Inhibition of bacteria and changes in susceptibility to antibiotics following the addition of plant essential oils

Abstract

Background – With antibiotic resistance, the difficulty in developing new antibiotics and synthetic antibacterial compounds causing toxicity to the patient, on the rise, many have turned to researching natural antibacterial alternatives, plant essential oils. Traditionally, these were used to treat minor ailments, but in time additional medicinal uses have been discovered which have aided in combating serious illnesses. This study aims to com-pare the efficacy of some essential oils and determine whether some of them are capa-ble of enhancing antibiotic sensitivity. Materials and methods – Antibacterial properties of 11 plant essential oils, seaweed E., seaweed A., tea tree, lavender, clove, lemon, basil, manuka, thyme, oak moss and peppermint, were initially investigated against four bacteria, Pseudomonas aeruginosa, Proteus vulgaris, Staphylococcus aureus and Bacillus subtilis. The disc diffusion method was used to compare the oils efficacy with each other by measuring zones of lysis at 1%, 10% and 100%. Afterwards, the MIC and sub-MIC were then determined by testing the oils at 1%, 0.5%, 0.25%, 0.125%, 0.0625%, 0.0312% and 0.016% against the bac-terium control. Only five oils, basil, manuka, peppermint, tea tree and clove, and two bacteria, Pseudomonas aeruginosa and Staphylococcus aureus, were selected to be tested with antibiotics at a sub-MIC to determine whether they can improve antibiotic activity. Results – In the higher concentrations, all the oils showed antibacterial efficacy at some point. Pseudomonas aeruginosa, displayed resistance to all but two out of the six anti-biotics tests, streptomycin and tetracycline. All oils that either increased or lowered the antibiotics’ sensitivity, only did it by a very small amount. When the oils were tested against Staphylococcus aureus, most of them helped to improve the antibacterial activity of the all the antibiotics, with clove oil showing extraordinary action. In contrast, there were oils, such as basil and manuka, that reduced the antibiotics potency or completely inhibited it from performing any antibacterial activity. Conclusion – All of the selected plant essential oils were capable of some antibacterial action at different concentrations. However, only some of the five chosen to be carried forward for antibiotic testing can improved antibiotic sensitivity, whereas others can re-duce or even entirely inhibit it. Clove oil appears to be particularly effective. Pseudo-monas aeruginosa is known for its resistance and shows that in this study as well. How-ever, some oils are capable of further improving antibacterial potency of the antibiotics it is not resistant to. Key words – Essential oils, antibiotic resistance, antibacterial activity.