Antimicrobial activity and chemical analysis of eucalyptus radiata leaf essential oil

Abstract

Eucalyptus radiata is one of the understudied medicinal Eucalyptus essential oils. It is an important aromatic oil, used for a variety of infectious conditions independently or in combination with other essential oils. Its antiinfective uses are an indicator of the antimicrobial potential of the essential oil and its compounds. However, the chemical composition and antimicrobial activities of this plant growing in South Africa is yet to be investigated. A basis of scientific evidence needs to be established in order to better understand its therapeutic properties, antiinfective potential and advocate its proper use in medicinal practices. Therefore, this study was designed to determine yield, chemical composition, antimicrobial activity and antiquorum sensing activity of the E. radiata leaf essential oil. Another aspect was to determine the influence of seasonal variation and leaf age on the essential oil yield and chemical composition over a 12 month period. Young and mature leaves of the E. radiata species were sampled monthly over a period of one year (January to December 2014). The essential oil was obtained using the hydro-distillation method. Higher yields were obtained in the seasons of summer and spring (0.90% - 4.31% w/w), characterized by high temperature and high rainfall in comparison to autumn and winter (0.14% - 2.83% w/w). The chemical composition was analysed by gas chromatography coupled to mass spectrometry (GC-MS). The major compounds identified within all samples regardless of seasonal variation and leaf maturity were; 1,8-cineole (64.1% ± 11.9), α-terpineol (12.4% ± 4.6) and limonene (3.6% ± 2.7). The minimum inhibitory concentration (MIC) assay was used to determine the antimicrobial activity of the essential oil independently, in comparison to commercial Eucalyptus essential oils and in 1:1 combinations with other essential oils. The Streptococci (0.19 - 2.00 mg/ml) and Lactobacillus acidophilus (0.19 - 1.75 mg/ml) showed the highest sensitivities. The E. radiata sample exhibited similar antimicrobial efficacy to commercial Eucalyptus essential oils. The antimicrobial activities of the major compounds were evaluated independently and in combination at 1:1 ratios and in various ratios relative to the arrangement in the E. radiata leaf essential oil. Independently, all major compounds; 1,8-cineole (2.00 mg/ml), α-terpineol (0.75 - 1.00 mg/ml) and S-(˗)-limonene (0.25 - 0.75 mg/ml) and R-(+)-limonene (0.25 - 0.63 mg/ml) displayed noteworthy antimicrobial activity. The sum of the fractional inhibitory concentration (ΣFIC) was used to determine the type of interactions observed from the compound combinations. The 1:1 combinations resulted in more synergistic interactions in comparison to combinations at relative ratios. Combinations with limonene resulted in better antimicrobial activity. When E. radiata essential oil was screened at 1:1 ratios with other oils, additive antimicrobial interactions were frequently demonstrated from the 1:1 combinations against Staphylococcus aureus (66.67%) in comparison to Pseudomonas aeruginosa (8.33%) and Candida albicans (16.67%). The broth macrodilution assay was used to screen for antiantiquorumquorum sensing activity against the biomonitor strain Chromobacterium violaceium (ATCC 12742) in dependently and at 1:1 combinations. Eucalyptus. radiata leaf essential oil displayed antiquorum sensing activity against C. violaceium with a 95.30% percentage violacein inhibition at a minimum quorum sensing inhibitory concentration (MQSIC) of 0.50 mg/ml. The 1:1 combination of E. radiata: Melaleuca alternifolia resulted was the most noteworthy outcome, thus the major compounds were investigated further. Two synergistic interactions were noted with the 1:1 combinations of α-terpinene and α-terpineol (ΣFQSIC 0.19), and 1,8-cineole and α-terpineol (FQSICI 0.19). This study demonstrated the in vitro antimicrobial properties of the E. radiata leaf essential oil, which may serve as credence for its use in the treatment of infectious conditions. The bioactivity of its major compounds highlights E. radiata leaf essential oil as a source of bioactive compounds with potential antimicrobial applications. This study also introduced E. radiata essential oil as a quorum sensing inhibitor.