The in vitro antimicrobial activity of Cymbopogon essential oil (Lemon Grass) and its interaction with silver ions

It is well known that Cymbopogon (Lemon grass) essential oil exhibits antimicrobial activity while the efficacy of silver ions as a disinfectant has also been known for centuries. In the present study, the chemical composition and in vitro antimicrobial activity of six different Cymbopogon essential oils(CEO) alone and in combination with silver ions (Ag⁺) against two Gram-positive (Staphylococcus aureus and Enterococcus faecalis), two Gram-negative (Escherichia coli and Morexella cattarhalis) and two yeast species (Candida albicans and Candidatropicalis)is presented.The nature of the interactions wasstudied by determining fractional inhibitory concentration indices (FICIs) for CEO and Ag⁺ calculated from microdilution assays and time-kill curves. Gas chromatography-mass spectrometry results showed that neral, geranial and geraniol are the major compounds in CEO. Minimum inhibitory concentration (MIC) results depicted that all the tested pathogens are variably susceptible to both CEO as well as Ag⁺. The MIC of CEO and Ag⁺againstall the tested pathogens ranged from 0.032 mg/ml to 1 mg/ml and 0.004 and 0.064 mg/ml respectively, whereas when assayed in combination the FICI values were drastically reduced to range between 0.258 and 2.186 mg/ml, indicating synergy, additive and indifferent interactions. The most prominent interaction was observed between Cymbopogon martini essential oil and Ag⁺ against E. faecalis with ∑FIC=0.254. The synergistic interactions were further confirmed through the construction of isobolograms and time-kill plots. Results suggest that CEO and Ag⁺ when used in combination offers an opportunity to the formulation scientist to produce novel combinations acting synergistically in the continued quest to control important infectious pathogens.