapeutic tools that can help support useful microbial populations and help to address the ongoing challenges of holobiont disease. We also caution that inappropriate sound, light, and/or vibration exposure can represent significant hazards that require greater recognition.

Synergistic bactericidal effect and mechanism of ultrasound combined with Lauroyl Arginate Ethyl against Salmonella Typhimurium and its application in the preservation of onions

In the present study, the synergistic bactericidal effect and mechanism of ultrasound (US) combined with Lauroyl Arginate Ethyl (LAE) against Salmonella Typhimurium were investigated. On this basis, the effect of US+LAE treatment on the washing of S. Typhimurium on the surface of onions and on the physical and chemical properties of onion during fresh-cutting and storage were studied. The results showed that treatment with US+LAE could significantly (P < 0.05) reduce the number of S. Typhimurium compared to US and LAE treatments alone, especially the treatment of US+LAE (230 W/cm2, 8 min, 71 μM) reduced S. Typhimurium by 8.82 log CFU/mL. Confocal laser scanning microscopy (CLSM), flow cytometry (FCM), protein and nucleic acid release and N-phenyl-l-naphthylamine (NPN) assays demonstrated that US+LAE disrupted the integrity and permeability of S. Typhimurium cell membranes. Reactive oxygen species (ROS) and malondialdehyde (MDA) assays indicated that US+LAE exacerbated oxidative stress and lipid peroxidation in cell membranes. Field emission scanning electron microscopy (FESEM) demonstrated that US+LAE treatment caused loss of cellular contents and led to cell crumpling and even lost the original cell morphology. US+LAE treatment caused a significant (P < 0.05) decrease in the number of S. Typhimurium on onions, but there was no significant (P > 0.05) effect on the color, hardness, weight and ascorbic acid content of onions. This study elucidated the synergistic antibacterial mechanism of US+LAE and verified the feasibility of bactericidal effect on the surface of onions, providing a theoretical basis for improving the safety of fresh produce in the food industry and to propose a new way to achieve the desired results.

Recent advances on the formation, detection, resistance mechanism, and control technology of Listeria monocytogenes biofilm in food industry

Listeria monocytogenes is an important foodborne pathogen that causes listeriosis, a severe and fatal condition. Biofilms are communities of microorganisms nested within a self-secreted extracellular polymeric substance, and they protect L. monocytogenes from environmental stresses. Biofilms, once formed, can lead to the persistence of L. monocytogenes in processing equipment and are therefore considered to be a major concern for the food industry. This paper briefly introduces the recent advancements on biofilm formation characteristics and detection methods, and focuses on analysis of the mechanism of L. monocytogenes biofilm resistance; Moreover, this paper also summarizes and discusses the existing different techniques of L. monocytogenes biofilm control according to the physical, chemical, biological, and combined strategies, to provide a theoretical reference to aid the choice of effective control technology in the food industry.

Adopting the "Missile boats-Aircraft carrier" strategy via human-contact friendly oxidized starch to achieve rapid-sustainably antibacterial paperboards

Polysaccharide-based antibacterial agents have received tremendous attention for the facile fabrication, low toxicity, and high compatibility with carbohydrate polymers. However, the antimicrobial mechanism, activity, and cytotoxicity for human-contact paperboards of oxidized starch (OST) with high carboxyl content, has not been explored. Herein, OST-27- 75 with 27- 75 wt% carboxyl contents were fabricated by H2O2 and coated on paperboards. Strikingly, OST-55 coating layer (16 g/m2) did not exfoliate from paperboard and possessed the rapid-sustainable antibacterial performance against Staphylococcus aureus and Escherichia coli. The soluble and insoluble components of OST-55 (OST55-S: OST55-IS mass ratio = 1: 2.1) presented different antimicrobial features and herein they were characterized by GC-MS, FT-IR, H-NMR, XRD, bacteriostatic activities, biofilm formation inhibition and intracellular constituent leakage to survey the antibacterial mechanism. The results revealed OST55-S displayed an amorphous structure and possessed superior antibacterial activity against S. aureus (MIC = 4 mg/mL) and E. coli (MIC = 8 mg/mL). Distinctively, OST55-S could rapidly ionize [H+] like "missile boats" from small molecule saccharides, while OST55-IS polyelectrolyte could continuously and slowly release for [H+] like an "aircraft carrier" to inhibit biofilm formation and disrupt cell structure. Eventually, the "Missile boats-Aircraft carrier" strategy provided a green methodology to fabricate polymeric antibacterial agents and expanded the use of cellulose-based materials.

Molecular and proteomic response of Pseudomonas fluorescens biofilm cultured on lettuce (Lactuca sativa L.) after ultrasound treatment at different intensity levels

Ultrasonic treatment is widely used for surface cleaning of vegetables in the processing of agricultural products. In the present study, the molecular and proteomic response of Pseudomonas fluorescens biofilm cultured on lettuce was investigated after ultrasound treatment at different intensity levels. The results show that the biofilm was efficiently removed after ultrasound treatment with intensity higher than 21.06 W/cm2. However, at an intensity of less than 18.42 W/cm2, P. fluorescens was stimulated by ultrasound leading to promoted bacterial growth, extracellular protease activity, extracellular polysaccharide secretion (EPS), and synthesis of acyl-homoserine lactones (AHLs) as quorum-sensing signaling molecules. The expression of biofilm-related genes, stress response, and dual quorum sensing system was upregulated during post-treatment ultrasound. Proteomic analysis showed that ultrasound activated proteins in the flagellar system, which led to changes in bacterial tendency; meanwhile, a large number of proteins in the dual-component system began to be regulated. ABC transporters accelerated the membrane transport of substances inside and outside the cell membrane and equalized the permeability conditions of the cell membrane. In addition, the expression of proteins related to DNA repair was upregulated, suggesting that bacteria repair damaged DNA after ultrasound exposure.

Relationship between Desiccation Tolerance and Biofilm Formation in Shiga Toxin-Producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) is a major concern in the food industry and requires effective control measures to prevent foodborne illnesses. Previous studies have demonstrated increased difficulty in the control of biofilm-forming STEC. Desiccation, achieved through osmotic stress and water removal, has emerged as a potential antimicrobial hurdle. This study focused on 254 genetically diverse E. coli strains collected from cattle, carcass hides, hide-off carcasses, and processing equipment. Of these, 141 (55.51%) were STEC and 113 (44.48%) were generic E. coli. The biofilm-forming capabilities of these isolates were assessed, and their desiccation tolerance was investigated to understand the relationships between growth temperature, relative humidity (RH), and bacterial survival. Only 28% of the STEC isolates had the ability to form biofilms, compared to 60% of the generic E. coli. Stainless steel surfaces were exposed to different combinations of temperature (0 °C or 35 °C) and relative humidity (75% or 100%), and the bacterial attachment and survival rates were measured over 72 h and compared to controls. The results revealed that all the strains exposed to 75% relative humidity (RH) at any temperature had reduced growth (p < 0.001). In contrast, 35 °C and 100% RH supported bacterial proliferation, except for isolates forming the strongest biofilms. The ability of E. coli to form a biofilm did not impact growth reduction at 75% RH. Therefore, desiccation treatment at 75% RH at temperatures of 0 °C or 35 °C holds promise as a novel antimicrobial hurdle for the removal of biofilm-forming E. coli from challenging-to-clean surfaces and equipment within food processing facilities.

Recent developments in ultrasound approach for preservation of animal origin foods

Ultrasound is a contemporary non-thermal technology that is currently being extensively evaluated for its potential to preserve highly perishable foods, while also contributing positively to the economy and environment. There has been a rise in the demand for food products that have undergone minimal processing or have been subjected to non-thermal techniques. Livestock-derived food products, such as meat, milk, eggs, and seafood, are widely recognized for their high nutritional value. These products are notably rich in proteins and quality fats, rendering them particularly vulnerable to oxidative and microbial spoilage. Ultrasound has exhibited significant antimicrobial properties, as well as the ability to deactivate enzymes and enhance mass transfer. The present review centers on the production and classification of ultrasound, as well as its recent implementation in the context of livestock-derived food products. The commercial applications, advantages, and limitations of the subject matter are also subject to scrutiny. The review indicated that ultrasound technology can be effectively utilized in food products derived from livestock, leading to favorable outcomes in terms of prolonging the shelf life of food while preserving its nutritional, functional, and sensory attributes. It is recommended that additional research be conducted to investigate the effects of ultrasound processing on nutrient bioavailability and extraction. The implementation of hurdle technology can effectively identify and mitigate the lower inactivation of certain microorganisms or vegetative cells.

Biofilm tolerance, resistance and infections increasing threat of public health

Microbial biofilms can cause chronic infection. In the clinical setting, the biofilm-related infections usually persist and reoccur; the main reason is the increased antibiotic resistance of biofilms. Traditional antibiotic therapy is not effective and might increase the threat of antibiotic resistance to public health. Therefore, it is urgent to study the tolerance and resistance mechanism of biofilms to antibiotics and find effective therapies for biofilm-related infections. The tolerance mechanism and host reaction of biofilm to antibiotics are reviewed, and bacterial biofilm related diseases formed by human pathogens are discussed thoroughly. The review also explored the role of biofilms in the development of bacterial resistance mechanisms and proposed therapeutic intervention strategies for biofilm related diseases.

Comparison of the efficacy of physical and chemical strategies for the inactivation of biofilm cells of foodborne pathogens

Biofilm formation is a strategy in which microorganisms generate a matrix of extracellular polymeric substances to increase survival under harsh conditions. The efficacy of sanitization processes is lowered when biofilms form, in particular on industrial devices. While various traditional and emerging technologies have been explored for the eradication of biofilms, cell resistance under a range of environmental conditions renders evaluation of the efficacy of control challenging. This review aimed to: (1) classify biofilm control measures into chemical, physical, and combination methods, (2) discuss mechanisms underlying inactivation by each method, and (3) summarize the reduction of biofilm cells after each treatment. The review is expected to be useful for future experimental studies and help to guide the establishment of biofilm control strategies in the food industry.

Biofilm formation in food processing plants and novel control strategies to combat resistant biofilms: the case of Salmonella spp

Salmonella is one of the pathogens that cause many foodborne outbreaks throughout the world, representing an important global public health problem. Salmonella strains with biofilm-forming abilities have been frequently isolated from different food processing plants, especially in poultry industry. Biofilm formation of Salmonella on various surfaces can increase their viability, contributing to their persistence in food processing environments and cross-contamination of food products. In recent years, increasing concerns arise about the antimicrobial resistant and disinfectant tolerant Salmonella, while adaptation of Salmonella in biofilms to disinfectants exacerbate this problem. Facing difficulties to inhibit or remove Salmonella biofilms in food industry, eco-friendly and effective strategies based on chemical, biotechnological and physical methods are in urgent need. This review discusses biofilm formation of Salmonella in food industries, with emphasis on the current available knowledge related to antimicrobial resistance, together with an overview of promising antibiofilm strategies for controlling Salmonella in food production environments.

Antibacterial and antibiofilm performance of low-frequency ultrasound against Escherichia coli O157:H7 and its application in fresh produce

Ultrasound technology has been focused on due to its unique advantages in biofilm removal compared with traditional antibacterial methods. Herein, the anti-biofilm properties of low-frequency ultrasound (LFUS) were studied against Enterohemorrhagic Escherichia coli O157: H7 (E. coli O157:H7). After ultrasonication (20 kHz, 300 W) for 5 min, the removal rate of biofilm from polystyrene sheets reached up to 99.999 %. However, the bacterial cells could not be inactivated completely even extending the duration of ultrasonic irradiation to 30 min. Fortunately, this study indicated that LFUS could efficiently weaken the metabolic capacity and biofilm-forming ability of bacterial cells separated from biofilm. It could be associated with the removal of cell surface appendages and damage to cell membrane induced by mechanical vibration and acoustic cavitation. Besides, the genetic analysis proved that the transcription level of genes involved in curli formation was significantly down-regulated during ultrasonic irradiation, thus impeding the process of irreversible adhesion and cells aggregation. Finally, the actual application effect of LFUS was also evaluated in different fresh produces model. The results of this study would provide a theoretical basis for the further application of ultrasound in the food preservation.

Antibacterial and antibiofilm efficacy of green synthesized ZnO nanoparticles using Saraca asoca leaves

Biofilms are made up of bacterial colonies and their extracellular polymeric substances (EPS) matrix, which protects the bacteria from adverse environmental conditions. The increasing drug resistivity of pathogenic bacteria is becoming an emergency for developing new antibacterial agents. In this study, we have synthesized the zinc oxide nanoparticles (ZnO NPs) using the leaf extract of Saraca asoca plant, and the antibacterial and antibiofilm activity of green synthesized ZnO NPs was measured against the biofilm-producing bacteria Bacillus subtilis. The disk diffusion data reveals that the zone of inhibition (ZOI) starts at a concentration of 0.5 mg/mL and minimum inhibition concentration (100 µg/mL) and minimum bactericidal concentration (150 µg/mL) values were also evaluated for green synthesized ZnO nanomaterials. Crystal violet test and microscopic examination were used to assess the impact of produced nanoparticles on biofilm development. The findings indicated a nearly 45%, 64%, and 83% suppression of biofilm development at 0.5 × MIC, 0.75 × MIC, and 1 × MIC value, respectively. The biofilm biomass of the preformed or matured biofilms by the ZnO NPs was evaluated to be 68%, 50%, and 33% at concentrations of 0.5 × MIC, 0.75 × MIC, and 1 × MIC which was concentration-dependent. Moreover, flow cytometry results suggest damage to the bacterial cell membrane. The data indicated that the proportion of dead cells increased with NP concentration in comparison to the control. Therefore, it can be concluded that the green synthetic ZnO nanoparticles showed excellent antibacterial and antibiofilm activity against the Bacillus subtilis bacteria that produce biofilms and that they could be a promising substitute agent for the treatment of biofilms and drug-resistant bacteria.

Advances in Natural Product Extraction Techniques, Electrospun Fiber Fabrication, and the Integration of Experimental Design: A Comprehensive Review

The present review explores the growing interest in the techniques employed for extracting natural products. It emphasizes the limitations of conventional extraction methods and introduces superior non-conventional alternatives, particularly ultrasound-assisted extraction. Characterization and quantification of bioactive constituents through chromatography coupled with spectroscopy are recommended, while the importance of method development and validation for biomarker quantification is underscored. At present, electrospun fibers provide a versatile platform for incorporating bioactive extracts and have extensive potential in diverse fields due to their unique structural and functional characteristics. Thus, the review also highlights the fabrication of electrospun fibers containing bioactive extracts. The preparation of biologically active extracts under optimal conditions, including the selection of safe solvents and cost-effective equipment, holds promising potential in the pharmaceutical, food, and cosmetic industries. Integration of experimental design into extraction procedures and formulation development is essential for the efficient production of health products. The review explores potential applications of encapsulating natural product extracts in electrospun fibers, such as wound healing, antibacterial activity, and antioxidant properties, while acknowledging the need for further exploration and optimization in this field. The findings discussed in this review are anticipated to serve as a valuable resource for the processing industry, enabling the utilization of affordable and environmentally friendly, natural, and raw materials.

Microbial decontamination assisted by ultrasound-based processing technologies in food and model systems: A review

Ultrasound (US) technology is recognized as one of the emerging technologies that arise from the current trends for improving nutritional and organoleptic properties while providing food safety. However, when applying the US alone, higher power and longer treatment times than conventional thermal treatments are needed to achieve a comparable level of microbial inactivation. This results in risks, damaging food products' composition, structure, or sensory properties, and can lead to higher processing costs. Therefore, the US has often been investigated in combination with other approaches, like heating at mild temperatures and/or treatments at elevated pressure, use of antimicrobial substances, or other emerging technologies (e.g., high-pressure processing, pulsed electric fields, nonthermal plasma, or microwaves). A combination of US with different approaches has been reported to be less energy and time consuming. This manuscript aims to provide a broad review of the microbial inactivation efficacy of US technology in different food matrices and model systems. In particular, emphasis is given to the US in combination with the two most industrially viable physical processes, that is, heating at mild temperatures and/or treatments at elevated pressure, resulting in techniques known as thermosonication, manosonication, and manothermosonication. The available literature is reviewed, and critically discussed, and potential research gaps are identified. Additionally, discussions on the US's inactivation mechanisms and lethal effects are included. Finally, mathematical modeling approaches of microbial inactivation kinetics due to US-based processing technologies are also outlined. Overall, this review focuses only on the uses of the US and its combinations with other processes relevant to microbial food decontamination.

Generation, mechanisms, kinetics, and effects of gaseous chlorine dioxide in food preservation

Food preservation is a critical issue in ensuring food safety and quality. Growing concern around industrial pollution of food and demand for environmentally sustainable food has led to increased interest in developing effective and eco-friendly preservation techniques. Gaseous ClO2 has gained attention for its strong oxidizing properties, high efficacy in microorganism inactivation, and potential for preserving the attributes and nutritional quality of fresh food while avoiding the formation of toxic byproducts or unacceptable levels of residues. However, the widespread use of gaseous ClO2 in the food industry is limited by several challenges. These include large-scale generation, high cost and environmental considerations, a lack of understanding of its mechanism of action, and the need for mathematical models to predict inactivation kinetics. This review aims to provide an overview of the up-to-date research and application of gaseous ClO2 . It covers preparation methods, preservation mechanisms, and kinetic models that predict the sterilizing efficacy of gaseous ClO2 under different conditions. The impacts of gaseous ClO2 on the quality attributes of fresh produce and low-moisture foods, such as seeds, sprouts, and spices, are also summarized. Overall, gaseous ClO2 is a promising preservation approach, and future studies are needed to address the challenges in large-scale generation and environmental considerations and to develop standardized protocols and databases for safe and effective use in the food industry.

Antimicrobial and Antibiofilm Efficacy and Mechanism of Oregano Essential Oil Against Shigella flexneri

The aim of this study was to assess the antimicrobial activity of oregano essential oil (OEO) against Shigella flexneri and eradication efficacy of OEO on biofilm. The results showed that the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of OEO against S. flexneri were 0.02% (v/v) and 0.04% (v/v), respectively. OEO effectively killed S. flexneri in Luria-Bertani (LB) broth and contaminated minced pork (the initial population of S. flexneri was about 7.0 log CFU/mL or 7.2 log CFU/g), and after treatment with OEO at 2 MIC in LB broth or at 15 MIC in minced pork, the population of S. flexneri decreased to an undetectable level after 2 or 9 h, respectively. OEO increased intracellular reactive oxygen species concentration, destroyed cell membrane, changed cell morphology, decreased intracellular ATP concentration, caused cell membrane depolarization, and destroyed proteins or inhibited proteins synthesis of S. flexneri. In addition, OEO effectively eradicated the biofilm of S. flexneri by effectively inactivating S. flexneri in mature biofilm, destroying the three-dimensional structure, and reducing exopolysaccharide biomass of S. flexneri. In conclusion, OEO exerts its antimicrobial action effectively and also has a valid scavenging effect on the biofilm of S. flexneri. These findings suggest that OEO has the potential to be used as a natural antibacterial and antibiofilm material in the control of S. flexneri in meat product supply chain, thereby preventing meat-associated infections.

The state-of-the-art research of the application of ultrasound to winemaking: A critical review

As a promising non-thermal physical technology, ultrasound has attracted extensive attention in recent years, and has been applied to many food processing operation units, such as involving filtration, freezing, thawing, sterilization, cutting, extraction, aging, etc. It is also widely used in the processing of meat products, fruits and vegetables, and dairy products. With regard to its application in winemaking, most of the studies available in the literature are focused on the impact of ultrasound on a certain characteristic of wine, lacking of systematic sorting of these literatures. This review systematically summarizes and explores the current achievements and problems of the application of ultrasound to the different stages of winemaking, including extraction, fermentation, aging and sterilization. Summarizing the advantages and disadvantages of ultrasound application in winemaking and its development in future development.

Inhibitory Effects of Trans-Cinnamaldehyde Against Pseudomonas aeruginosa Biofilm Formation

Pseudomonas aeruginosa biofilm formation has been considered to be an important determinant of its pathogenicity in most infections. The antibiofilm activity of trans-cinnamaldehyde (TC) against P. aeruginosa was investigated in this study. Results demonstrated that the minimum inhibitory concentration (MIC) of TC against P. aeruginosa was 0.8 mg/mL, and subinhibitory concentrations (SICs) was 0.2 mg/mL and below. Crystal violet staining showed that TC at 0.05-0.2 mg/mL reduced biofilm biomass in 48 h in a concentration-dependent mode. The formation area of TC-treated biofilms was significantly declined (p < 0.01) on the glass slides observed by light microscopy. Field-emission scanning electron microscopy further demonstrated that TC destroyed the biofilm morphology and structure. Confocal laser scanning microscopic observed the dispersion of biofilms and the reduction of exopolysaccharides after TC treatment stained with concanavalin A (Con-A)-fluorescein isothiocyanate conjugate and Hoechst 33258. Meanwhile, TC caused a significant decrease (p < 0.01) in the component of polysaccharides, proteins, and DNA in extracellular polymeric substance. The swimming and swarming motility and quorum sensing of P. aeruginosa was also found to be significantly inhibited (p < 0.01) by TC at SICs. Furthermore, SICs of TC repressed the several genes transcription associated with biofilm formation as determined by real-time quantitative polymerase chain reaction. Overall, our findings suggest that TC could be applied as natural and safe antibiofilm agent to inhibit the biofilm formation of P. aeruginosa.

Inhibitory effects of hexanal on acylated homoserine lactones (AHLs) production to disrupt biofilm formation and enzymes activity in Erwinia carotovora and Pseudomonas fluorescens

Erwinia carotovora and Pseudomonas fluorescens were two bacteria commonly caused the spoilage of vegetables through biofilm formation and secretion of extracellular enzymes. In this study, N-(3-oxohexanoyl)-L-homoserine lactone (3-oxo-C6-HSL) and N-Octanoyl-L-homoserine lactone (C8-HSL) were confirmed as acylated homoserine lactones (AHLs) signal molecule produced by E. carotovora and P. fluorescens, respectively. In addition, quorum sensing inhibitory (QSI) effects of hexanal on AHLs production were evaluated. Hexanal at 1/2 minimum inhibitory concentration (MIC) was achieved 76.27% inhibitory rate of 3-oxo-C6-HSL production in E. carotovora and a inhibitory rate of C8-HSL (60.78%) in P. fluorescens. The amount of biofilm formation and activity of extracellular enzymes treated with 1/2 MIC of hexanal were restored with different concentrations (10 ng/mL, 50 ng/mL, 100 ng/mL) of exogenous AHLs (P < 0.05), which verified QSI effect of hexanal on biofilm and extracellular enzymes were due to its inhibition on AHLs production. Molecular docking analysis showed that hexanal could interact with EcbI and PcoI protein to disrupt AHLs production. Furthermore, results showed that sub-MICs of hexanal could suppress expressions of ecbI and pcoI genes in AHL-mediated QS system of E. carotovora and P. fluorescens. This study provides theoretical support for the application of essential oils as QS inhibitors in the preservation of vegetables.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-022-05624-9.

The Use of Natural Methods to Control Foodborne Biofilms

Biofilms are large aggregates of various species of bacteria or other microorganisms tightly attached to surfaces through an intricate extracellular matrix. These complex microbial communities present quite the challenge in the food processing industry, as conditions such as raw meats and diverse food product content in contact with workers, drains, machinery, and ventilation systems, make for prime circumstances for contamination. Adding to the challenge is the highly resistant nature of these biofilm growths and the need to keep in mind that any antimicrobials utilized in these situations risk health implications with human consumption of the products that are being processed in these locations. For that reason, the ideal means of sanitizing areas of foodborne biofilms would be natural means. Herein, we review a series of innovative natural methods of targeting foodborne biofilms, including bacteriocins, bacteriophages, fungi, phytochemicals, plant extracts, essential oils, gaseous and aqueous control, photocatalysis, enzymatic treatments, and ultrasound mechanisms.

Antibiofilm Effects of Bacteriocin BMP32r on Listeria monocytogenes

Listeria monocytogenes is a well-known foodborne pathogen that usually lives as biofilm to cope with unfavorable surroundings. Bacteriocins have been reported as antimicrobial compounds, and their bactericidal actions have been extensively studied, but their antibiofilm actions have rarely been studied. Previous study indicated that bacteriocin BMP32r has a broad-spectrum antibacterial activity. In this study, the efficacy of BMP32r against the planktonic bacteria, inhibition of forming biofilm, destruction of mature biofilm, and kill persisters of L. monocytogenes ATCC 15,313 was determined. BMP32r exhibited the bactericidal effect on L. monocytogenes planktonic bacteria. Crystal violet staining showed that sub-minimum inhibitory concentrations (SICs) of BMP32r (1/32 × MIC and 1/16 × MIC) significantly (p < 0.001) inhibit the biofilm formation. In addition, the results of CCK-8, plate count, ruthenium red staining, scanning electron microscopy, and real-time quantitative PCR assay showed that SICs of BMP32r reduced cell adhesion, exopolysaccharide production, quorum sensing, and virulence genes expression in biofilm formation. Moreover, higher concentrations of BMP32r (2 × MIC and 4 × MIC) disrupt the mature biofilm by killing the bacteria in the biofilm and kill L. monocytogenes persisters bacteria effectively. Therefore, BMP32r has promising potential as an antibiofilm agent to combat L. monocytogenes.

Ultrasound-Assisted Slightly Acidic Electrolyzed Water in Aquatic Product Sterilization: A Review

Ultrasound has been confirmed as the propagation process of mechanical vibrations in a medium, with a frequency significantly higher than 20 kHz. Moreover, it has an effect of sterilization on foods. In general, ultrasonic sterilization medium is manifested as a liquid. Ultrasonic treatment technology has certain advantages in aquatic product processing. It is noteworthy that this technology will have better effects of sterilization if used in combination with other treatment methods. Slightly acidic electrolyzed water (SAEW) is characterized by high-efficiency broad-spectrum sterilization operation, low cost, and environmental protection, among other properties, and has a positive effect on aquatic product sterilization and preservation. Selecting acidic electrolyzed water with a low concentration coupled with low-power ultrasonic waves for combined sterilization exerts a more potent sterilization effect, and acidic electrolyzed water combined with ultrasonic sterilization is expected to be a potentially environment-friendly alternative. In this study, the sterilization mechanisms of ultrasonic and SAEW methods used both individually and as a synergistic treatment, the effect on microbial growth, and the research progress of the application of the combined effect in the sterilization and refrigeration of aquatic products are reviewed. Furthermore, this study looks forward to the future development trend, with a view to its application in aquatic products, while providing a reference for research and application in the field of processing and safety.

Ultrasound-assisted production of alcoholic beverages: From fermentation and sterilization to extraction and aging

Ultrasound is sound waves above 20 kHz that can be used as a nonthermal ''green'' technology for agri-food processing. It has a cavitation effect, causing bubbles to form and collapse rapidly as they travel through the medium during ultrasonication. Therefore, it inactivates microorganisms and enzymes through cell membrane disruption with physicochemical and sterilization effects on foods or beverages. This emerging technology has been explored in wineries to improve wine color, taste, aroma, and phenolic profile. This paper aims to comprehensively review the research on ultrasound applications in the winery and alcoholic beverages industry, discuss the impacts of this process on the physicochemical properties of liquors, the benefits involved, and the research needed in this area. Studies have shown that ultrasonic technology enhances wine maturation, improves wine fermentation, accelerates wine aging, and deactivates microbes while enhancing quality, as observed with better critical aging markers such as phenolic compounds and color intensity. Besides, ultrasound enhances phytochemical, physicochemical, biological, and organoleptic properties of alcoholic beverages. For example, this technology increased anthocyanin in red wine by 50%. It also enhanced the production rate by decreasing the aging time by more than 90%. Ultrasound can be considered an economically viable technology that may contribute to wineries' waste valorization, resource efficiency improvement, and industry profit enhancement. Despite numerous publications and successful industrial applications discussed in this paper, ultrasound up-scaling and applications for other types of liquors need further efforts.

Dual-Species Biofilms Formed by Escherichia coli and Salmonella Enhance Chlorine Tolerance

Outbreaks of Escherichia coli and Salmonella in food might be associated with the cross-contamination of biofilms on food-contact surfaces. The knowledge of the sanitization of mono-species biofilm on the food-contact surface is well established, while mixed-species biofilm occurs more naturally, which could profoundly affect the efficacy of sanitizer.

Lifestyle of Listeria monocytogenes and food safety: Emerging listericidal technologies in the food industry

Listeria monocytogenes, a causative agent of listeriosis, is a major foodborne pathogen. Among pathogens, L. monocytogenes stands out for its unique ecological and physiological characteristics. This distinct lifestyle of L. monocytogenes has a significant impact on food safety and public health, mainly through the ability of this pathogen to multiply at refrigeration temperature and to persist in the food processing environment. Due to a combination of these characteristics and emerging trends in consumer preference for ready-to-eat and minimally processed food, there is a need to develop effective and sustainable approaches to control contamination of food products with L. monocytogenes. Implementation of an efficient and reliable control strategy for L. monocytogenes must first address the problem of cross-contamination. Besides the preventive control strategies, cross-contamination may be addressed with the introduction of emerging post packaging non-thermal or thermal hurdles that can ensure delivery of a listericidal step in a packed product without interfering with the organoleptic characteristics of a food product. This review aims to present the most relevant findings underlying the distinct lifestyle of L. monocytogenes and its impact on food safety. We also discuss emerging food decontamination technologies that can be used to better control L. monocytogenes.

First-passage time for the g-subdiffusion process of vanishing particles

Subdiffusion equation and molecule survival equation, both with Caputo fractional time derivatives with respect to other functions g_{1} and g_{2}, respectively, are used to describe diffusion of a molecule that can disappear at any time with a constant probability. The process can be interpreted as an "ordinary" subdiffusion and "ordinary" molecule survival process in which timescales are changed by the functions g_{1} and g_{2}. We derive the first-passage time distribution for the process. The mutual influence of subdiffusion and molecule-vanishing processes can be included in the model when the functions g_{1} and g_{2} are related to each other. As an example, we consider the processes in which subdiffusion and molecule survival are highly related, which corresponds to the case of g_{1}≡g_{2}.

Combination of ultrasound-peracetic acid washing and ultrasound-assisted aerosolized ascorbic acid: A novel rinsing-free disinfection method that improves the antibacterial and antioxidant activities in cherry tomato

Traditional ultrasound (US)-assisted disinfection is only effective during washing. Coating is an effective method to control microbial growth after washing; however, cross-contamination can occur during immersion in the coating aqueous solution. Tap water (TW) rinsing is generally used to remove sanitizer residues after US-assisted washing; however, the Food and Drug Administration stated that rinsing is unnecessary when the peracetic acid (PAA) concentration does not exceed 80 ppm. In this study, we proposed a novel US-assisted hurdle technology of 80 ppm PAA combined with low-frequency US (25 kHz) during washing, followed by US-assisted aerosolization processing (nonimmersion coating). Ascorbic acid (AA), a safe and low-cost agent, was selected as the aerosolization solution. Cherry tomatoes were selected as the model, and the proposed method was compared with traditional US-assisted disinfection methods (US-10 ppm free chlorine washing + TW rinsing and US-5 ppm chlorine dioxide washing + TW rinsing) to analyze the disinfection efficacy and quality changes. During storage, US-PAA + 1%AA facilitated additional 0.7-0.9, 0.6-0.8, 0.7-1.0, and 0.5-1.0 log CFU/g reductions in the counts of Escherichia coli O157:H7, Salmonella Typhimurium, aerobic mesophilic counts, and molds and yeasts, respectively, as compared with traditional US-assisted methods. Sensory properties, color index, total soluble solids, titratable acidity, and weight loss were not negatively affected by any of the treatments. Firmness was slightly reduced after all treatments; however, the firmness of the samples was maintained during storage, in contrast with the decreased firmness observed in the control. Phenolic content and antioxidant activity significantly increased after all treatments. Further analysis of two key enzymes (phenylalanine ammonia-lyase and 4-coumarate-CoA ligase) involved in phenolic synthesis showed that their levels significantly increased following all treatments, leading to an increase in phenolic content and antioxidant activity. This result also indicated that US-assisted washing could act as an abiotic elicitor to increase nutritional content. Overall, US-PAA + 1%AA treatment served as an effective method for disinfecting produce during washing and for controlling microbial growth after washing without prolonging the processing time, which is an advantage over traditional US-assisted washing.

In vitro and in silico approaches to investigate antimicrobial and biofilm removal efficacies of combined ultrasonic and mild thermal treatment against Pseudomonas fluorescens

A combined ultrasonic and thermal (US-TM) treatment was developed in this study to achieve a high efficacy of P. fluorescens biofilm control. The present study demonstrated that combined a moderate ultrasound treatment (power ≥ 80 W) and a mild heat (up to 50 °C) largely destroyed biofilm structure in 15 min and removed>65.63% of biofilm from a glass slide where cultivated the P. fluorescens biofilm. Meanwhile, the viable cell count was decreased from 10.72 to 6.48 log10CUF/mL. Differences in biofilm removal and lethal modes of US-TM treatment were confirmed through microscopies analysis in vitro. The ultrasound first contributed to releasing the bacteria in the biofilm to the environment and simultaneously exposing inner bacteria at the deep layer of biofilm depending on shear force, shock waves, acoustic streaming, etc. When the biofilm structure was destroyed, US-TM treatment would synergistically inactivate P. fluorescens cells. In silico studies adopted COMSOL to simulate acoustic pressure and temperature distribution in the bioreactor; both of them were significantly influenced by various factors, such as input power, sonotrode position, materials and volume of container, etc. Facing the biofilm issue existing on the surface of container, boundary conditions were exported and thereby pointing out potential "dead ends" where the ultrasound may not be effectively transduced. Both in vitro and in silico results may inspire the food industry to adopt US-TM treatment to achieve biofilm control.

The combination of hexanal and geraniol in sublethal concentrations synergistically inhibits Quorum Sensing of Pseudomonas fluorescens - in vitro and in silico approaches

AIM

Hexanal and geraniol are essential oil components with anti-Quorum Sensing (QS) activity against Pseudomonas fluorescens. This study demonstrated that QS inhibition (QSI) efficacy of hexanal and geraniol combination (HG) was significantly increased compared with their mono-counterparts at the same concentration.

METHODS AND RESULTS

Tests on P. fluorescens motility, biofilm formation, acyl-homoserine lactones (AHLs) production, gene expression in vitro, and molecular docking in silico were conducted to evaluate the synergistic effect of hexanal and geraniol on QSI. HG mixture at 0.5 minimal inhibitory concentration (MIC) showed a strong synergistic inhibition of biofilm formation (51.8%), motility (60.13%), and extracellular protease activity (58.9%) of P. fluorescens. The synthesis of AHLs, e.g. C₈ -HSL and C₁₂ -HSL was inhibited by hexanal, geraniol, and HG; both AHLs are responsible for regulating virulence factors in P. fluorescens. The expression of pcoI and gacA genes regulating AHLs synthetase and sensor kinase was significantly down-regulated by HG (0.29 and 0.38-fold) at 0.5 MIC. Hexenal and HG showed significant inhibition of pcoR and gacS genes expression regulating AHLs receptor protein and response regulator; however, geraniol failed to down-regulate the two genes. Molecular docking in silico also supported these findings. Hexenal inserted into minor groove of pcoI/pcoR DNA fragments to inhibit genes expression. Both hexanal (-31.487 kcal/mol) and geraniol (-25.716 kcal/mol) had a higher binding affinity with PcoI protein than halogenated furanone C30 (-24.829 kcal/mol) as a known competitor of AHLs. Similarly, hexenal and geraniol would also strongly bind to the PcoR protein.

CONCLUSIONS

It was found that HG at 0.5 MIC would effectively inhibit QS through suppressing pcoR/gacS and gacA/gacS genes expression and therefore, inhibit motility and biofilm formation in P. fluorescens.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study indicated that HG at sub-MIC as QS inhibitor could be further developed as a new preservative of agri-food products.