Comparative assessment of essential oils for antioxidant and anti-microbial potential against common enteric pathogens

Six essential oils, oregano (Origanum vulgare) oil, cinnamon (Cinnamomum verum) leaf oil, cinnamon (C. zeylanicum) bark oil, clove (Syzygium aromaticum) leaf oil, ajwain (Trachyspermum ammi) oil, and clove (S. aromaticum) bud oil were assessed for their antimicrobial and antioxidant properties. The antibacterial activity was determined by measuring the diameter of the zone of inhibition (ZOI), minimum inhibitory concentration (MIC) and fractional inhibitory concentration (FIC). Oregano, cinnamon bark, and clove leaf oils showed larger zone of inhibition and lower MIC against the pathogenic bacteria viz., strains of Escherichia coli and Salmonella sps. compared to other essential oils. These three most effective oils were checked for their antioxidant activity by DPPH, ABTS and total nitric oxide assay. The results revealed that cinnamon bark oil had higher antioxidant activity than oregano and clove leaf oils. Further, some combinations of these three oils showed a synergistic anti microbial effect (FIC<0.5) against selective pathogenic bacteria tested. Overall among all the six essential oils, cinnamon bark oil showed the lowest MIC with higher zone of inhibition (ZOI) for most of the pathogens studied. Among the three best oils, cinnamon bark oil was superior to oregano oil and clove leaf oil for antioxidant activity.