Synergistic Efficacy Of Gallic Acid And Thymol Against Major Foodborn Pathogens On Fresh-cut Tomatoes And Their Antibacterial Mechanism

Although chlorine is one of the most common sanitizers used in the fresh-cut fruits and vegetables industry to remove agricultural debris and reduce the microbial load,formation of carcinogenic by-products increases the health risks to workers.Plant-derived compounds for inactivating pathogenic microorganisms have received renewed attention dur to its safety,efficiency and economy.However,the use of naturally derived antibacterial may exceed acceptable flavour thresholds and the antibacterial mechanism of these natural plants extracts against remains unknown,which restrict its application.The inhibitory effects of five natural agents against E.coli O157:H7 and S.aureus were evaluated by determining the minimum inhibitory concentration（MIC）and the fractional inhibitory concentration index（FICI）.A combination of thymol and gallic acid(S_TGA)showed synergistic effect with a FICI value of 0.5 on both E.coli O157:H7 and S.aureus,under the condition that both compounds are at the concentration of their 1/4 MIC.The growth curve of E.coli O157:H7 and S.aureus confirmed the synergistic effect.With the treatment of S_TSA,proportions of E.coli O157:H7 and S.aureus staining with propidium iodide（PI）also increased compared with thymol and gallic acid alone.Microscopy results showed different degrees of morphological change.The permeability and integrity of the bacterial cell membrane were disrupted treated with S_TGA.Fresh-cut tomatoes as the experimental material were treated with S_TGA under different conditions.S_TGA with a concentration of 1/2MIC+1/2MIC at 40℃for 10min was able to completely inactivate both E.coli O157:H7 and S.aureus incubated artificially on fresh-cut tomatoes.The effect of S_TGAon texture,color,Vc and total phenols during storage are evaluated on fresh-cut tomatoes.The results showed that the treatment of S_TGAhad no negative impact on odour and taste of produces compared with the control.In addition,the exposure of S_TGAmaintained the quality of fresh-cut tomatoes and prolonged the storage time.RNA-sequencing transcriptomic was employed to determining the adaptive strategies of foodborne pathogens to food.The antibacterial mechanism of S_TGA against E.coli O157:H7was also elucidated.A total of 1551 differentially expressed genes were identified in E.coli O157:H7 subjected to fresh-cut tomatoes.In order to survive on fresh-cut tomatoes,E.coli O157:H7 increased expression of genes associated with ribosome biosynthesis,molecular chaperones,stress proteins,and two-component regulatory systems.The fresh-cut tomato culture also displayed characteristics of anaerobic metabolism to reduce energy consumption,and activated acid resistance system to maintain p H homeostasis within cells.A total of 533differentially expressed genes were identified subjected to Treatment-1 vs Treatment-2.S_TGAtreatment inhibited tryptophan biosynthesis,and induced the dysfunction of the membrane,causing the changes in membrane permeability and integrity.Moreover,the S_TGA treatment also induced oxidative stress and the chaperones that help refold proteins complexes are depressed.Genes related to the transcription,translation and repair of DNA are downregulated.This research provides fundamental information on the antibacterial mechanism of S_TGAagainst E.coli O157:H7.The S_TGA antibacterial agent for control foodbome pathogens on fresh-cut produces was developed,which can effectively control the infection of foodbome pathogens and maintains the sensory quality of fresh-cut fruits and vegetables during the process of packaging,storage,logistics and sales.