Enhanced synergistic antioxidant and antibacterial effects of stable
nanoemulsions prepared from blends of thyme, black pepper, cinnamon, and
garlic essential oils.
Abstract
The emergence of multidrug-resistant bacteria has increased the need for
potent antibacterial agents. Essential oils (EO) are promising
alternatives; however, their volatile, unstable, and hydrophobic nature
challenges their bioavailability. In this study, we describe enhanced
synergistic antioxidant and antibacterial effect of stable oil-in-water
(O/W) nanoemulsions (NE) formulated from blends of thyme, cinnamon bark,
black pepper, and garlic essential oils, using a high-energy
ultrasonication incorporating a surfactant (tween 80, HLB=15) and
co-surfactant (glycerol). Average droplet size of ~44-88
nm and polydispersity index of 0.2-0.3 were determined using dynamic
light scattering. The spherical shape and size of the nanoemulsions were
confirmed by transmission electron microscopy. The nanoemulsions were
thermodynamically stable up to three months after the heating-cooling
cycle (40↔ 4°C) and freeze-thaw cycle (-21°C) respectively. GC-MS and
FT-IR analysis identified structures and functional groups of diverse
volatile components. DPPH antioxidant assay demonstrated
>16-fold enhanced free radical scavenging capacity of NE
compared to blended EO and pure oils; ABTS assay exhibited similar
trend. Antibacterial effects was enhanced, with MIC values halved when
the blended EO was nano-encapsulated, against a resistant
extended-spectrum lactamase (ESBL) strain of K. pneumoniae from
1.25 to 0.625 mg/ml , and 0.625 to 0.312 mg/ml for E. coli,
S. aureus and S. typhimurium, while it remains unchanged for
P. aeruginosa (1.25 mg/ml). Scanning electron microscopy
highlighted significant alteration in treated bacteria’s membrane
integrity. Therefore, we have successfully formulated stable
nanoemulsions exhibiting enhanced synergistic antioxidant and
antibacterial activities with potential applications in the food and
feed industry.