Synergistic antioxidant and antimicrobial activities of essential oils of some selected medicinal plants in combination and with synthetic compounds

Regardless of the well-known antioxidant and antimicrobial properties of essential oils, requirement of their large doses is the major bottleneck in using them as food preservatives. In the present study, possible synergistic interactions on antioxidant and antibacterial capacity of essential oils from six selected medicinal plants namely Callistemon lanceolatus (Sm.) Sweet, Ocimum gratissimum L., Cymbopogon winterianus Jowitt ex Bor. (Citronella java), Cymbopogon flexuosus (Nees ex Steud.) W. Watson, Mentha longifolia (L.) L., and Vitex negundo L., in combination as well as with synthetic antioxidant and antibiotics were evaluated. Antibacterial synergistic effect was determined against three Gram positive (Staphylococcus aureus, Micrococcus luteus and Bacillus subtilis) and two Gram negative (Escherichia coli and Klebsiella pneumoniae) bacteria using broth dilution method. Antioxidant combination effect was evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging method. The types of interactions in different binary combinations were determined by calculating the fractional inhibitory concentration indexes (FICI). Synergistic interactions were observed between essential oil combinations showing enhanced antioxidant and antibacterial activity. Further it was observed that essential oils not only enhanced the antioxidant and antibacterial efficacy of synthetic compounds but also resulted in significant reduction of their active doses due to synergy.