Blended formulations of oregano-sage essential oils: antimicrobial, phytotoxic, and anti-quorum sensing investigations

There is a growing interest in the potential use of essential oils (EOs) as a possible alternative to synthetic pesticides. The formulation of bioinsecticides with different EOs could improve their bioactivities through synergic mechanisms. This study aimed to evaluate the biological activities of three blended oil formulations (BOFs) derived from oregano (Origanum vulgare L.) and sage (Salvia officinalis L.). The chemical composition of the individual EOs was investigated using GC-MS analysis. The BOFs were prepared as follows: i) 25% oregano EO + 25% sage EO (BOF-I); ii) 25% oregano EO + 5% sage EO (BOF-II); iii) 5% oregano EO + 25% sage EO (BOF-III). The BOFs were tested for their phytotoxic effects on Lepidium sativum, Solanum lycopersicum, and Lactuca sativa as well as their antimicrobial activity against some phytopathogens. The tested BOFs were evaluated for their possible anti-quorum sensing activity against Chromobacterium violaceum Schröter. GC-MS analysis revealed that the oregano EO is mainly composed of thymol (76%), p-cymene (5.7%) and carvacrol (3.2%). Whereas the dominant constituents of sage EO were transthujone and camphor. The results demonstrated that all tested BOFs possess an antimicrobial effect higher than each parent EO. In particular, BOF-II showed the highest effect against all tested bacteria and fungi. In addition, the three BOFs showed notable phytotoxic effects against all tested plants, particularly BOF-I. Whereas, the single sage EO at 25% showed the lowest significant phytotoxic effect, indicating its possible use as a natural herbicide. All examined BOFs showed promising quorum quenching activity against C. violaceum, especially at a concentration of 100%.