Composition and antimicrobial activity of the essential oil from the branches of Jacaranda cuspidifolia Mart. growing in Sichuan, China

Abstract This study represents the first report on the chemical composition and antimicrobial activity of the essential oil from the branches of Jacaranda cuspidifolia Mart. Thirty-three compounds were identified by Gas Chromatography-Mass Spectrometry (GC-MS) and the major constituents of the essential oil were Palmitic acid (31.36%), (Z) − 9,17-Octadecadienal (12.06%), Ethyl palmitate (3.81%), Perhydrofarnesyl acetone (2.07%), γ-Maaliene (1.88%), and Cedro (1.42%) and 9,12-Octadecadienoic acid ethyl ester (1.42%). The in vitro antimicrobial activities of the essential oil were evaluated by the disc diffusion method, and the inhibition zones against Escherichia coli, Staphylococcus aureus and Candida albicans were 7.10, 8.20 and 7.25 mm, respectively. The oil showed moderate activities against E. coli, S. aureus and C. albicans with minimum inhibition concentration (MIC) values of 17.3 mg/mL, 12.9 mg/mL and 16.0 mg/mL, respectively.


Plant material
The branches of the J. Cuspidifolia were collected at Xinjin District, Sichuan Province, China in April, 2016, and identified by associate Prof. Liang-Ke Song, Southwest Jiaotong University.
The specimen (JYLHY01) was deposited at the Natural Products Chemistry Laboratory of Southwest Jiaotong University.

Isolation of esential oil
The powders of the branches of J. Cuspidifolia were submitted to hydrodistillation for 3 hours in a Clevenger-type apparatus according to the Chinese Pharmacopoeia. Then the essential oil was collected, dried with anhydrous Na 2 SO 4 , measured, and transferred to glass flasks that were filled to the top and kept at a temperature of -4°C for further analysis.

Gas Chromatographic (GC)
Gas Chromatographic analysis was performed with a Thermo Trace Gas Chromatograph (HP6890, USA) equipped with a DB-1 fused-silica capillary column (30 m × 0.25 mm, 0.25 μm film thickness). The chromatographic conditions were as follows: The oven temperature increased from 50°C to 280°C at a rate of 5°C/min. The injector and detector temperatures were 280°C and 250°C, respectively. Helium used as the carrier gas was adjusted to a linear velocity of 32 cm/s. The samples were injected using split sampling technique by a ratio of 1:10. Quantitative data was obtained from electronic integration of peck areas without the use of correction factors.

Gas chromatography-Mass spectrometry (GC-MS)
GC-MS analysis was performed with a Thermo Trace Gas Chromatograph equipped with a Polaris-Q External ion Trap Mass Spectrometer (HP6890-HP5973, USA). A DB-1 fused-silica capillary column (30 m × 0.25 mm, 0.25 μm film thickness) was used with a Helium carrier gas at 1.0 ml/minute; split mode (1:10); 0.1μl as injection volume; 250°C as injection temperature. The oven temperature programme is described in section Gas chromatography. An ionisation mode with electronic impact at 70 eV over a scan range of 30-450 atomic mass units was used.
Data is processed by HP MSD Chemistry Station, and then follows the peak area normalization method to calculate the percentage of part of the chemical composition of the volatile oil. The constituents of the essential oils were identified by comparing their retention indices relative to n-alkanes (C8-C20). Individual compounds were identified by comparing their recorded mass spectra with those stored in the NIST2008 mass spectral library of the GC-MS.

Antimicrobial activity
This study used Escherichia coli (ATCC, 25922), Staphylococcus aureus (ATCC, 25923), Pseudomonas aeruginosa (ATCC, 27853) and Candida albicans (ATCC, 10231) in the antimicrobiall activity assay with the agar disc diffusion method (Wei et al. 2016). These strains were purchased from West China School of Pharmacy, Sichuan University. Amoxicillin (Kunming baker NORTON pharmaceutical co., LTD, China) was used as positive controls. The concentration of the test sample and the reference substance all was 0.1g/ml (by test tube method). Filter paper discs (6.0mm in diameter, 20 μl in water absorption) were placed on the surface of culture dish, which were soaked with the essential oil. Each test was carried out in triplicate. After incubation at 37°C (Bacterial) or 30°C (Fungi) for 24 hours, the diameters of the inhibition zones were measured in millimetres. Pseudomonas aeruginosa ---Notes: a Includes diameter of disc (6.0mm). b For all tests, 0.1g/mg of essential oil were applied. c "-" means that the compounds were inactive against the investigated bacterium.

Determination of the minimum inhibitory concentration
The minimum inhibitory concentration (MIC) was calculated with the cylinder plate method (Boney et al. 2008). A serial two-fold dilution of the oil in DMSO was carried out. Cups were made in inoculated agar surface which was filled with a different concentration of the essential oil.
The plates were incubated overnight at temperatures 37ºC (Bacteria) and 30ºC (Fungi). Diameters of inhibition zones were plotted against log concentration to obtain a standard calibration curve from which log MIC was determined by extrapolation at cup diameter (10 mm). The same method was applied for the standard antibiotics.