Development of novel and effective antibacterial technologies to ensure the microbiological safety of food items has gained considerable attention, mainly driven by numerous outbreaks. Essential oil nanoemulsions have been proven to have stronger antimicrobial effects compared to the essential oil alone or coarse emulsion. Sonoporation could be the promising candidate to trigger a synergistic effect with thyme essential oil nanoemulsion (TEON) and produce a more effective antibacterial efficacy. Therefore, in our studies, the bactericidal effects of ultrasound (US) in combination with TEON treatments against Escherichia coli (E. coli) O157:H7 and Staphylococcus aureus (S. aureus) were investigated in both liquid simulated system and real food matrix. The optimal ultrasonication and emulsifier concentration conditions for the preparation of three different TEON with enhanced dispersion characteristics and antibacterial activity were obtained by response surface methodology. The results showed that TEON-CPC obtained at 350 W, 14.44 min, and 0.15 mg/mL exhibited excellent antibacterial activity through electrostatic interaction. Afterward, the optimal TEON was selected for the further investigation of antimicrobial effect and mechanism of its combination with US on the microbial inactivation. The combined treatment reduced the E. coli and S. aureus population by 2.16-7.10 and 3.21-4.51 log CFU/mL, respectively, indicating the effects of ultrasonic field on facilitating the antibacterial efficacy of TEON. Moreover, remarkable synergistic effect of the combined treatment was also achieved, which decontaminated the E. coli populations by 4.49–6.72 log CFU/g on the surface of cherry tomatoes. As for the antibacterial mechanism, the combined treatment led to drastically larger extent of membrane disintegration, fluidity reduction, membrane depolarization, and membrane composition modification than the single use of ultrasound and TEON. It could be concluded that US could induce physical damage on cell morphology, facilitating the entry of TEON into the bacteria; reduce the bacterial membrane hydrophobicity that would increase the odds of TEON interacting with the cellular membrane. Overall, US combined with TEON was proved to be a promising and feasible alternative for the reduction of microbiological contaminants in food industry.

Mingming Guo

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