Optimization of antibacterial activity of essential oil mixture obtained from three medicinal plants: Evaluation of synergism with conventional antibiotics and nanoemulsion effectiveness

The present study aimed to optimize the antibacterial activity of essential oil (EO) mixture obtained from Ammodaucus leucotrichus Cosson, Thymus vulgaris L. and Lavandula maroccana (Murb.) against three multi-drug resistant bacteria, using augmented simplex-centroid design. GC–MS analysis showed that A. leucotrichus EO was characterized by the abundance of l-perillaldehyde (50.39%), d-limonene (21.61%) and bornyl angelate (7.09%). T. vulgaris EO was dominated by thymol (42.34%), p-cymene (19.71%) and γ-terpinene (11.50%), while carvacrol (71.62%) represents the main component of L. maroccana EO. Regarding antibacterial assays, individual EOs displayed moderate activity with MIC and MBC values ranging from 0.06% to 5.00%, while the coefficient terms of the different tested combinations revealed synergistic interactions. The results of predicted optimization showed that the optimal EOs mixture composed by 55% of T. vulgaris, 41% of L. maroccana and 4% of A. leucotrichus, significantly decreased the MICs against Staphylococcus aureus (0.0225%), Escherichia coli (0.05%) and Pseudomonas aeruginosa (0.688%). The optimal mixture exhibited a significant decrease in the MICs of gentamicin (4-fold) and amoxicillin (2- to 64-fold), and achieve the highest antibacterial effect in nanoemulsion formulation. The optimized EO mixture based on the studied plants should be considered as a potential alternative to control foodborne, and resistant bacteria.