Antimicrobial Activity of Formulated Origanum and Thyme Essential Oil Nanoemulsions - A Comparative Study

Introduction: This study focused on the formulation processing and antimicrobial activities of Origanum and Thyme essential oils (OEO) (TEO) and their nanoemulsions (OEON) (TEON) on gram-positive and gram-negative bacteria and yeast. Optimal nanoemulsion formulation and stability were achieved through balancing between requisite hydrophilic-lipophilic balance (rHLB) and surfactant concentration (Smix) of nanoemulsions. Methods: The smallest droplet sizes of OEON: (z -185.1 nm +/- 0.85) and TEON (z -130.1 nm +/- 0.60), were achieved by using 10 g/100 mL of OEO and TEO with 15 g/100 mL Smix at ultrasonication for 2 min with no phase separation. The size of nanoemulsion droplet and PDI was found to be influenced by HLB value, treatment time, and Smix concentration. Antimicrobial analyses, including a zone of inhibition, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and time-kill kinetics assay, were performed against Escherichia coli, Staphylococcus aureus, and Saccharomyces cerevisiae. Results: The antimicrobial findings demonstrated that OEO exhibited higher antimicrobial activity compared to TEO (P<0.05). Meanwhile, the OEON and TEON substantially reduced the MIC and MBC values, compared to OEO and TEO against all tested microorganisms (P<0.05). Throughout the time-kill assay, E. coli was reduced by 5 log CFU/ml within 120 minutes, while S. cerevisiae and S. aureus were eradicated after 60 and 120 minutes of incubation with OEON, respectively. Conclusion: In turn, TEON reduced E. coli by 5 log CFU/ml and S. cerevisiae by 4 log CFU/ml within 120 minutes, while S. aureus was inhibited within the same time of incubation with TEON. The nanoemulsion formulations of OEO and TEO considerably enhanced the antimicrobial properties, which provides a promising alternative plant-derived antimicrobial for pharmaceuticals and food applications.