Combined antibacterial effect of essential oils from three Indian medicinal plants and antibiotic tetracycline on...

Methicillin-resistant Staphylococcus aureus (MRSA) is a major global pathogen, recognized for its high level of antimicrobial resistance. Essential oils have been extensively studied in both observational and clinical research for their promising antimicrobial properties. This study aimed to characterize the essential oils Satureja montana (SMEO), Syncarpia glomerulifera (SGEO), and Valeriana officinalis (VOEO) using GC-MS analysis and their synergistic antibacterial activity in combination with tetracycline evaluated through an augmented simplex centroid mixture design. Mixture design is a statistical phenomenon which is applied to obtain the maximum desired outcomes through minimizing number of experiments. GC-MS identified major components such as benzene, 4-ethyl-1,2-dimethyl (15.70%) in SMEO, copaene (20.95%) in SGEO, and β-maaliene (12.19%) in VOEO. SMEO exhibited the highest antioxidant activity (IC₅₀ 2.68 µL/mL) followed by VOEO and SGEO in DPPH assay. Antibacterial activity of EOs against MRSA showed in terms of zone of inhibition (ZOI) and minimum inhibitory concentrations (MIC), where SGEO showed strong inhibition against SACI-4 with ZOI;14 ± 1.7 mm, and MIC 25 μL/mL) whereas, VOEO demonstrated weaker activity against SACI-4 with ZOI up to 11.66 ± 0.57 mm, MIC 25 μL/mL). PCA correlated the antibacterial activity against MRSA with the chemical composition of the essential oils. Further, mixture design represents coefficients such as β₁₄, β₃₄, and β₂₃₄ which represent interaction effects between the respective components. These indicate how combinations of variables influence the response beyond their individual effects. Results showed the most effective binary combinations against MRSA, excluding β₁₄ (SGEO-TET) and β₃₄ (SMEO-TET), while the ternary combination of VOEO, SMEO, and TET (β₂₃₄) had a highly significant effect on SACI-1 (p = 0.0001). These findings suggest tested essential oils as potential natural sources of antimicrobials against the MRSA. Their combinations with antibiotics can be used to develop a formulation which could be useful in biopharmaceutical products and combat the rise of bacteria resistance.