MESABIH MounaBENYOUCEF FatimaRAHMANI Khaled2026-09-202026-09-202026https://dspace.univ-temouchent.edu.dz/handle/123456789/7654This work aims to study the antimicrobial activity of individual essential oils as well as cross- mixtures of three aromatic medicinal plants: rosemary (Rosmarinus officinalis L.), common sage (Salvia officinalis), and bay laurel (Laurus nobilis), whose essential oils were extracted by hydrodistillation. The antimicrobial activity revealed variable results between individual essential oils and cross- mixtures, with marked differences in efficacy depending on the type of target microbe and the nature of the composition used. Bay laurel oil proved to be the most effective against Gram-negative bacteria, particularly against Escherichia coli, with good activity against Gram-positive bacteria. Rosemary oil showed overall moderate activity with relative distinction against yeasts. In contrast, sage oil exhibited generally limited efficacy, with moderate activity only against Staphylococcus aureus. Binary and ternary compositions showed complex interactions ranging from strong synergy to marked antagonism. The binary mixture (rosemary + bay laurel) achieved the strongest synergy against most of the studied microbes, significantly surpassing the individual activity of each oil. The binary mixture (rosemary + sage) also recorded a strong and unexpected synergy against Klebsiella pneumoniae, while showing pronounced antagonism against other microbes. As for the ternary mixture, it yielded varied positive results with moderate to strong synergy against the majority of microbes. These results show that bay laurel oil is distinguished by high efficacy against Gram-negative bacteria, while rosemary oil stands out for its activity against yeasts. The study also confirms that cross-mixtures, particularly the binary mixture (rosemary + bay laurel), can generate strong synergy exceeding the individual activity of the oils, thus opening promising perspectives for the development of natural antimicrobial formulations. However, the appearance of antagonistic interactions in certain compositions calls for caution in the selection of mixtures and invites a deeper study of the chemical and biochemical mechanisms responsible for these interactions.frCross-mixtureshydrodistillationAntimicrobialSynergistic.essential oilsThesis