The problem the researchers set out to solve is a familiar one in the age of antibiotic resistance. Staphylococcus aureus is a common resident of the human nasal cavity and skin, but when it colonizes persistently, it can drive severe chronic rhinosinusitis, a debilitating inflammation of the nasal passages and sinuses. Worse, the bacterium increasingly shrugs off frontline drugs such as methicillin and amoxicillin, and it has a second line of defense: the ability to grow in aggregated communities called biofilms, which are sheathed in protective extracellular polymers. These biofilms can tolerate antibiotics at concentrations up to 1,000 times higher than what kills free-floating planktonic cells, making chronic infections notoriously difficult to eradicate and frequently leading to treatment failure.

Essential oils, the concentrated aromatic extracts of medicinal and aromatic plants, have long been known to possess antimicrobial properties. Roughly 85 percent of traditional crude medicines are plant-derived, and around 60 percent of the world’s population still relies on plant-based remedies for primary healthcare. Yet essential oils suffer from a fundamental chemical handicap: they are highly hydrophobic, meaning they dissolve poorly in water, permeate cells inefficiently, and show low bioavailability. In laboratory tests, their high viscosity and poor miscibility in growth media can even lead to inaccurate assessments of their true antimicrobial power. This solubility barrier has kept many promising plant oils out of serious clinical consideration despite centuries of traditional use.

The team, led by researchers affiliated with the University of Adelaide and the Basil Hetzel Institute in South Australia, focused on three plants with rich ethnobotanical histories. Eremophila mitchellii, known as false sandalwood, was used by indigenous Australians to treat colds, fevers, and rheumatism. Artemisia vulgaris, or mugwort, and Rhododendron anthopogon have been staples of Himalayan and traditional herbal medicine, valued for their antimicrobial, anti-inflammatory, and respiratory benefits, with the rhododendron oil traditionally serving as a cough medicine. Oils were sourced from Nepal and South Australia and formulated into oil-in-water nanoemulsions using the FDA-approved surfactant Pluronic F127 and the cosurfactant propylene glycol, with probe sonication producing droplets small enough to behave very differently from bulk oil.

The physicochemical characterization revealed just how successful the formulation was. All three nanoemulsions displayed average droplet diameters below 240 nanometers, with the Eremophila formulation measuring around 230 nanometers, the Artemisia around 217 nanometers, and the Rhododendron around 193 nanometers. Polydispersity index values, a measure of size uniformity, all came in below 0.3, indicating well-distributed droplet populations. Crucially, the nanoemulsions remained physically stable for up to six months of refrigerated storage, with minimal changes to their size, charge, and uniformity. This shelf stability matters enormously for any future pharmaceutical application, since many colloidal formulations degrade or separate within days.

The antibacterial results were striking. Against the standard laboratory strain and two multidrug-resistant clinical isolates recovered from chronic rhinosinusitis patients, the crude essential oils required concentrations ranging from 16 to more than 512 milligrams per milliliter to inhibit growth. Once nanoemulsified, those requirements plummeted. The Eremophila mitchellii nanoemulsion proved the star performer, inhibiting bacterial growth at concentrations of 1 milligram per milliliter or less, an improvement of at least sixteen-fold over the raw oil. The Rhododendron nanoemulsion showed minimum inhibitory concentrations of 4 to 8 milligrams per milliliter, while the Artemisia formulation required somewhat higher doses. Notably, the blank emulsion containing only the surfactants showed no antibacterial activity whatsoever, confirming that the enhancement came from better delivery of the oils themselves rather than the excipients.

Against established biofilms, the nanoemulsions delivered the study’s most dramatic results. When the Eremophila nanoemulsion was applied at 10 milligrams per milliliter, bacterial viability within biofilms dropped by 3.5 to 8.3 orders of magnitude depending on the strain, with complete eradication observed in some clinical isolates. The Artemisia and Rhododendron nanoemulsions each achieved reductions of roughly three to four orders of magnitude. Confocal microscopy using live-dead staining confirmed the dose-dependent kill, showing that at 10 milligrams per milliliter of the Eremophila nanoemulsion, no living cells remained in the biofilm. Scanning electron microscopy went further, revealing biofilm matrices torn apart and individual staphylococcal cells visibly distorted, with damaged cell surfaces and walls, in stark contrast to the plump, round morphology of untreated control bacteria.

The mechanism, the researchers propose, hinges on physics as much as chemistry. The ultrafine droplets of a nanoemulsion carry an enormous surface-to-volume ratio, presenting far more antimicrobial molecules to the bacterial cell wall than a viscous droplet of bulk oil ever could. The team suggests that while crude oil can only attack the top layer of a biofilm where bacteria make direct contact, nanoemulsified active compounds can diffuse through the biofilm matrix and reach bacteria buried deep within. The negatively charged droplets may also fuse electrostatically with bacterial membranes, destabilizing them and disrupting metabolic processes. The Eremophila nanoemulsion carried the strongest negative zeta potential at minus 18.4 millivolts, which the authors suggest may partly explain its superior antibiofilm performance. Because essential oils contain dozens of compounds acting in synergy, improved solubility likely raises the delivered concentration of multiple bioactive constituents simultaneously.

Safety data offered cautious encouragement. Using an MTT assay on human foreskin fibroblast cells, the team found that the Artemisia nanoemulsion maintained more than 90 percent cell viability even at 64 milligrams per milliliter, well above its effective antibacterial dose, and retained over 60 percent viability at double that concentration. The Eremophila and Rhododendron nanoemulsions showed dose- and time-dependent toxicity, with viability falling below 60 percent at their higher test concentrations, a signal that dosing windows will need careful optimization. The authors also note a technical caveat: some essential oil components may interfere with the MTT dye itself, potentially producing false toxicity readings, so ex vivo and in vivo studies will be essential to establish the true safety profile of these formulations.

The implications reach well beyond sinusitis. The researchers argue that nanoemulsification overcomes the core limitations that have sidelined essential oils, including poor solubility, high viscosity, and inconsistent biological activity, while preserving the synergistic interplay of their complex chemistry. All excipients used are FDA-approved food additives, and the formulation process is straightforward enough to scale. If future animal and clinical studies validate these in vitro findings, nanoemulsified plant oils could emerge as topical or localized treatments for biofilm-driven infections of the skin, wounds, and airways, offering a non-antibiotic weapon in the fight against resistant bacteria while opening new markets for producers of traditional medicinal oils around the world.

Subject of Research: Nanoemulsion-enhanced antibacterial and antibiofilm activity of plant essential oils against multidrug-resistant Staphylococcus aureus

Article Title: Nanoemulsification Promotes Antibacterial Activity of Selected Essential Oils Against Multidrug‐Resistant Staphylococcus aureus Biofilms In Vitro

Article References: Karki, S., Ramezanpour, M., Cooksley, C. M., Barry, E., Prestidge, C. A., Nepal, R., Feizi, S., Correll, D., Wormald, P.-J., Vreugde, S., & Awad, M. (2026). Nanoemulsification Promotes Antibacterial Activity of Selected Essential Oils Against Multidrug‐Resistant Staphylococcus aureus Biofilms In Vitro. MicrobiologyOpen, 15(5), Article e70407. https://doi.org/10.1002/mbo3.70407

Image Credits: AI Generated

DOI: 10.1002/mbo3.70407

Keywords: nanoemulsion, essential oils, Staphylococcus aureus, biofilm, antimicrobial resistance, chronic rhinosinusitis, Artemisia vulgaris, Eremophila mitchellii, Rhododendron anthopogon, nanotechnology, cytotoxicity, drug delivery