Essential oils (EOs) and their active compounds are gaining interest as alternatives to synthetic antibiotics because of their antimicrobial activity [1]. Unfortunately, EOs have several drawbacks, such as low solubility in the aqueous phase and instability. [2]. The encapsulation of EOs in suitable drug delivery systems, such as nanoemulsions, is an appropriate way to enhance their bioavailability and increase their biological activities [3]. In this study, a stable nanoemulsion of cinnamaldehyde and 1,8-cineol was prepared using a sonicator. In continue, the antibacterial activity of the stable nanoemulsion was assessed against E. coli, P. aeruginosa and S. aureus using broth dilution assay. In addition, the synergistic effects of the stable nanoemulsion and traditional antibiotics (chloramphenicol, amoxicillin, and azithromycin) were analyzed and the fractional inhibitory concentration index (FICI) was determined. Our results revealed that the stable nanoemulsion of cinnamaldehyde and 1,8-cineol (particle size = 27.76 ± 0.37 nm) had noticeable antibacterial activity against selected microorganisms with the minimum inhibitory concentrations (MICs) values of 1,1 and 2 mg/ml against E. coli, P. aeruginosa, and S. aureus, respectively. Calculation of the FICI revealed a partial synergy between the nanoemulsion and azithromycin against E. coli and P. aeruginosa. This synergistic effect between nanoemulsions and traditional antibiotics is a potential option for improving the effectiveness of antimicrobial agents.
