Inactivation of Staphylococcus aureus using ultrasound in combination with thyme essential oil nanoemulsions and its synergistic mechanism

The present study investigated the antimicrobial effect and mechanism of ultrasound (US) with thyme essential oil nanoemulsions (TEON) on the inactivation of Staphylococcus aureus (S. aureus) cells. A remarkable increase in reducing the population of S. aureus was observed when cells were subjected to combined US (3.33 W/mL, 3 min) and TEON (0.375 mg/mL) treatment, which exhibited synergistic inactivation efficiency and caused a 3.21-log reduction in viability. The most severe damage to cell morphology was visualized in US + TEON group using scanning electron microscopy. Other proposed mechanisms involved that ultrasound reduced the membrane hydrophobicity, which increased the exposure of cell membrane to TEON. Moreover, the greatest degree of membrane permeabilization was evaluated, along with the massive leakage of intracellular contents. Depolarization in membrane potential was also observed and confirmed by the excessive outflow of K+ and influx of Ca2+. On the other hand, a significant decrease in membrane fluidity was monitored and further verified by the modifications in bacterial membrane fatty acid profile. This study also highlighted the impact of US + TEON treatment on bacterial membrane compositions involving proteins, nucleic acids, and phospholipids by infrared spectroscopy. Current study provides a proof-of-concept into the synergistic mechanisms of the integrated technology for enhancing S. aureus inactivation.

Automated summary

The study demonstrated the synergistic antimicrobial effect of ultrasound combined with thyme essential oil nanoemulsions on the inactivation of Staphylococcus aureus. The mechanisms involved changes in membrane properties, increased exposure to essential oil, and alterations in membrane fatty acid profile. This integrated technology effectively enhanced bacterial inactivation by targeting membrane compositions.