| Salmonella is a foodborne pathogenic microorganism commonly found in dairy,meat and egg products,and foodborne illnesses caused by Salmonella pose a significant health risk to the general population.Research into effective techniques for the control of Salmonella in food has become a focus of interest.Amongst other things,research has found that ultrasonic sterilisation has shown its superiority over traditional thermal sterilisation techniques in terms of energy savings,efficiency and flavour retention.However,ultrasound treatment has been found to have limited sterilisation effects,is not effective in ensuring food safety,and has high equipment and energy requirements.This further limits the further processing and application of ultrasonic sterilisation technology in practical food processing.Based on this,this study investigates the physiological metabolism of Salmonella cells under ultrasound treatment to reveal the mechanism behind ultrasound treatment and its effect on the inhibition of Salmonella by chlorine dioxide.The main findings are as follows:Firstly,the survival mechanism of Salmonella under high intensity ultrasound(HIU)treatment was investigated.It was found that high intensity ultrasound reduced the number of culturable cells of Salmonella to some extent.Preliminary results from electron microscopy indicate that high intensity ultrasound causes damage to the cellular structure of Salmonella and even disintegration,while the increase in conductivity values(13.47-18.10%)and the increase in protein content(27.62-38.30 fold)in the supernatant of the bacterial suspension further indicate that high intensity ultrasound can cause damage to the cellular structure of Salmonella and consequently leakage of intracellular material.In addition,high-intensity ultrasound also caused abnormal energy metabolism in bacterial cells,in which citrate synthase and malate dehydrogenase,key proteins of the TCA cycle,were significantly downregulated after high-intensity ultrasound,resulting in the blockage of intracellular ATP accumulation,which in turn affected the repair of stress damage and substance uptake in Salmonella cells,leading to the gradual death of bacterial cells.Finally,surviving Salmonellae may be enhancing their adaptation to adverse stress by upregulating their environmental perception(two-component system),chemotaxis(bacterial chemotaxis),substance uptake(ABC transporter)and ATP production(oxidative phosphorylation)to high intensity ultrasound.The effect of low-intensity ultrasound treatment on the physiological state and physiological regulatory processes of Salmonella was further investigated.It was found that both treatments,probe ultrasound(US)and water bath ultrasound(WUS),disrupted the cellular structure of Salmonella,increased the permeability of its bacterial outer membrane(US: 9.00%,WUS: 11.96%)and caused the accumulation of intracellular reactive oxygen species(US: 13.95%,WUS: 4.34%).It also caused a decrease in intracellular ATP(US:15.22%,WUS: 14.15%)and ATPase activity(US: 3.13%,WUS: 26.06%).This series of adverse effects leads to the disruption of the material and energy metabolic processes of Salmonella cells,which in turn affects the normal physiological state of the bacterial cells.At the same time,the bacterial cell also undergoes a complex metabolic regulation to effectively cope with the adverse effects of ultrasound,such as lipid metabolism,small molecule metabolism and energy metabolism,which may be the reason why neither of the two low intensity ultrasounds resulted in a significant reduction in the number of culturable cells of Salmonella.Finally,the potential mechanisms by which low-intensity ultrasound enhanced the effectiveness of chlorine dioxide in inhibiting Salmonella were explored in depth.Electron microscopy results showed that low intensity ultrasound enhanced the damage to Salmonella cell membrane structure by chlorine dioxide,which in turn led to a further increase in the permeability of the bacterial outer cell membrane(CLO2: 31.23%,CLO2+WUS: 66.25%)and a further increase in conductivity in the supernatant of the bacterial suspension(CLO2:6.57%,CLO2+WUS: 10.28%).It was also found that low-intensity ultrasound enhanced the dysregulation of energy supply within Salmonella cells following chlorine dioxide treatment,where the combined treatment resulted in a further reduction in ATP content in bacterial cells(CLO2: 78.63%,CLO2+WUS: 91.66%),as well as a significant decrease in ATPase activity(CLO2: 14.32%,CLO2+WUS: 38.06%).In addition,metabolomic findings suggest that ultrasound further exacerbates the disruption of important physiological processes such as small molecule metabolism and nucleotide metabolism in Salmonella cells by chlorine dioxide.This study investigates the inhibition effect and mechanism of different conditions of ultrasound alone and in combination with chlorine dioxide on Salmonella through proteomic and metabolomic techniques,further exploring the specific effect and application potential of ultrasound against Salmonella,and thus providing a theoretical and practical basis for the expansion of combined ultrasound decontamination technology to more application scenarios. |
