Antioxidative activity and synergistic effect of Thymus saturejoides Coss. essential oils with cefixime against selected food-borne bacteria

Thymus saturejoides is a perennial shrub widely distributed in the arid and semi arid parts of the Moroccan mountains. Essential oils (EOs) of this species are used extensively in food and pharmaceutical industries because of their wide biological and pharmacological properties. To our knowledge, no data are available on the antibacterial and antioxidant activities of T. saturejoides EOs in relation to the chemical variability of the species. Also, the potential synergistic interaction of EOs from this species as natural antimicrobial agent with conventional antibiotics against food-borne bacteria has not been investigated. Thus, the aim of the present study was to determine the volatile oil constituents of EOs from three wild Moroccan T. saturejoidespopulations collected from Northwest–Southeast of Morocco (June, 2011). Antioxidant activity and their antibacterial potency (singly and in combination with cefixime) against selected food-borne bacteria was then determined. Gas chromatography–mass spectrometry (GC–MS) analysis revealed 30 compounds, representing 98.6 to 99.5% of the total oils. The major constituents identified were carvacrol (45.3%), p-cymene (8.9%), linalool (8.4%), and borneol (7.5%) in oil from the Er-Rich population (arid site); carvacrol (26.5%), borneol (20.1%), camphene (8.0%), and γ-terpinene (5.6%) in oil from Ourika population (medium arid site) and carvacrol (25.3%), borneol (19.7%), camphene (7.6%), and p-cymene (6.6%) in oil from Taws population (less arid site). Essential oil obtained from Er-Rich population showed the highest antioxidant activity as measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging ability, reductive potential, and β-carotene/linoleic acid assays with IC₅₀ values of 44.54 ± 0.92 μg/mL, 22.90 ± 0.16 μg/mL and 19.17 ± 0.01 μg/mL, respectively. Antibacterial tests showed that the oils from this species had a high inhibitory activity against tested bacteria, with the exception of Pseudomonas aeruginosa. The most potent activity was observed with oil obtained from the Er-Rich population with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of 0.14 to 0.55 mg/mL and 0.28 to 0.55 mg/mL, respectively. Out of 21 combinations tested between EOs and cefixime, 67% showed total synergism, 19% had partial synergistic interaction and 14% showed no effect. Oil from the Er-Rich population exhibited the highest synergistic effect with antibiotic (FIC index values of 0.29 to 0.5). The synergy displayed by this combination of EO and cefixime may be of potential benefit for control of food-related bacteria, consequently permitting the use of lower doses of standard antibiotics.