IN VITRO ANTIMICROBIAL AND ANTIOXIDANT ACTIVITIES OF THE ESSENTIAL OILS OF OCIMUM GRATISSIMUM, O. SANCTUM

The growing crisis of antimicrobial resistance (AMR) and consumer interest in natural essential oilshas increased the interest in research on plant-based essential oils. The medicinal plants Ocimum gratissimum(clove basil) and Ocimum sanctum (holy basil/Tulsi) are traditionally venerated but the chemical profile andbioactivity of their essential oils in the biodiversity-rich Western Ghats of North Karnataka have not been studied.To describe volatile chemical composition and measure in vitro antimicrobial and antioxidant activity of essentialoils of O. gratissimum and O. sanctum growing in this understudied area and their key isolated compounds. Theessential oils were obtained through the process of steam distillation of fresh flowering aerial parts. GC and GC-MSwere used to characterize the chemical. Column chromatography was used to isolate the major constituents whichwere eugenol of O. gratissimum and methyl eugenol of O. sanctum, and confirmed by NMR. The tube-dilutionmethod was used to determine minimum inhibitory concentrations (MICs) of 18 microbial strains (Gram-positive,Gram-negative, and fungi) in antimicrobial activity. The antioxidant activity was assessed by the DPPH and ABTSradical scavenging tests. O. gratissimum produced 1.24% (w/w) pale yellow oil that was dominated by eugenol(68.4) and O. sanctum produced 0.98% (w/w) light brown oil that was dominated by methyl eugenol (57.6). The O.gratissimum oil, which is rich in eugenol, exhibited high antimicrobial activity, especially against Gram-positivebacteria (MIC: 0.15-0.62 mg/ml), with purified eugenol being more active (MIC: 0.08-0.31 mg/ml against S. aureusand S. epidermidis). Methyl eugenol was much less efficient (MIC: 0.622.50 mg/ml). O. gratissimum oil andeugenol were moderate against fungi (P. chrysogenum MIC: 0.31 mg/ml), whereas methyl eugenol was not veryactive (MIC >5.0 mg/ml). Eugenol had a high radical scavenging capacity (DPPH IC 50: 0.18 mg/ml; ABTS IC 50:0.12mg/ml) in the antioxidant assays and was similar to Trolox. Methyl eugenol was more than 25 times less active(DPPH IC 0: 4.80 mg/ml) and O. sanctum oil had low antioxidant capacity (DPPH IC 0: 5.20 mg/ml). Free phenolichydroxyl group of eugenol is of significant importance to both the strong antimicrobial and antioxidant propertiesbut the methylation of eugenol to methyl eugenol drastically decreases bioactivity. The chemotype that has higheugenol content in O. gratissimum in the Western Ghats of North Karnataka is a good source of bioactivecompounds. This omnipresent oil and its principal component eugenol should be further investigated as aprospective drug, preservative in natural foodstuffs, and complementary medicine in the battle against AMR andoxidative stress disease.