Unravelling the antibacterial properties and mechanisms of action of essential oil nanoemulsions: A literature review

The rising demand for natural antimicrobials for application as alternatives to synthetic chemicals has spurred significant interest in essential oils (EOs) formulation. However, the practical application of EOs is hampered by varied limitations, including high volatility, poor solubility in water, and sensitivity to degradation. Nanoemulsion (NE) technology has emerged as a promising delivery system to overcome these challenges and enhance the antibacterial efficacy of EOs. This review comprehensively examines the formulation and mechanisms of action (MsOA) of EO-based NEs. We detail two primary preparation methods: high-energy (HE) techniques, often favored in food-grade applications for their lower surfactant requirements, and low-energy (LE) methods, prized for their efficiency and lower instrumental complexity. The core of this review emphasizes the multifaceted antibacterial mechanisms of EO NE. The nanometric droplet size enhances antibacterial activity not by altering the chemical composition of the EOs, but by profoundly improving their physical properties and interactions with bacterial cells. Key mechanisms discussed include: (1) the dramatic increase in surface area for efficient cell membrane (CM) disruption; (2) enhanced cellular uptake through porin channels or passive absorption, driven by an improved concentration gradient; (3) improved solubilization and targeted delivery of hydrophobic compounds; and (4) enhanced disruption of cell membrane integrity via the “like dissolves like” principle. Smaller NE droplets prove more effective against bacteria. Additionally, their range of droplet sizes enhances versatility, enabling it to inhibit diverse bacteria. The antibacterial action is further elaborated as a combination of primary mechanisms, such as membrane disintegration leading to content leakage and inhibited biofilm formation, and secondary mechanisms, including the induction of oxidative stress, metabolic interference, disrupting efflux function, targeting ribosomal assembly, and inhibition of gene expression and virulence factors. The potential for synergistic effects with conventional antibiotics is also highlighted.

In conclusion, NE formulation significantly boosts the antimicrobial potency of EOs by enabling controlled release, enhancing bioavailability, and promoting targeted delivery. Given their broad-spectrum activity, ability to combat biofilms, and potential to counteract multidrug-resistant pathogens, essential oil NE represent an innovative and viable strategy across diverse fields, including human and veterinary medicine, agriculture, and food safety.