Ultrasonication induced nano-emulsification of thyme essential oil: Optimization and antibacterial mechanism against Escherichia coli

This study optimized the ultrasound emulsification process using response surface methodology (RSM) to obtain thyme essential oil nanoemulsions (TEON) with excellent dispersion characteristics and antibacterial activity. Based on the single-factor experiments, the effects of ultrasonic power (350–550 W), time (5–15 min) and emulsifier [Tween 80 (T80), sodium dodecyl sulfate (SDS), and cetylpyridinium chloride (CPC)] concentrations were determined on response variables, including droplet size, PDI, zeta-potential, MIC, and MBC. Further analysis of stability and antibacterial mechanism were also conducted based on the optimal results. TEON-T80, obtained at 527.45 W, 9.97 min, and 5.94 mg/mL with good stability, killed E. coli through extracellular release, whereas TEON-CPC obtained at 350 W, 14.44 min, and 0.15 mg/mL exhibited excellent antibacterial activity through electrostatic interaction. TEON-SDS obtained at 350 W, 15 min, and 1 mg/mL was poor in stability and antibacterial activity. All nanoemulsions destroyed cell morphology and caused severe cytoplasm leakage. Through a preliminary antibacterial mechanism study, we hypothesized a controlled in vitro release for TEON-T80, electrostatic interaction for TEON-CPC, and self-assembly together with cell penetration for TEON-SDS. Considering all the results, TEON-CPC was determined as the best nanoemulsion for optimal antibacterial activity. Also, optimizing the ultrasonication process is useful for preparing TEON with enhanced dispersion characteristics and antibacterial activity.