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Hexanal is an essential oil component which has quorum sensing (QS) inhibitor effect on vegetable spoilage bacteria such as Erwinia carotovora (E. carotovora) and Pseudomonas fluorescens (P. fluorescens). However, the QS inhibitory (QSI) effect of hexanal on Autoinducer‐2 (AI‐2), biofilm formation, and enzyme activity is not well understood. Our results showed that the contents of AI‐2 produced by E. carotovora were higher than P. fluorescens, and the 4,5‐Dihydroxy‐2,3‐pentanedione (DPD) concentrations were significantly (p <.05) inhibited by hexanal. Adding DPD exogenously could form more biofilm when inhibited by 1/2 of minimum inhibitory concentration (MIC) of hexanal, which showed that QSI effect of hexanal on biofilm was due to its inhibition on AI‐2. There was no significant increase in enzyme activity, which suggested that enzyme inhibition by hexanal may not due to AI‐2. Understanding the relationship between QSI effect of hexanal on AI‐2, biofilm formation, and enzyme activity can improve to suppress decomposition of vegetables after harvest. Practical applications: Hexanal is an EO component which has shown good QSI effect on biofilm formation and exoenzyme activities of P. fluorescens and E. carotovora. However, the reason for the QSI effect of hexanal was still unclear. In this study, hexanal as a QS inhibitor on these two vegetable spoilage bacteria was studied from the perspective of AI‐2 signal molecule. Understanding the QS inhibition of hexanal on AI‐2 signal molecule of spoilage bacteria and its relationship with biofilm formation and enzyme activity was necessary for the application of more EO components as QS inhibitor to suppress corruption of vegetables after harvest. [ABSTRACT FROM AUTHOR]

Biofilm formation is mainly controlled by quorum sensing (QS). To prevent biofilm formation on stainless steel, the QS inhibitory (QSI) effect of 10 essential oil (EO) components on dual‐species bacterial biofilm (Erwinia carotovora and Pseudomonas fluorescens) were screened. Geraniol showed the best QSI effect on dual‐species biofilm with an inhibition rate of 61.27%. Meanwhile, geraniol at the sub‐MIC significantly restrained the swimming, swarming, twitching motilities, exopolysaccharide production, and biofilm biomass of the two bacterial biofilms. The reduction of QS regulatory factors was possibly attributed to the inhibition of AI‐2 signal molecule and thereby controlled the formation of biofilm. The viable cells in the dual‐species biofilm formed on stainless steel were also decreased by using geraniol at sub‐MICs. Such an inhibition also showed a good dose‐dependence. Results of this study indicated that geraniol can be utilized as a QSI to achieve biofilm control on foods. Practical applications: EOs can be utilized as a group of natural food preservers. However, high value and significant flavoring impact restrict their applications in the food industry. Facing these issues, taken Eos as QSI would be helpful to achieve microbial control at a low concentration level. Geraniol is an important component in many essential oils. In this study, the QSI effect of geraniol on the AI‐2 signal molecule production, and biofilm formation in Erwinia carotovora and Pseudomonas fluorescens were elucidated. These findings would support that geraniol can be further developed as a promising strategy to achieve postharvest preservation of vegetables. [ABSTRACT FROM AUTHOR]

There has been a growing concern for methicillin-resistant Staphylococcus aureus (MRSA) contamination in the food industry. In the present study, the highest inactivation effect (5.80 log CFU/mL) was observed in simultaneous treatment compared with sequential treatment of ultrasound (US)-assisted cinnamaldehyde (CA). The absence of sublethal bacteria indicated US-assisted CA did not induce the tolerance or adaptation to MRSA cells. The results of SEM analysis indicated that the application of US-assisted CA resulted in severe damage to the cell membrane, as demonstrated by the highest fluorescence intensity of propidium iodide dye, intracellular substance leakage, and molecular docking analysis, as well as ATPase activity. Furthermore, US-assisted CA significantly disrupted energy metabolism, as evidenced by a reduction in ATP concentration, succinate dehydrogenase, and malate dehydrogenase activities. The exacerbation of ROS-induced damage to MRSA cells was observed due to the reduction in catalase and peroxidase activities. Additionally, a significant decrease (p < 0.05) in extracellular carbohydrates and proteins within the MRSA biofilm was noted on both glass and stainless-steel surfaces exposed to US-assisted CA. Ultimately, US-assisted CA treatment resulted in a maximum reduction (p < 0.05) of MRSA cells in beef meat, while maintaining color, pH, and sensory properties during an 8-day storage period at 4 °C. These findings suggested that simultaneous US-assisted CA treatment may serve as an alternative method for ensuring microbiological safety. [Display omitted] • Ultrasound-assisted cinnamaldehyde did not cause tolerance of MRSA. • Ultrasound assisted rapid destruction of cell membrane by cinnamaldehyde. • Disordered energy metabolism aggravated the accumulation of ROS. • Negligible changes of quality attributes in beef meat subjected to simultaneous treatment was observed. [ABSTRACT FROM AUTHOR]

To prevent the spoilage of vegetables in food processing, hexanal, a quorum-sensing (QS) inhibitor, was first used to inhibit the formation of Erwinia carotovora and Pseudomonas fluorescens biofilms. Their growth curves were not markedly inhibited by hexanal below the minimum inhibitory concentration (MIC), but the biofilm formation could be inhibited. Motility, exopolysaccharide production, and biofilm formation of E. carotovora and P. fluorescens were significantly inhibited by hexanal at sub-MICs (P < 0.05). There was a positive dose-dependent relationship (r = 0.8167–0.9998) between the inhibition ratio and concentrations of hexanal during the critical biofilm-formation stage (1–3 d), as well as a negative relationship (r = −0.8168 to −0.9998) between the inhibition ratio and culture time. The biomass, as well as average and maximum of biofilm thicknesses, decreased significantly (P < 0.05) relative to those of the control group. Luminescence, which represents the AI-2 molecule, decreased significantly with the increase in hexanal concentration (P < 0.05), indicating that inhibition of motility, exopolysaccharide production, and biofilm formation of E. carotovora and P. fluorescens was related to the decrease in the autoinducer AI-2. Therefore, hexanal could be developed as a new QS inhibitor for preventing decay of vegetables in the food industry. • Biofilm formation was inhibited with good dose-dependent under MIC. • Inhibition occurred significantly in the earlier stage of biofilm formation. • Hexanal inhibited biofilm formation due to the reduction of AI-2. [ABSTRACT FROM AUTHOR]

The effect of 10 essential oil (EO) components on biofilm formation for vegetable spoilage Erwinia carotovora and Pseudomonas fluorescens at sub-MICs were investigated. Chromobacterium violaceum CV026 (CV026) was selected as quorum sensing (QS) biosensor bacterium. The swimming, swarming, twitching motility, exopolysaccharide (EPS) production and biofilm formation were determined. All tested EO components showed QS inhibition of CV026, E. carotovora and P. fluorescens . Biofilms were inhibited by all EO components by inhibiting their motilities and EPS production. Salicylic acid and thymol showed the best inhibitory effect on biofilm formation of P. fluorescens (37.61%) and E. carotovora (47.24%) respectively. Hexanal as QS inhibitor for biofilm formation was first reported, with inhibition ratio of 25.86% and 20.18%, which was confirmed by scanning electron and fluorescence microscopy. Inhibition of biofilm formation by anti-QS EO components could be novel intervention strategy to enhance safety and quality of vegetables in the food industry. [ABSTRACT FROM AUTHOR]

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. • Pseudomonas fluorescens biofilm was treated by ultrasound at different intensity levels. • Molecular and proteomic response of P. fluorescens biofilm was revealed. • Intensity >21.06 W/cm2 efficiently removed biofilm and suppressed bacterial growth. • P. fluorescens biofilm was stimulated by ultrasound at intensity <18.42 W/cm2. • P. fluorescens initiate stress response and damage repair after ultrasound stimulation. [ABSTRACT FROM AUTHOR]

Pollution of foodborne microbial biofilms is a serious problem in the food industry. Microorganisms in the biofilms become insensitive to environmental stresses and increase tolerance to antimicrobial agents, therefore, making them extremely hard to be inactivated by conventional methods. Ultrasound-involved emerging strategies offer options for effectively controlling the biofilms formed on either food contact surfaces or real foods. This review emphasizes the significances of either ultrasonication alone or combined with other strategies for controlling foodborne microbial biofilms. Ultrasound as an emerging technology would effectively destroy biofilm structure and partially inactivate microorganisms in the biofilms; however, stimulated the growth of microbes may happen after treatment of low-frequency and low-intensity ultrasound. Combined ultrasound (especially low-frequency and high-intensity ultrasound) and chemical disinfectants shows a synergistic effect with a relatively high proportion of inactivated microbes in the biofilms compared with that adopted one strategy alone. Ozone and electrolyzed water are also developed for inactivating microbes and removing the biofilms after combining with the ultrasound. Combined treatment of ultrasonication and chelating agents or enzymes is proved to effectively remove the biofilms instead of achieving a strong bactericidal effect. Mechanical oscillation, localized high temperature and pressure, as well as free radicals generated by cavitation during the ultrasonication can partially destroy the basic structure of biofilms, and furthermore, increase the penetration and diffusion of chemicals into the deeper layer of biofilms for achieving a synergistic effect on the biofilm control. • Ultrasound is used for controlling biofilms formed on foods and food contact surfaces. • Combined ultrasound and other strategies show a synergistic effect on biofilm control. • Mechanical oscillation and cavitation contribute to destruction of biofilm structure. • Bactericidal effect in biofilms depends on intensity and frequency of ultrasound. • Future studies are recommended regarding ultrasound-involved emerging strategies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Combined ultrasonic and thermal treatment against Pseudomonas fluorescens was studied. • P. fluorescens biofilm was efficiently removed by ultrasound with power > 80 W. • Ultrasound with a mild heat at 50 °C synergistically inactivate P. fluorescens cells. • Acoustic pressure and temperature distribution were simulated using COMSOL. • Potential "dead ends" were identified due to reflectivity of the sound waves. 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 log 10 CUF/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. [ABSTRACT FROM AUTHOR]

• Acoustic pressure was simulated as a standardized parameter to reflect sono-effect. • High-intensity ultrasound efficacy against Staphylococcus aureus biofilm was studied. • Bactericidal rate of high-intensity ultrasound (HIU, 20 kHz, 240 W) was limited. • Detachment of S. aureus biofilm was efficient by using the HIU. • HIU (60 W, 15 h) stimulated S. aureus biofilm formation and enhanced its resistance. This study evaluated efficacy of high-intensity ultrasound (HIU) on controlling or stimulating Staphylococcus aureus biofilm. Acoustic pressure distribution on the surface of glass slide cultivated S. aureus biofilm was first simulated as a standardized parameter to reflect sono-effect. When the power of HIU was 240 W with acoustic pressure of −1.38×105 Pa, a reasonably high clearance rate of S. aureus biofilm was achieved (96.02%). As an all-or-nothing technique, the HIU did not cause sublethal or injury of S. aureus but inactivate the cell directly. A further evaluation of HIU-induced stimulation of biofilm was conducted at a low power level (i.e. 60 W with acoustic pressure of −6.91×104 Pa). The low-power-long-duration HIU treatment promoted the formation of S. aureus biofilm and enhanced its resistance as proved by transcriptional changes of genes in S. aureus , including up-regulations of rbf , sigB , lrgA , icaA , icaD , and down-regulation of icaR. These results indicate that the choose of input power is determined during the HIU-based cleaning and processing. Otherwise, the growth of S. aureus and biofilm formation are stimulated when treats by an insufficiently high power of HIU. [ABSTRACT FROM AUTHOR]

This study investigated the synergistic efficacy of combined high-intensity ultrasound (HIU, 20 kHz, 60 W) and chlorine dioxide (ClO 2 , 4 mg/L) on the inactivation of viable cells and clearance of Staphylococcus aureus biofilms followed by comparing with those when treated by either HIU and ClO 2. The bactericidal rate after the ClO 2 treatment at 10 min was 82.62%; however, half of S. aureus remained sublethal or injured instead of fully dead as proved by flow cytometric assay. The bactericidal rate of the HIU treatment was even worse (18.72%). Inactivation rate of viable S. aureus cells after the combined treatment was 99.03% at 10 min and all bacteria were completely inactivated. Therefore, a strongly synergistic effect between HIU and ClO 2 treatment was determined with a synergistic value of 1.24 (>1.15 as the critical limit); and the synergistic effect is mainly attributed to mechanical oscillation and cavitation generated by HIU which promote penetration and diffusion of ClO 2 into the biofilm structure. Furthermore, transcriptional changes of biofilm-related genes in S. aureus indicate that the combined treatment reduced the biofilm formation through mediating quorum sensing (QS) signaling molecules, cell-cell adhesion, and polysaccharide intercellular adhesin (PIA) production; meanwhile, the secretion of enterotoxin in S. aureus was also suppressed after the combined treatment. These findings indicate that the HIU–ClO 2 combination synergistically reduced the number of sublethal cells and efficiently detached the biofilm. Lastly, surface properties, especially roughness of food contact materials showed a significantly negative correlation with the clearance rate of biofilms after the combined treatment. This result suggests that the efficacy of biofilm detachment by using the combined treatment should be further evaluated based on the surface properties of food contact materials. • Staphylococcus aureus biofilm was resistant to chlorine dioxide (ClO 2) at 4 mg/L. • Bactericidal effect of high-intensity ultrasound (HIU, 20 kHz, 60 W) was limited. • Combined HIU and CIO 2 treatment fully inactivated S. aureus without sublethal damage. • Combined treatment reduced biofilm formation and enterotoxin secretion in S. aureus. • Roughness of material is prominent that negatively correlates with biofilm clearance. [ABSTRACT FROM AUTHOR]