Combination effects of essential oils with antimicrobials

Antimicrobal resistance is a leading cause of treatment failure for infectious diseases, resulting in increased mortality and morbidity. Several attempts have been made to combine for example essential oils, which are known to have antimicrobial effects, with conventional antibiotics in order to achieve synergistic activity [1].

The antibacterial effects of pine oil (Pinus sylvestris), peppermint oil (Mentha piperita) and thyme oil (Thymus vulgaris) were tested individually and in combinations with rifampicin and tetracycline in a preliminary agar diffusion assay. Thyme oil and pine oil were further investigated. A MIC determining assay was conducted with serial dilutions in 96-well plates. Fractional inhibitory concentration values were calculated and presented as isobolograms.

Synergy was detected in one combination of rifampicin and peppermint oil, whilst several other combinations presented additive results. However, the main part of the tested combinations demonstrated strong antagonism. Thyme oil demonstrated the strongest antimicrobial effect on its own whilst peppermint oil contributed mostly to synergistic interactions.

The results demonstrate that the combination of antibiotics with essential oils can be a promising approach to increase the effectivity of antibiotics, although further studies must be conducted to determine the adequate antibiotic-to-oil ratios.

• References

• 1 Langeveld WT, Veldhuizen EJ, Burt SA. Synergy between essential oil components and antibiotics: a review. Crit rev microbiol 2014; 40: 76-94