Antibacterial and antioxidant activities in Sideritis italica (Miller) Greuter et Burdet essential oils

Abstract

Sideritis italica (Miller) Greuter et Burdet is a widespread Lamiacea in the Mediterranean region used in traditional medicine. Essential oils were antibacterial against nine ATCC and as many clinically isolated Gram-positive and Gram-negative bacterial strains. Antibacterial activity was also found against Helicobacter pylori: a dose-dependant inhibition was shown between 5 and 25 g/ml. The antibacterial activity of the oils was expressed as MICs (minimum inhibitory concentrations) and MBCs (minimum bactericidal concentrations). At a concentration between 3.9 and 250 µg/ml the oils showed a significant antibacterial effect against both Gram-negative and Gram-positive bacteria. In particular the ATCC strains Pseudomonas aeruginosa (MIC = 3.9 µg/ml and 7.8 for flowerheads and leaves, respectively), Proteus mirabilis (MIC = 15.6 and 7.8 µg/ml), Salmonella typhi (MIC = 7.8 µg/ml) and Proteus vulgaris (MIC = 15.6 µg/ml) were the most inhibited. Only Pseudomonas aeruginosa showed MBC at a concentration between 62.6 and 125 µg/ml. The antioxidant activity of the essential oils was evaluated by two cell free colorimetric methods: ABTS and DMPD; leaf oil is more active (4.29±0.02 trolox equivalents and 4.53±0.67 ascorbic acid equivalents by ABTS and DMPD, respectively). Finally the antioxidant activity of the essential oils was also evaluated by their effects on human whole blood leukocytes (WB) and on isolated polymorphonucleate (PMN) chemiluminescence. Comparing the effects of the oils from leaves and flowerheads on both PMN and WB chemiluminescence emission, we found no significant differences. Essential oils showed a dose-dependent and linear inhibitory activity on isolated PMN as well as on WB CL emission when PMA-stimulated. On the contrary, the inhibitory activity on resting cells was nonlinear. Our data represent an answer to the continual demand for new antibiotics and antioxidants for the continuous emergence of antibiotic-resistant strains and the growing interest in the substitution of synthetic antioxidants with natural ones. Furthermore, our finding of antibacterial and antioxidant activities in the essential oils from Sideritis italica flowerheads and leaves validated the use of the plant for medical purposes.