Mixture-efficient identification of high-order herbal essential oil synergy using a normalized DiaMOND design

Abstract

In this study, the effect of high-order combinations of Rosmarinus officinalis L. (R) EO with other essential oils obtained from selected Moroccan medicinal and aromatic plants, namely, Thymus satureioides Coss. (T), Origanum compactum Benth. (O), Pelargonium roseum L. (P), Artemisia herba-alba Asso. (A), and Salvia officinalis L. (S), was evaluated. The chemical composition of EOs was determined by GC/MS. Their antibacterial effect was determined using the microdilution method, whereas their antioxidant activity was evaluated by the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging and ferric reducing antioxidant power (FRAP) methods. The type of interaction between EOs was evaluated by the diagonal measurement of n-way drug interactions (DiaMOND). Antioxidant and antibacterial activity results demonstrate that O. compactum, T. satureioides, and P. roseum EOs tested alone showed significant efficacy. Using Principal Component Analysis (PCA) and hierarchical clustering analysis (HCA) modeling, one three-way combination (RST) was found with particularly strong synergistic DPPH radical scavenging (FIC3 = 0.50) and Fe3+ reducing (FIC3 = 0.48) properties. This combination also produced the highest activity against four bacteria strains (FIC3 = 0.38 for S. aureus, FIC3 = 0.38 for E. faecalis, FIC3 = 0.44 for E. coli, and FIC3 = 0.79 for P. aeruginosa). This mixture was, therefore, combined with conventional antibiotics. The results of all combinations showed a synergistic effect against four bacteria strains, which were tested with FIC2 ranging between 0.11 and 0.97. Our findings showed that a three-way combination of RST EOs constitutes a promising approach for possible application in the food and pharmaceutical industries.